Written StudyGuide CCIE Routing

Routing and Switching Written Qualification Exam (350-001)

Table of Contents
Cisco Device Operation .................................................................................................................................................. 7
Commands..................................................................................................................................................................... 7
Infrastructure.................................................................................................................................................................. 7
Configuration Register................................................................................................................................................ 7
Configuration Register................................................................................................................................................ 8
Software Configuration Bit Meanings......................................................................................................................... 8
Bunch of Bits (some of the more interesting Configuration Register Bits, and what they do) .................................. 9
More Bits .................................................................................................................................................................. 10
Seeing and Changing Configuration Register Settings............................................................................................ 11
Boot Command ........................................................................................................................................................ 11
My simplistic description of the boot sequence........................................................................................................ 11
Operations.................................................................................................................................................................... 11
Password recovery................................................................................................................................................... 11
Copying and Backing up Configuration Files ........................................................................................................... 11
Configuring a new router .......................................................................................................................................... 12
Security & Passwords .............................................................................................................................................. 12
General Networking Theory.......................................................................................................................................... 13
OSI Models .................................................................................................................................................................. 13
MAC Addressing ...................................................................................................................................................... 13
General Routing Concepts........................................................................................................................................... 14
Standards..................................................................................................................................................................... 15
Ethernet Cable Specifications .................................................................................................................................. 15
Protocol Mechanics...................................................................................................................................................... 16
Transmission Control Protocol (TCP) ...................................................................................................................... 16
Fragmentation & MTU .............................................................................................................................................. 17
Bridging and LAN Switching ........................................................................................................................................ 17
Transparent Bridging (TB) ........................................................................................................................................... 17

Translational Bridging............................................................................................................................................... 18
Integrated Routing and Bridging (IRB) ..................................................................................................................... 18
Bridge ACL & Filtering.............................................................................................................................................. 18
Multiple-Instance Spanning Tree Protocol (MISTP)................................................................................................. 19
Source-Route Bridging (SRB)...................................................................................................................................... 19
Data Link Switching (DLSw) and DLSw+ ................................................................................................................. 20
Source-Route Transparent Bridging (SRT) and Source-Route Translational Bridging (SR/TLB) .......................... 20
LAN Switching.............................................................................................................................................................. 21


Routing and Switching Written Qualification Exam (350-001)
Switching Technique Types ..................................................................................................................................... 21
Command-Line Interface (CLI)................................................................................................................................. 21
Trunking.................................................................................................................................................................... 22
Virtual LAN (VLAN) .................................................................................................................................................. 23
VLAN Trunk Protocol (VTP) ..................................................................................................................................... 23
Spanning-Tree Protocol (STP) ................................................................................................................................. 23
Root Bridges and Switches ...................................................................................................................................... 24
Bridge Protocol Data Units (BPDUs)........................................................................................................................ 24
How STP Works ....................................................................................................................................................... 24
STP Timers .............................................................................................................................................................. 24
Ports in an STP domain will progress through the following states: ........................................................................ 24
Notes about STP Port States: .................................................................................................................................. 25
STP Enhancements: ................................................................................................................................................ 25
DISL.......................................................................................................................................................................... 26
Fast Ether Channel (FEC)........................................................................................................................................ 26
Cisco Discovery Protocol (CDP) .............................................................................................................................. 26
CGMP....................................................................................................................................................................... 26
Security ........................................................................................................................................................................ 26
802.1X ...................................................................................................................................................................... 27

Multi-Layer Switching (MLS) ........................................................................................................................................ 27
Multi-Layer Switching (MLS) ........................................................................................................................................ 28
Internet Protocol (IP) ..................................................................................................................................................... 28
IP Addressing............................................................................................................................................................... 28
Subnetting ................................................................................................................................................................ 28
Subnetting Tricks...................................................................................................................................................... 29
Route Summarization............................................................................................................................................... 29
Services & Applications ............................................................................................................................................... 30
DNS .......................................................................................................................................................................... 30
ARP & RARP............................................................................................................................................................ 30
BOOTP & DHCP ...................................................................................................................................................... 30
ICMP......................................................................................................................................................................... 31
NAT .......................................................................................................................................................................... 31
HSRP & VRRP ......................................................................................................................................................... 31
Telnet........................................................................................................................................................................ 32
FTP & TFTP ............................................................................................................................................................. 32
SNMP ....................................................................................................................................................................... 32
Access Control Lists (ACL) .......................................................................................................................................... 32


Routing and Switching Written Qualification Exam (350-001)
Access list types are designated by the list Numbers:............................................................................................. 33
Internet Protocol Version 6 (IPv6)................................................................................................................................ 33
IP Routing....................................................................................................................................................................... 34
Routing Protocol Concepts .......................................................................................................................................... 34
Distance-Vector Routing Protocols .......................................................................................................................... 34
Link State Routing Protocols .................................................................................................................................... 34
Hybrid Routing Protocols.......................................................................................................................................... 34
Distribution Lists ....................................................................................................................................................... 35
Routing Loops .......................................................................................................................................................... 35

Administrative Distance............................................................................................................................................ 36
Open Shortest Path First (OSPF) ................................................................................................................................ 36
Area 0 ....................................................................................................................................................................... 37
OSPF Area Types: ................................................................................................................................................... 37
Stub and Totally Stubby Area Similarities: ............................................................................................................... 37
Stub and Totally Stubby Area Differences: .............................................................................................................. 38
Router Types: ........................................................................................................................................................... 38
Traffic Types:............................................................................................................................................................ 38
NMBA Networks ....................................................................................................................................................... 38
LSA Types:............................................................................................................................................................... 39
Routing Authentication ............................................................................................................................................. 39
Border Gateway Protocol (BGP).................................................................................................................................. 39
Synchronization/Full Mesh ....................................................................................................................................... 40
Next-Hop-Self Command ......................................................................................................................................... 40
BGP Path Selection.................................................................................................................................................. 40
Scalability Problems (and Solutions) with IBGP....................................................................................................... 41
Configuring Neighbors & Networks .......................................................................................................................... 41
Route Dampening .................................................................................................................................................... 41
Enhanced Interior Gateway Routing Protocol (EIGRP) ............................................................................................... 42
Tables:...................................................................................................................................................................... 42
Choosing routes: ...................................................................................................................................................... 43
Intermediate System-to-Intermediate System (IS-IS).................................................................................................. 43
Access-Control & Filtering ........................................................................................................................................... 44
Distribution Lists ....................................................................................................................................................... 44
Route-Maps .............................................................................................................................................................. 44
Policy Routing .......................................................................................................................................................... 45
Redistribution ........................................................................................................................................................... 45
Route-Tagging.......................................................................................................................................................... 45

