Chapter 1 Introduction to Routing and Packet ForwardingRouting Protocols and Concepts quangkien@gmail.com.Topicsl Inside the Router Ÿ Routers are computers Ÿ Router CPU and Memory Ÿ Internetwork Operating System Ÿ Router Bootup Process Ÿ Router Ports doc

Chapter 1
Introduction to Routing and
Packet Forwarding
Routing Protocols and Concepts

2
Topics
l Inside the Router
Ÿ Routers are computers
Ÿ Router CPU and Memory
Ÿ Internetwork Operating
System
Ÿ Router Bootup Process
Ÿ Router Ports and Interfaces
Ÿ Routers and the Network
Layer
l Path Determination and
Switching Function
Ÿ Packet Fields and Frame
Formats
Ÿ Best Path and Metrics
Ÿ Equal Cost Load Balancing
Ÿ Path Determination
Ÿ Switching Function
l CLI Configuration and
Addressing
Ÿ Implementing Basic
Addressing Schemes
Ÿ Basic Router
Configuration
l Building the Routing Table

Ÿ Introducing the Routing
Table
Ÿ Directly Connected
Networks
Ÿ Static Routing
Ÿ Dynamic Routing
Ÿ Routing Table Principles
Inside the Router
l Routers are computers
l Router CPU and Memory
l Internetwork Operating System
l Router Bootup Process
l Router Ports and Interfaces
l Routers and the Network Layer
4
Routers are Computers
l A router is a computer:
Ÿ CPU, RAM, ROM, Operating System
l The first router: used for the Advanced Research Projects Agency
Network (ARPANET):
Ÿ IMP (Interface Message Processor)
Ÿ Honeywell 516 minicomputer that brought the ARPANET to life
on August 30, 1969.
Leonard Kleinrock and the first IMP.
5
l Routers forwarding packets (packet switching):
Ÿ From the original source to the final destination.
Ÿ Selects best path based on destination IP address
l A router connects multiple networks:
Ÿ Interfaces on different IP networks

6
l Router interfaces:
Ÿ LAN
Ÿ WAN
7
Routers Determine the Best Path
l The router’s primary responsibility:
Ÿ Determining the best path
Ÿ Forwarding packets toward their destination
8
Routers Determine the Best Path
l Routing table
Ÿ Determines best path.
Ÿ Best match between destination IP address and network
address in routing table
IP Packet enters router’s Ethernet interface.
Router examines the packet’s destination IP address.
Router searches for a best match between packet’s destination IP address and
network address in routing table.
Using the exit-interface in the route, the packet is forwarded to the next router or
the final destination.
9
Router
CPU and
Memory
l CPU - Executes operating system instructions
l Random access memory (RAM)
Ÿ running copy of configuration file
Ÿ routing table
Ÿ ARP cache

l Read-only memory (ROM)
Ÿ Diagnostic software used when router is powered up.
Ÿ Router’s bootstrap program
Ÿ Scaled down version of operating system IOS
l Non-volatile RAM (NVRAM)
Ÿ Stores startup configuration. (including IP addresses, Routing protocol)
l Flash memory - Contains the operating system (Cisco IOS)
l Interfaces - There exist multiple physical interfaces that are used to connect
network. Examples of interface types:
Ÿ Ethernet / fast Ethernet interfaces
Ÿ Serial interfaces
Ÿ
Management interfaces
10
Router physical characteristics
11
Cisco IOS - Internetwork
Operating System
l Responsible for managing the hardware and software resources:
Ÿ Allocating memory
Ÿ Managing processes
Ÿ Security
Ÿ Managing file systems
l Many different IOS images.
l An IOS image is a file that contains the entire IOS for that router.
Ÿ Router model
Ÿ IOS features
l Example IPv6 or a routing protocol such as Intermediate System–
to–Intermediate System (IS-IS).
12

Router Bootup Process (more in later course)
13
Bootup Process
running-config
IOS (running)
startup-config IOS
ios (partial)
Bootup program
14
running-config
IOS (running)
startup-config IOS
ios (partial)
Bootup program
Where is the permanent configuration file stored used during boot-up? NVRAM
Where is the diagnostics software stored executed by hardware modules? ROM
Where is the backup (partial) copy of the IOS stored? ROM
Where is IOS permanently stored before it is copied into RAM?
FLASH
Where are the bootsystem commands stored which are used to locate
the IOS?
NVRAM
15
running-config
IOS (running)
startup-config IOS
ios (partial)
Bootup program
?
?