Routing and Switching Written Qualification Exam (350-001)
Dial-on-Demand Routing (DDR) .................................................................................................................................. 45
DDR has two important applications: ....................................................................................................................... 45
Encapsulation Methods for DDR:............................................................................................................................. 45
Dial Backup .............................................................................................................................................................. 45
Interior Gateway Routing Protocol (IGRP) ............................................................................................................... 46
Router Information Protocol (RIP) Version 1 and 2 ................................................................................................. 46
QoS ................................................................................................................................................................................. 46
Fancy Queuing............................................................................................................................................................. 46
Weighted Fair Queuing (WFQ) ................................................................................................................................ 46
Priority Queuing........................................................................................................................................................ 47
Custom Queuing ...................................................................................................................................................... 47
Packet over SONET/SDH (PoS) and IP Precedence.................................................................................................. 47
Class of Service (CoS)................................................................................................................................................. 47
Random Early Detection (RED) and Weighted RED (WRED) .................................................................................... 48
Weighted Round-Robin (WRR)/Queue Scheduling..................................................................................................... 48
Weighted Round-Robin (WRR)/Queue Scheduling..................................................................................................... 49
Shaping vs. Policing / Committed Access Rate (CAR)................................................................................................ 49
Committed Access Rate (CAR)................................................................................................................................ 49
Network-Based Application Recognition (NBAR) ........................................................................................................ 50
Configuring NBAR .................................................................................................................................................... 50
802.1x....................................................................................................................................................................... 51
Differentiated Services Code Point (DSCP) ................................................................................................................ 51
WAN ................................................................................................................................................................................ 51
Integrated Services Digital Network (ISDN)................................................................................................................. 51
ISDN Specifics ......................................................................................................................................................... 52
Channels .................................................................................................................................................................. 53
Flavors of ISDN ........................................................................................................................................................ 53
Point-to-Point Protocol (PPP)................................................................................................................................... 53
OSPF and ISDN ....................................................................................................................................................... 53

Frame Relay ................................................................................................................................................................ 53
Types of Circuits....................................................................................................................................................... 54
Data Link Connection Identifier (DLCI) .................................................................................................................... 54
Local Management Interface (LMI) .......................................................................................................................... 54
Encapsulation........................................................................................................................................................... 54
Frame-Relay Traffic Shaping (FRTS) ...................................................................................................................... 54
Frame-Relay Compression ...................................................................................................................................... 55
Frame-Relay Mapping.............................................................................................................................................. 55


Routing and Switching Written Qualification Exam (350-001)
Split Horizon and Frame Relay Interfaces ............................................................................................................... 55
Speed Elements........................................................................................................................................................... 55
Asynchronous Transfer Mode (ATM)........................................................................................................................... 55
ATM is comprised of four major layers:.................................................................................................................... 56
ATM Adaptation Layer (AAL) ................................................................................................................................... 56
IISP and PNNI .......................................................................................................................................................... 56
NSAP Format ATM Addresses ................................................................................................................................ 57
Service-Specific Connection-Oriented Protocol (SSCOP)....................................................................................... 57
RFC 1483 & RFC 2684 – Multiprotocol Encapsulation over AAL5 .......................................................................... 57
ATM Mapping ........................................................................................................................................................... 57
Physical Layer.............................................................................................................................................................. 58
Serial Interface Abbreviations .................................................................................................................................. 58
Is Your Interface a DTE or a DCE?.......................................................................................................................... 58
RS-232 ..................................................................................................................................................................... 58
V.35 Interface ........................................................................................................................................................... 59
Troubleshooting Serial Links .................................................................................................................................... 59
Show Controllers Command .................................................................................................................................... 61
Serial Line Conditions .............................................................................................................................................. 62
Debug Commands ................................................................................................................................................... 62

Increasing Output Drops .......................................................................................................................................... 63
Increasing Input Drops ............................................................................................................................................. 63
Excessive Aborts...................................................................................................................................................... 64
Clocking Problems ................................................................................................................................................... 64
Increasing Interface Resets on a Serial Link............................................................................................................ 65
Increasing Carrier Transitions Count on Serial Link ................................................................................................ 65
CRC and Framing Errors.......................................................................................................................................... 66
SONET / SDH .......................................................................................................................................................... 66
T1 Encoding ............................................................................................................................................................. 66
Leased Line Protocols.............................................................................................................................................. 67
HDLC........................................................................................................................................................................ 67
PPP .......................................................................................................................................................................... 67
Packet over SONET (PoS)....................................................................................................................................... 67
DPT / SRP ................................................................................................................................................................ 67
LAN ................................................................................................................................................................................. 68
Ethernet/FE/GE............................................................................................................................................................ 68
Ethernet/Fast Ethernet/Gigabit Ethernet .................................................................................................................. 68
Fast EtherChannel (FEC)......................................................................................................................................... 68


Routing and Switching Written Qualification Exam (350-001)
Carrier Sense Multiple Access Collision Detect (CSMA/CD)................................................................................... 68
Wireless/802.11 ........................................................................................................................................................... 69
Deployment issues for wireless include: .................................................................................................................. 69
Wireless Security...................................................................................................................................................... 69
Important wireless networking terms:....................................................................................................................... 70
Radio Frequency (RF) Terms:.................................................................................................................................. 70
Cisco Deployments .................................................................................................................................................. 70
Multiservice .................................................................................................................................................................... 71
Voice/Video .................................................................................................................................................................. 71

Coder-decoders (Codecs)............................................................................................................................................ 71
Signaling System 7 (SS7) ............................................................................................................................................ 71
Signaling System 7 (SS7) ............................................................................................................................................ 72
Real-Time Transport Protocol (RTP) ........................................................................................................................... 72
Real-Time Transport Control Protocol (RTCP)............................................................................................................ 72
Session Initiation Protocol (SIP) .................................................................................................................................. 72
Multiprotocol Label Switching (MPLS) ......................................................................................................................... 72
Definitions follow for the MPLS terms: ..................................................................................................................... 73
MPLS Operations ..................................................................................................................................................... 73
How the LFIB is Propagated .................................................................................................................................... 74
Quality of Service and Traffic Engineering............................................................................................................... 74
IP Multicast..................................................................................................................................................................... 74
Addressing ................................................................................................................................................................... 75
Translate Multicast Addresses into Ethernet MAC addresses................................................................................. 76
Internet Group Management Protocol (IGMP) and Cisco Group Management Protocol (CGMP).............................. 77
IGMP ........................................................................................................................................................................ 77
CGMP....................................................................................................................................................................... 78
IGMP Snooping ........................................................................................................................................................ 78
Multicast Distribution Trees.......................................................................................................................................... 79
Protocol Independent Multicast (PIM).......................................................................................................................... 79
PIM-Spare Mode Mechanics........................................................................................................................................ 80
PIM-SM Joining & Pruning ....................................................................................................................................... 80
IP Multicast Routing Table (mroute)......................................................................................................................... 80
Distribution Trees......................................................................................................................................................... 80
Rendezvous Points ...................................................................................................................................................... 80
Bootstrap Router (BSR) ........................................................................................................................................... 81


Routing and Switching Written Qualification Exam (350-001)

Cisco Device Operation
Commands
Cisco routers are configured and maintained primarily through the issuing of IOS commands. If you have reached the
point of preparing for the CCIE Written exam, I must assume that you have spent considerable time configuring Cisco
routers and switches. You should, however, make sure you have a complete understanding of how the different
technologies are configured, and thorough knowledge of the show and debug commands that are used to troubleshoot
them.
A note on debug commands: you should know that debug commands can seriously stress the resources of a router,
and they should be used carefully and as conservatively as possible when working in a production environment.

Infrastructure
The infrastructure of a Cisco router includes the main board, memory, CPU, Flash and interfaces. You should
understand what each of these devices does, and how they interact. The most commonly misunderstood are:
RAM (Random Access Memory) – In all but a few low-end routers like 2500’s, the RAM holds the running version of
the IOS and the current running configuration. This is also where the routing tables, caches, and queues are stored.
Remember that when the router is powered-off, everything in RAM is lost.
ROM (Read-Only Memory) – Holds some basic router commands and usually a limited version of Cisco IOS
(Internetwork Operating System). It also houses the power-on diagnostics and the bootstrap program. The ROM is
read-only and cannot be changed.
NVRAM (Non-Volatile Random Access Memory) – This is where the router’s saved configuration file is stored. This
information will not be lost if the router is powered down.
Flash memory – Home for the router’s IOS image and microcode. Prior to installing any IOS, ensure that you have
enough flash to support the proposed image. Depending on the version and feature set of the IOS, the image can be
of various sizes. Newer versions with more powerful features will often require additional flash. Remember that files
deleted from flash can remain in place, marked for deletion, until the “squeeze” command is issued.


Routing and Switching Written Qualification Exam (350-001)

Configuration Register

Early Cisco routers had a set of hardware switches that controlled certain aspects of the router’s performance, such as
the boot sequence. This was phased out some time ago, but there is now a software equivalent, the sixteen-bit
Software Configuration Register, which is written into nonvolatile memory.
Common reasons for modifying the register include:
Recovering a lost password
Changing the router boot configuration to allow Flash or ROM boot
Loading an image into Flash memory
Enabling or disabling the console break key
Here are some of the common Configuration Register values:
0x2102 – The most common value, which establishes booting to flash and NVRAM
0x2142 – The value used most commonly to recover passwords
0x2100 – Boots using the bootstrap found in ROM

Software Configuration Bit Meanings

* Please note that a boot system global command in the router’s NVRAM configuration will override the
default net-boot filename.


Routing and Switching Written Qualification Exam (350-001)

Bunch of Bits (some of the more interesting Configuration Register Bits,
and what they do)
Bits 0,1,2 and 3 are known collectively as the boot field, and determine where the router will load its IOS image from.
If the boot field value is 0x0, you will need to boot the operating system manually by entering the “b” command
at the bootstrap prompt.
If the boot field value is 0x1 (the factory default), the router will boot using the default ROM software.
If the boot field has any other value, the router uses the resulting number to form a default boot filename for
network booting, which is created as part of the automatic configuration process. To form the boot filename, the
server starts with the word “cisco”, attaches the octal equivalent of the boot field number, then a dash, and finally

the processor-type name. The following table lists the default boot filenames for boot field values between 0x2
and 0xf on an IGS router.
Default Boot Filenames
Bit 3 Bit 2

Bit 1 Bit 0 Hex Value

Net-boot
Filename

0

0

1

0

0x2

cisco2-igs

0

0

1

1

0x3

cisco3-igs

0

1

0

0

0x4

cisco4-igs

0

1

0

1

0x5

cisco5-igs

0

1

1

0

0x6

cisco6-igs

0

1

1

1

0x7

cisco7-igs

1

0

0

0

0x8

cisco10-igs

1

0

0

1

0x9

cisco11-igs

1

0

1

0

0xa

cisco12-igs

1

0

1

1

0xb

cisco13-igs

1

1

0

0

0xc

cisco14-igs

1

1

0

1

0xd

cisco15-igs

1

1

1

0

0xe

cisco16-igs

1

1

1

1

0xf

cisco17-igs

It’s important to remember that the boot sequence, baring the involvement of “boot system” commands in the
configuration, is Flash, Network, ROM.

Routing and Switching Written Qualification Exam (350-001)

More Bits
Bit 4 enables "Fast Boot", which is only supported on a dual RSP chassis. This allows the "slave" RSP to reload
without going through an IOS load sequence; just reload the config file and go. The documentation says it will
accomplish a fast boot in approximately 30 sec.
Bit 6 determines whether the router should load its startup config from NVRAM (1) or not (0). This is the key bit
used for recovering a lost password. If it is turned on, the startup configuration (usually in NVRAM) is ignored.
This will allow you to log in without using a password and display the startup configuration passwords.
Bit 7 allows Cisco boot messages to be suppressed when IOS is licensed to another manufacturer.
Bit 8 controls the console Break key. Setting bit 8 on (the factory default) causes the processor to ignore the
console Break key. Clearing bit 8 causes the processor to interpret the break as a command, which forces the
system into the bootstrap monitor, halting normal operation. Remember that a break can be issued anytime
during the first 60 seconds of booting to go to ROM mode, regardless of the configuration settings.
Bit 10 controls the host portion of the IP broadcast address. Setting bit 10 causes the processor to use all zeros;
clearing bit 10 (the factory default) causes the processor to use all ones. Bits 10 and 14 interact to control the
network and subnet portions of the broadcast address. This table shows how these settings are configured.

Bit 14

Bit 10

Address
(<net><host>)

Off

Off

<ones><ones>

Off

On

<zeros><zeros>

On

On

<net><zeros>

On

Off

<net><ones>

Bits 11 and 12 determine the baud rate of the console port. The default setting is 9600 (00). The most common
reason for changing the speed is to increase the speed at which you can transfer a new IOS version through the
console port connection. Here are the possible combinations of these two bits, and the speeds they represent:

Bit 12 Bit 11 Baud Rate
0

0

9600

0

1

4800

1

0

1200

1

1

2400

Bit 13 determines the router’s response to a boot load failure. If the bit is turned on (1), it causes the server to
load IOS from ROM after five unsuccessful attempts to load a boot file from the network. If the bit is set to “0”
(factory default), the router will continue trying to load a boot file from the network indefinitely. The important
thing to remember is that if the bit is (0) and no IOS is found the router will hang. If the bit is (1), and no IOS is
found, the router will boot from ROM.