?
?
?
?
?
16
running-config
IOS (running)
startup-config
IOS
ios (partial)
Bootup program
startup-config
IOS
Bootup program
ios (partial)
running-config
IOS (running)
17
1. ROM
1. POST
2. Bootstrap code executed
3. Check Configuration Register value (NVRAM)
0 = ROM Monitor mode
1 = ROM IOS
2 - 15 = startup-config in NVRAM
2. Check for IOS boot system commands in startup-config file (NVRAM)
If boot system commands in startup-config
a. Run boot system commands in order they appear in startup-config to locate the IOS
b If boot system commands fail, use default fallback sequence to locate the IOS (Flash, TFTP, ROM)

3. Locate and load IOS, Default fallback sequence: No IOS boot system commands in startup-config
a. Flash (sequential)
b. TFTP server (netboot) - The router uses the configuration register value to form a filename from
which to boot a default system image stored on a network server.
c. ROM (partial IOS) or keep retrying TFTP depending upon router model
- If no IOS located, get partial IOS version from ROM
4. Locate and load startup-config configuration
a. If startup-config found, copy to running-config
b. If startup-config not found, prompt for setup-mode
c. If setup-mode bypassed, create a “skeleton” default running-config (no startup-config)
Router Boot Process –
Details (later)
18
Verify the router boot-up process
l show version command is used to view information about the
router during the bootup process (later).
19
Ports and Interfaces
l Port - normally means one of the management ports used for
administrative access
l Interface normally refers to interfaces that are capable of sending
and receiving user traffic.
l Note: However, these terms are often used interchangeably in the
industry and even with IOS output.
20
Management
Ports
Console port
Ÿ Terminal
Ÿ PC running terminal emulator software

l No need for network access
l Used for initial configuration
Auxiliary (AUX) port
l Not all routers have auxiliary ports.
Ÿ At times, can be used similarly to a console port
Ÿ Can also be used to attach a modem.
l Note: Auxiliary ports will not be used in this curriculum.
21
Router Interfaces
l Interfaces - Receive and forward packets.
Ÿ Various types of networks
Ÿ Different types of media and connectors.
Ÿ Different types of interfaces.
l Fast Ethernet interfaces - LANs
l Serial interfaces - WAN connections including T1, DSL, and ISDN
22
Router Interfaces
l Router Interface:
Ÿ Different network
Ÿ IP address and subnet mask of that network
l Cisco IOS will not allow two active interfaces on the same
router to belong to the same network.
FastEthernet 0/0
MAC: 0c00-3a44-190a
192.168.1.1/24
FastEthernet 0/0
MAC: 0c00-41cc-ae12
10.1.0.1/16
Serial 0/0
172.16.1.1/24

Serial 0/1
172.16.1.2/24
23
LAN Interfaces
l Ethernet and Fast Ethernet interfaces
l Connects the router to the LAN
Ÿ Layer 2 MAC address
Ÿ Participates in the Ethernet
Ÿ Address Resolution Protocol (ARP):
Ÿ Maintains ARP cache for that interface
Ÿ Sends ARP requests when needed
Ÿ Responds with ARP replies when required
l Typically an RJ-45 jack (UTP).
Ÿ Router to switch: straight-through cable
Ÿ Router to router: crossover cable
24
WAN Interfaces
l Point-to-Point, ISDN, and Frame Relay interfaces
l Connects routers to external networks.
l The Layer 2 encapsulation can be different types including:
Ÿ PPP
Ÿ Frame Relay
Ÿ HDLC (High-Level Data Link Control).
l Note: MAC addresses are used only on Ethernet interfaces and are
not on WAN interfaces.
l Layer 2 WAN encapsulation types and addresses are covered in a
later course.
25
Routers at
the

Network
Layer
l Layer 3 device because its primary forwarding
decision is based on the information in the Layer 3 IP
packet (destination IP address).
l This is known as routing.