Routing and Switching Written Qualification Exam (350-001)
Bit 14 controls the network and subnet portions of the broadcast address and allows subnet or directed
broadcasts. It should be seen as being related to the function of bit 10.

Bit 15 in a hardware configuration register causes NVRAM configuration files to be ignored. This is not true of
virtual configuration registers.

Seeing and Changing Configuration Register Settings
To display the current configuration register value and the value that will be used next time the router is loaded (if the
two values are different) use the “show version” enable command.
The “config-register” global command is used to modify configuration register settings while the operating system is
running. Remember that configuration register changes only take effect when the router is rebooted.

Boot Command
You can alter the boot sequence by using the “boot” global configuration command. Here are several possible
configurations:
Boot from a specific Flash image (using the boot system flash filename command).
Boot from an undefined network server by sending broadcast TFTP requests (using the boot system filename
command).
Boot from a specific network server by sending a direct TFTP request to a specific IP address (using the boot
system filename address command).

My simplistic description of the boot sequence
The main thing to remember is that with standard configuration register settings (last four bits are between 0x2 and
0xF), and if there are “boot system” commands present in the startup, the boot sequence will not attempt to boot from
the network using the default image name. If there aren’t any “boot system” commands, it will attempt a network boot:
With “boot system” commands in the configuration - Flash, ROM
Without “boot system” commands in the configuration - Flash, Network, ROM

Operations
Password recovery
For every family of Cisco routers and switches, there is a procedure for hacking out the password when it is lost. To
develop a basic understanding of how this is done you should review the procedures for several devices, including the
2600 and 3700 routers, and the Cat3550 switches. These are explained in detail on the Cisco website at

If you have physical access to this equipment, I would recommend
following the procedures several times to get familiar with the process.

Copying and Backing up Configuration Files
You can and should understand (and practice) backing up the running configurations on your routers. This can be
done to Flash as the startup configuration, or even better, to an off-router TFTP server.


Routing and Switching Written Qualification Exam (350-001)

Configuring a new router
There are several ways to prepare a new router for production, including:
Connecting to the console port of the router with a rolled cable, and
running the Setup dialog that appears when the router first boots up.
Connecting to the console port with a rolled cable, bypassing the
Setup dialog, and manually typing the configuration commands.
Connecting to the console port with a rolled cable, defining a minimum
configuration, and using TFTP to download an existing predefined
configuration file.
Use BOOTP with SLARP/RARP to download an existing configuration
file.

Security & Passwords
Below are the different types of router passwords:
Privileged Mode / Enable Password – There are two types of
passwords that allow you to move from user mode to privileged mode.
They are the enable password and the enable secret password.
Enable – this is an unencrypted password used to allow the
movement into privileged mode. From privileged mode, you
could move into global configuration mode. To configure an

enable password you would type:
Router(config)# enable password cisco
•

Secret - this is an encrypted password used to allow the
movement into privileged mode. From privileged mode, you
could move into global configuration mode. If you configure
a secret password and do a “show running-configuration”,
you will not be able to see your password as it will be in an
encrypted form. To configure a secret password you would
type:
Router(config)# enable secret cisco

Although you can have both passwords configured, the enable
secret overrides the enable password.
Console Password – to protect the console from unauthorized
access, you would configure a console password. To configure a
console password you would type:
Router(config)# line console 0
Router(config-line)# login
Router(config-line)# password cisco
The login commands enable password checking on the line. Without
the login command, the password can be configured but you are not
prompted to enter the password.
Vty Password – inbound telnet lines to the router/switch are called vty
lines (virtual TTY lines). To protect these lines from unauthorized
network access, you would configure a vty password. By default, there


Routing and Switching Written Qualification Exam (350-001)

are 5 of these lines (zero through four). To configure a vty password, on all 5 lines, you would type:
Router(config)# line vty 0 4
Router(config-line)# login
Router(config-line)# password cisco

General Networking Theory
OSI Models
Most people who attempt the CCIE Written have either gone through the CCNA and CCNP exams, or already have a
solid background in networking. In either case, I’m sure you have a solid grasp on the OSI model; but it’s on the
blueprint and therefore deserves at least a quick review.
The OSI model is a common tool for conceptualizing how network traffic is handled. For the CCIE track, the bulk of
your focus will be on the three lower levels. Just a reminder, you can use the old mnemonic “All People Seem To
Need Data Processing” as a way to help remember the sequence. The seven layers of the OSI model are:
Application –Provides services directly to applications.
Presentation –Provides a variety of coding and conversion functions that ensure information sent from the
application layer of one system will be readable by the application layer of another.
Session –Establishes, manages, maintains, and terminates communication sessions between applications.
Transport – Segments and reassembles data into data streams, and provides for both reliable and unreliable
end-to-end data transmission.
Network – Applies logical addressing to provide routing and related functions to allow multiple data links to be
combined into an internetwork. Network layer protocols include routing and routed protocols (make sure you
know the difference between these).
Data Link – The data link layer provides for reliable transmission of data across physical media. The Data link
layer is commonly subdivided into two sub-layers, known as the Media Access Control (MAC) Layer and the
Logical Link Control (LLC) layer.
LLC – The LLC sub-layer manages communications between devices over a single link of a network.
It provides error control, flow control, framing, and MAC sub-layer addressing.
MAC –The MAC layer manages addressing and access to the physical layer.
Physical – The electrical, mechanical, procedural, and functional specifications for activating, maintaining, and
deactivating the physical link between communicating network systems.

Note: Remember that routing is handled at Layer-3 of the OSI model, while bridging is handled at Layer-2 of the OSI
model.

MAC Addressing
Media Access Control (MAC) is the lower of the two sub-layers of the Data Link Layer defined in the OSI model, which
provides access to the shared media. MAC addresses are the standard, unique address that every networked device
must have; it is the true burned-in physical address of the Network Interface Card (NIC) in a host, server, router
interface or other device on a network. They are 6 bytes (48 bits) long and are controlled by the IEEE. They can be
broken down into two sub-fields:
The first three bytes (24 bits) are called the Organization Unique Identifier (OUI) field and are issued in series to
manufacturers.


Routing and Switching Written Qualification Exam (350-001)
The second part of the MAC address, the last three bytes (24 bits), is a unique identifier burned into the device
by the manufacturer from the series issued to it.

General Routing Concepts
Link-State – Link state routing protocols use a complex algorithm to calculate the best route. Each router
calculates its own routing table. Examples of Link-State routing protocols are OSPF and NLSP.
Distance Vector – Routing protocols that use hop counts to select the best path. Examples are RIP and IGRP.
Distance vector routing protocols are best for small networks.
Switching vs. Routing – switching works at OSI Layer 2 (data-link) by keeping track of L2 addresses and
sending out frames to only the ports where the destination MAC address has been seen. Routing, on the other
hand, uses OSI Layer 3 (Network) addresses to determine the interface that the packet will exit the router.
Autonomous Systems (ASs) - A group of routers sharing a single routing policy; run under a single technical
administration; and commonly, with a single Interior Gateway Protocol (IGP). Each AS has a unique identifying
number between 1 and 65,535 (64,512 through 65,535 are set aside for private use) usually assigned by an
outside authority.
Convergence – The process of bringing the routing tables on all the routers in the network to a consistent state.

Load Balancing – Load balancing allows the transmission of packets to a specific destination over two or more
paths.
Metrics – All routing protocols use metrics to calculate the best path. Some protocols use simple metrics, such
as RIP, which uses hop count. Others, such as EIGRP, use more meaningful information.
Passive-Interface – Prevents interfaces from sending routing updates. They will, however, continue to listen for
updates. This command is applied in the router configuration, and specifies a physical interface.
Redistribution - The process of sharing routes learned from different sources (usually routing protocols). For
instance, you might redistribute the routes learned through OSPF to a RIP domain, in which case you might
have problems with VLSM; or you might redistribute routes learned through static entries into EIGRP.
Redistribution is just the sharing of information learned from different sources, and it must be manually
configured.
Route Flapping – The frequent changing of preferred routes as an interface or router goes into and out of
operation (error condition). This process can create problems in a network, especially in complex OSPF
networks, as this information will cause the routers to constantly recalculate their OSPF database and flood the
network with LSAs (Link State Advertisements).
Static Routing –Static routes can point to a specific host, a network, a subnet, or a super-net. You can also
have floating static routes: routes that have an Administrative Distance (AD) set higher than the dynamic routing
protocol in use.
Split-Horizon - Split-horizon is used by Distance Vector routing protocols to block information about routes from
being advertised to the same interface from which the information originated. This can be a problem with
nonbroadcast networks (such as Frame Relay and SMDS), where spokes on a hub-and-spoke environment will
have trouble learning about each other. For these situations, you may choose to disable split-horizon.
Routing Loops - Routing loops occur when the routing tables of some or all of the routers in a given domain
route a packet back and forth without ever reaching its final destination. Routing loops often occur during route
redistribution, especially in networks with multiple redistribution points.
Tunneling – Tunneling is the transmission of one network’s data inside packets of another network. Usually, this
is done when you send a private network’s data over a public network. The private network’s data is
encapsulated inside the public network’s packets, transmitted over the public network, and unencapsulated.

Routing and Switching Written Qualification Exam (350-001)

Standards
There are several organizations that have taken responsibility for developing and documenting network standards,
including:
The Institute of Electrical and Electronics Engineers (IEEE) – A professional organization that develops
communications and network standards. For example, details of all the 802.x protocols can be found on their
excellent website at www.ieee.org.
The Internet Engineering Task Force (IETF) – An international community of network designers, operators,
vendors, and researchers concerned with the evolution of the Internet architecture and the smooth operation of
the Internet. You will find a list of the current and developing Requests for Comment (RFCs) on their website at
/>
Ethernet Cable Specifications
Some facts to note about Ethernet cabling are:
10Base-T
•

Runs at 10Mb/sec

•

Maximum cable length is 100 meters, or about 300 feet.

•

Uses Unshielded Twisted Pair (UTP) cable
Uses CSMA/CD standard
Can run on cabling as low as Category 3

100Base-T (Fast Ethernet)

Runs at 100Mb/sec
Requires UTP Category 5
Uses a RJ-45 connector, just like 10Base-T
Uses only two pairs of the 4-pair UTP cabling
100Base-FX
Same as 100Base-T but runs over Fiber optic cabling
Operates on two strands of multimode or single mode fiber cabling
Does not have the same 100 meter distance limitation as UTP cabling
1000Base-T (Gig-Ethernet)
Based on the 802.3ab standard for GE over copper Category 5 UTP cabling. Although, Category 5e or
Category 6 cabling is highly recommended.
Different from 10 & 100base-T as it uses 4 pairs of a UTP cable (8 Very similar to 10Base-T and
100Base-T as it uses CSMA/CD, offer half and full duplex, RJ45 connectors, and maximum cable
length is still 100 meters.
Very similar to 10Base-T and 100Base-T as it uses CSMA/CD, offer half and full duplex, RJ45
connectors, and maximum cable length is still 100 meters.


Routing and Switching Written Qualification Exam (350-001)

Protocol Mechanics
Transmission Control Protocol (TCP)
TCP is a connection-oriented Layer-4 (transport layer) protocol designed to provide reliable end-to-end transmission of
data in an IP environment. It groups bytes into sequenced segments, and then passes them to IP for delivery.
These sequenced bytes have forward acknowledgment numbers that indicate to the destination host what next byte it
should see. Bytes not acknowledged to the source host within a specified time period are retransmitted, which allows
devices to deal with lost, delayed, duplicate, or misread packets.
TCP hosts establish a connection-oriented session with one another through a "three-way handshake" mechanism,
which synchronizes both ends of a connection by allowing both sides to agree upon initial sequence numbers. Each
host first randomly chooses a sequence number to use in tracking bytes within the stream it is sending and receiving.

Then, the three-way handshake proceeds in the following manner:
1. The initiating host (Host-A) initiates a connection by sending a packet with the initial sequence number ("X")
and SYN bit (or flag) set to make a connection request of the destination host (Host-B).
2. Host-B receives the SYN bit, records the sequence number of "X", and replies by acknowledging the SYN
(with an ACK = X + 1).
3. Host-B includes its own initial sequence number ("Y"). As an example: An ACK of "20" means that Host-b has
received bytes 0 through 19, and expects byte 20 next. This technique is called forward acknowledgment.
4. Host-A then acknowledges all bytes Host-B sent, with a forward acknowledgment indicating the next byte Host
A expects to receive (ACK = Y + 1).
5. Data transfer can now begin.

You will find an excellent clarification of this process at:
/>
There is an acknowledgment process associated with TCP. Here is a sample sequence to show how this works:
1. The sender (Host-A) has a sequence of ten bytes ready to send (numbered 1 to 10) to a recipient (Host-B)
who has a defined window size of five.
2. Host-A will place a window around the first five bytes and transmit them together, then wait for an
acknowledgment.
3. Host-B will respond with an "ACK = 6", indicating that it has received bytes 1 to 5, and is expecting byte 6 next.
4. Host-A then moves the sliding window five bytes to the right and transmits bytes 6 to 10.
5. Host-B will respond with an "ACK = 11", indicating that it is expecting sequenced byte 11 next. In this packet,
the receiver might indicate that its window size is 0 (because, for example, its internal buffers are full). Host-A
won't send any more bytes until Host-B sends a subsequent packet with a window size greater than 0.

TCP also has a mechanism called "slow start" that is designed to expand and contract the window size based on flow
control needs, starting with small window sizes and increasing over time as the link proves to be reliable. When TCP
sees that packets have been dropped (ACKS are not received for packets sent), it tries to determine the rate at which
it can send traffic through the network without dropping packets. Once data starts to flow again, it slowly begins the
process again. This may create oscillating window sizes if the main problem has not been resolved, so the window
size is slowly expanded after each successful ACK is received.

Routing and Switching Written Qualification Exam (350-001)

Fragmentation & MTU
Although the maximum size of an IP packet is usually 64k, most technologies enforce a smaller maximum
transmission unit. For instance, the MTU of Ethernet is 1514 Bytes. Because of the different MTU’s along the path that
a packet travels, the packet may be fragmented into smaller packets. When the multiple smaller packets arrive at their
destination, they must be reassembled into the original data.
In the IP packet header, there are flags that specify “don’t fragment” or “more fragments”. RFC 791 specifies the
mechanics of IP Fragmentation.
For an excellent explanation of how Fragmentation, Reassembly and MTU works, see this Cisco whitepaper:
/>
Bridging and LAN Switching
Transparent Bridging (TB)
Found predominantly in Ethernet environments, the operation of a Transparent Bridge is transparent to the network
end-devices concerned; the hosts are completely unaware that they are not local to one another when they
communicate.
A TB learns the network's topology by reading the source address of incoming frames from all attached networks, and
caches that information in a forwarding table. TB’s never change the make-up of a frame. The fully intact frame is
either forwarded or filtered based on its destination MAC address. If the destination MAC address has not previously
been seen (and, thus, is not in the CAM table) then the frame is flooded out all ports on the switch/bridge.
The three functions of a bridge/switch are:
Learn the MAC addresses of all Ethernet devices and their ports
Send incoming frames to their destination port, based on previously learned frames
Drop incoming frames whose destination is the same as the sort port
The name of the table that Cisco switches store the learned MAC addresses & there ports in is the CAM table. CAM
stands for Content Addressable Memory.

Routing and Switching Written Qualification Exam (350-001)
Routers can be configured to bridge, just as a switch or bridge can. To transparently bridge packets on an IOS router,
you would do:
Router(config)# bridge 1 protocol ieee
Router(config-if)# bridge group 1
One of the problems, inherent with this type of layer-2 technology, is loops. The Spanning Tree Protocol (STP), based
on the Spanning Tree Algorithm (STA), provides the bridge-to-bridge communication necessary to have the desired
redundancy, while not causing bridges to fail.
Bridge Protocol Data Units (BPDUs) are passed between the bridges at fixed intervals, usually every one to four
seconds. If a bridge fails, or a topology change occurs, the lack of BPDUs will be detected and the STA calculation will
be re-run. Since topology decisions are made locally as the BPDUs are exchanged between neighboring bridges,
there is no central control on the network topology. The tools for fine-tuning an STP domain include adjusting the
bridge priority, port priority and path cost parameters.
There are two major disadvantages to TB:
The forwarding tables must be cleared each time STP reconfigures, which can trigger a broadcast storm as the
tables are being reconstructed.
The volume of broadcasts can overwhelm low-speed serial interfaces when the network is flooded with unknown
frames.
Cisco supports Transparent Bridging over DDR (Dial-on-Demand Routing) and Frame Relay networks.

Translational Bridging
A translational bridge is a bridge that can forward frames between different types of network technologies. For
instance, a translational bridge would send frames between an Ethernet network and a Token-Ring network or
between a FDDI Network and an Ethernet Network.

Integrated Routing and Bridging (IRB)
With IRB, a packet can be routed between routed interfaces and bridged between bridged interfaces. A Bridge Virtual
Interface (BVI) is created to represent the bridge group it corresponds to. The number of the bridge group is also the
number of the BVI. The BVI interface has networking features, like an IP address and subnet mask.
When you turn on routing for a protocol on the BVI, packets from routed networks but destined for hosts on the bridged

network are sent to the BVI. From the BVI, this traffic is sent to the bridged network. On the other hand, any traffic
destined for routed networks from a bridged network is sent to the BVI and then sent to the routed networks.
When configuring IRB, you must configure which protocols will be routed and which protocols will be bridged.

Bridge ACL & Filtering
To filter bridged packets, it is done in one of the following ways:
By MAC address with this command:
Router(config)# bridge {bridge-group} address {mac-address} {forward | discard} [interface]
By Vendor code with this command:
Router(config)# access-list {number} {permit | deny} {address} {mask}
Then, you would apply it to an interface with this command:
Router(config-if)# bridge-group {bridge-group} input-address-list {access-list number}


Routing and Switching Written Qualification Exam (350-001)
OR
Router(config-if)# bridge-group {bridge-group} output-address-list {access-list number}
Or by Protocol type with this command:
Router(config)# access-list {number} {permit | deny} {type-code} {wild-mask}
Then, you would apply it to an interface with this command:
Router(config-if)# bridge-group {bridge-group} input-type-list {access-list number}
OR
Router(config-if)# bridge-group {bridge-group} output-type-list {access-list number}

Multiple-Instance Spanning Tree Protocol (MISTP)
MISTP is a Cisco-proprietary spanning-tree mode on Cisco switches. MISTP allows a switch to running a separate
Spanning-Tree instance (process) for different groups of ports on the switch. Thus, with MISTP, not every port on the
bridge is under the same spanning-tree process. With MISTP, you can scale your bridged network much larger.

Source-Route Bridging (SRB)

Developed by IBM for its Token Ring environment, and further enhanced by the IEEE, SRB provides a means by
which multiple rings can be connected together through bridges. SRB’s use the routing information field (RIF) in the
MAC header to determine which Token Ring network segments the frame must transit. The source station inserts the
RIF into the MAC header immediately following the source address field in every frame destined for a remote host,
giving this style of bridging its name. The destination station reverses the routing field to reach the originating station.
There are two flavors of SRB - IBM and IEEE. The primary difference between them being that IBM allows only seven
bridges, while IEEE allows 13. Newer IBM bridge software programs, combined with new LAN adapters, support 13
hops.
A RIF is included only in those frames destined for other rings. The first single bit of the first byte of the source MAC
address will tell the processing device if there is a RIF present. The presence of the routing information indicator (RII)
bit indicates it is a RIF frame: If the RII value is 0, the RIF is absent; if the value is 1, there is a RIF present.
The RIF is made up of two fields:
Routing Control field – Provides information about the RIF, including the length and direction. There is always
one, and only one RC per RIF.
Route Descriptors - Made up of alternating sequences of ring and bridge numbers. A single RIF will contain
one or more routing descriptor fields.

Cisco’s source-route bridging implementation provides three types of explorer packets to collect RIF information:
Directed frame - A data frame that already contains the defined path across the network.
All-routes explorer packets (also known as all-rings explorer packets) - All route explorers go through the
whole network looking for Source-Route Bridges; all SRB’s they encounter forward the frame to every port,
except the one on which it was learned. This is how RIF’s are developed.
Spanning explorer packets (also known as single-route, or limited-route explorer packets) - Explorer
packets pass through a predetermined path constructed by a spanning tree algorithm in the bridges. A station
should receive only one single route explorer from the network. SR/TLB uses this to define an Ethernet domain
to the SRB domain.
I have created a document specifically about reading RIFs, which you can obtain free at www.laganiere.net.


Routing and Switching Written Qualification Exam (350-001)

Data Link Switching (DLSw) and DLSw+
DLSw was developed as an advanced tool for transporting Systems Network Architecture (SNA) and other nonroutable protocols over campus or wide-area networks. DLSw+ is Cisco’s version of DLSw, which offers more options
and greater functionality than RSRB and has many enhancements over non-Cisco DLSw implementations, including:
Dynamic peers, peers on demand, backup peers and the ability to load balance connections.
DLSw+ also provides a mechanism for dynamically searching a network for SNA or NetBIOS resources, and includes
caching algorithms that help to minimize broadcast traffic. It can work with Token Ring, Ethernet, FDDI and Serial
interfaces, but not ATM.
The methods of encapsulation methods for DLSw+ are similar to RSRB, with one addition:
Direct Encapsulation – This method uses HDLC (High-Level Data Link Control) and is the simplest type of
remote peering. It adds little overhead, but lacks reliability. The two routers must be directly attached to each
other, with no intermediate hops, through HDLC- encapsulated serial, FDDI, Ethernet or Token Ring interfaces.
Direct Encapsulation is fast-switched.
Fast-Sequenced Transport (FST) – This method encapsulates SRB packets within IP packets. The primary
advantage is that FST allows the link to traverse multiple hops. The IP encapsulation adds more overhead, but
does not provide the reliability of TCP. FST is fast-switched.
Transport Control Protocol (TCP) – This is the most commonly used encapsulation type, and the only
encapsulation method supported by RFC 1795. The primary advantage being that TCP encapsulation provides
for the reliable delivery of packets. TCP has greater overhead, both in actual bandwidth and router processor
cycles, than either direct or FST encapsulation methods. TCP is process-switched.
DLSw Lite (also known as LLC2 or Frame Relay encapsulation) - This method supports many DLSw+
features, but requires less overhead (16 bytes in a normal DLSw header, against 4 bytes in LLC2). It is currently
supported over Frame Relay. DLSw Lite is process-switched.
SRB is an end-to-end protocol, which puts significant load on slow WAN links, especially while waiting for the return of
acknowledgements and keepalives. DLSw+ terminates the LLC2 connection at the local switch so that traffic does not
need to traverse the link. Moving this traffic off the WAN link conserves bandwidth, and allows the local switch to
provide acknowledgement so that timeout issues are avoided.
When providing connectivity between Token Ring and Ethernet, DLSw+ handles the problems of bit ordering, MTU
sizes, and MAC address translation differences. Other limitations of SRB and RSRB include the hop count, and the
lack of flow control and prioritization. DLSw+ has greater scalability, as the RIF terminates locally in the virtual ring,

allowing a maximum of seven SRB hops on each side of the WAN cloud. This comes at the cost of end-to-end RIF
visibility, since each side of the WAN cloud builds its own RIF. Virtual ring numbers need not be the same on the two
end routers.
DLSw+ uses Switch-to-Switch Protocol (SSP) to communicate between routers (called data-link switches) at the data
link layer. This provides the mechanism to establish DLSw+ peer connections, locate resources, forward data, handle
flow control, and perform error recovery. SSP uses TCP as the preferred reliable transport among data link switches.
McGraw-Hill’s “Configuring Cisco Routers for Bridging, DLSw+, & Desktop Protocols” by Tan Nam-Kee is an
excellent resource for learning more about DLSw+, and bridging in general.

Source-Route Transparent Bridging (SRT) and Source-Route
Translational Bridging (SR/TLB)
SRT bridges can create a one spanning-tree between source-route nodes and transparent bridging nodes. It does this
by using a Routing Information Indicator (RII) to determine which nodes are SRB and which are TB. Here is how the
SRT bridge determines this:
If the node is a SRB node, the RII = 0. This means that a RIF is being used.
If the node is a TB node, the RII = 1. This means that a RIF is not being used.


Routing and Switching Written Qualification Exam (350-001)
What SR/TLB provides is the ability to create a single spanning-tree and perform source-route bridging between
translational bridged networks. That means that you have a Token Ring and an Ethernet network and are performing
bridging between them. As you know, there are many differences between how an Ethernet network and Token Ring
network functions. Some of these differences are: Bits of MAC addresses are reversed, MTU sizes are different, Token
Ring uses a RIF, different spanning-tree algorithms, etc.

LAN Switching
Layer-2 switches are sometimes called micro-segmentation devices because you can think of them as bridges with
dozens of ports, sometimes having as few as one host per collision domain. Because switches facilitated the move
away from shared media for end-devices, they had the affect of increasing available bandwidth without increasing
complexity. They have the following features:

Each port on a switch is a separate collision domain.
Each port can be assigned a VLAN (Virtual Local Area Network) membership, which creates controllable
broadcast domains.
While switch ports are more expensive than shared media, they are generally much cheaper than Router ports.

Switching Technique Types
Store-and-forward – Receives the complete frame before forwarding. Copies the entire frame into the buffer
and then checks for CRC errors. Higher latency than other techniques. This technique is used on Cat5000s.
Cut-through – Checks the destination address as soon as the header is received and immediately forwards it
out, lowering the latency level.
Fast switching - The default switching type. It can be configured manually through use of the “ip route-cache”
command. The first packet is copied into packet memory, while the destination network or host information is
stored in the fast-switching cache.
Process Switching - This technique doesn’t use route caching, so it runs slow; however, slow usually means
SAFE. To enable, use the command “no protocol route-cache”.
Optimum Switching – From its name you can understand what it is – high performance! This is the default on
7500’s.

Command-Line Interface (CLI)
One of the nicest things about working on Cisco routers is the transparency of IOS. Because a similar command set
has been developed for each family of routers, the knowledge gained from working on one router is applicable to
others.
This nicety does not carryover into the world of Cisco switches. Because there are several families of switches that
were acquired from disparate places, the Command Line Interface (CLI) differs significantly between the families of
switches.
Menu Configurable - Found primarily on older low-end switches, there are several different menu based
systems, such as those found on the 1900 or 3900 series switches. These are meant to be intuitive, but have
their own configuration problems awaiting the uninitiated, not the least of which is figuring out what keys the
menu expects you to use to select between options.
IOS-Like - Another common CLI is the IOS-like version found on many Access-layer switches, like the 2950 and

3550 series. Those who have worked on Cisco routers in the past will find that the command nomenclature is
familiar and, other than a few new commands, the same rules generally apply.


Routing and Switching Written Qualification Exam (350-001)
Set-based - The most common CLI is that which was brought into the Cisco family with the acquisition of
Crescendo Communications in 1993. It is found on the Catalyst 4000/5000/6000 series of switches, and is often
called XDI, CatOS, or the Set-based CLI. This is what you will find on most of the Core and Distribution layer
switches, and most new products use this CLI. XDI is based on the Unix csh or c-shell prompt, and the reason it
is commonly called the Set-based CLI is that “Set” is one of the three primary commands used. Most commands
start with one of the following keywords:
Set – Implements configuration changes
Show – Verifies and provides information on the configuration
Clear – Removes configuration elements
In a separate document intended for people studying for the Cisco Switching exam, I put together a list of which
models use what interface, and a sample configuration for each type. I think this document is also useful for CCIE
Written exam candidates who want to review the basics of switch configuration. This document can be found at
www.laganiere.net.

Trunking
Trunks transport the packets of multiple VLANs over a single network link using either IEEE 802.1Q or Cisco’s
proprietary Inter-Switch Link (ISL). IEEE has become common in Cisco networks because it gives you the flexibility to
include other vendor’s equipment, and because of the reduced overhead when compared to ISL, which is
encapsulated with a 26-byte header that transports VLAN IDs between switches and routers.
Note that not all Cisco switches support all encapsulation methods; for instance the Cat2948G and Cat4000 series
switches support only 802.1Q encapsulation. In order to determine whether a switch supports trunking, and what
trunking encapsulations are supported, look to the hardware documentation or use the "show port capabilities"
command.
Trunks are configured for a single Fast-Ethernet, Gigabit Ethernet, or Fast- or Gigabit EtherChannel bundle and
another network device, such as a router or second switch. Notice that I specifically excluded 10Mb Ethernet ports,

which cannot be used for trunking. For trunking to be enabled on EtherChannel bundles, the speed and duplex
settings must be configured the same on all links. For trunking to be auto-negotiated on Fast Ethernet and Gigabit
Ethernet ports, the ports must be in the same VTP domain.
To help understand how trunks negotiate, this chart tells where they will form, based on the settings of the ports:

Trunk Negotiation
Ports

On

Off Auto Desirable Non-Negotiate

On

Yes No Yes

Yes

Yes

Off

No

No

No

Auto

Yes No No

Yes

No

Desirable

Yes No Yes

Yes

Yes

Yes

Yes

No No

Non-Negotiate Yes No No


Routing and Switching Written Qualification Exam (350-001)

Virtual LAN (VLAN)
A VLAN is an extended logical network that is configured independent of the
physical network layout. Each port on a switch can be defined to join
whatever VLAN suits the Network Architect’s plans.

VLAN Trunk Protocol (VTP)
VTP is a layer-2 messaging protocol that centralizes the management of
VLANs on a network-wide basis, simplifying the management of large
switched networks with many VLANs.
Switches defined as part of a VTP domain can be configured to operate in
any of three VTP modes:
Server – Advertise VLAN configuration to other switches in the same
VTP domain and synchronize with other server switches in the domain.
You can create, modify, and delete VLANs, as well as modify VLAN
configuration parameters such as VTP version and VTP pruning for the
entire domain. This is the default mode for a switch.
Client – Advertise VLAN configuration to other switches in the same
VTP domain and synchronize their VLAN configuration with other
switches based on advertisements received over trunk links; however,
they are unable to create, change, or delete VLAN configurations.
Transparent – Does not advertise its VLAN configuration and does
not synchronize its VLAN configuration with other switches. If the
switch is running VTP version 2, it does forward VTP advertisements,
while still not acting on them.
Switches can only belong to one VTP domain, but if you have more than one
group of switches, and each group has a different set of VLANs that it has to
recognize, you could use a separate domain for each group of switches.
There is a second version of VTP that has features not supported in version
one, including Token Ring LAN Switching and VLANs, unrecognized Type
Length Value, Version Dependent Transparent Mode and Consistency
Checks. Please note that all switches in the VTP domain must run the same
VTP version. In general, don’t enable VTP version 2 in the VTP domain
unless you are ready to migrate all the switches to that version. However, if
the network is Token Ring, you will need VTP version 2.

Spanning-Tree Protocol (STP)
Spanning-Tree Protocol (STP) is a Layer 2 link management protocol
designed to run on bridges and switches to provide path redundancy and
prevent undesirable loops from forming in the network. It uses the Spanning
Tree Algorithm (STA) to calculate the best loop-free path through a switched
network.


Routing and Switching Written Qualification Exam (350-001)

Root Bridges and Switches
The key to STP is the election of a root bridge, which becomes the focal point in the network. All other decisions in the
network, such as which ports are blocked and which ports are put in forwarding mode, are made from the perspective
of this root bridge.
When implemented in a switched network, the root bridge is usually referred to as the "root switch." Depending on the
type of spanning-tree enabled, each VLAN may have its own root bridge/switch. In this case, the root for the different
VLANs may all reside in a single switch, or it can reside in varying switches, depending on the estimates of the
Network Architect.
You should remember that selection of the root switch for a particular VLAN is extremely important. You can allow the
network to decide the root based on arbitrary criteria, or you can define it yourself.

Bridge Protocol Data Units (BPDUs)
All switches exchange information to use in the selection of the root switch, as well as for subsequent configuration of
the network. This information is carried in Bridge Protocol Data Units (BPDU).
The primary functions of BPDUs are to:
Propagate bridge IDs in order for the selection of the root switch.
Find loops in the network.
Provide notification of network topology changes.
Remove loops by placing redundant switch ports in a backup state.

How STP Works
When the switches first come up, they start the root switch selection process with each switch transmitting BPDU to its
directly connected switch neighbors on a per-VLAN basis.
As the BPDUs go through the network, each switch compares the BPDU it sent out to the ones it has received from its
neighbors. From this comparison, the switches determine the root switch. The switch with the lowest priority in the
network wins this election process. (Remember, there may be one root switch identified per VLAN, depending on the
type of STP selected.)

STP Timers
Hello timer - How often the switch broadcasts Hello messages to other switches.
Forward delay timer - Amount of time a port will remain in the listening and learning states before going into the
forwarding state.
Maximum age timer – How long protocol information received on a port is stored by the switch.

Ports in an STP domain will progress through the following states:
Blocking – Listens for BPDUs from other bridges, but does not forward them or any traffic.
Listening – An interim state while moving from blocking to learning. Listens for frames and detects available
paths to the root bridge, but will not collect host MAC addresses for its address table.
Learning – Examines the data frames for source MAC addresses to populate its address table, but no user data
is passed.