Giáo trình CCNA - Chương 3

CCNA – Semester3
Module 1
Introduction to Classless Routing
Objectives
• Define VLSM and devide major network with VLSM
• Configure VLSM
• Differences between RIPv1 and RIPv2
• Configure RIPv2
VLSM
Why VLSM?
• The addressing crisis
• Rapid increase in the size of routing tables
• IP address solutions:
– Short term extensions to IPv4
– Subnetting 1985
– Variable length subnetting 1987
– Classless Interdomain Routing 1993
– Private IP address
– Network Address Translation(NAT)
–IPv6
What is VLSM?
• With Variable-Length Subnet Masks (VLSM), a
network administrator can use a long mask on
networks with few hosts, and a short mask on
subnets with many hosts
• In order to use VLSM, a network administrator
must use a routing protocol that supports it:
–OSPF
–IS-IS
– EIGRP
–RIPv2

–BGP
– Static routing
VLSM
A waste of space
• It has become acceptable practice to use the first and
last subnets in a subnetted network in conjunction with
VLSM.
• With ip subnet-zero command, network has 8 usable
subnets.
Subnetting with VLSM
Using VLSM
VLSM Practice
Subnet the network
address 192.168.1.0/24
192.168.1.0/26
192.168.1.64/27
192.168.1.96/28 192.168.112.0/28
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Route aggregation with VLSM
• When using VLSM, try to keep the subnetwork
numbers grouped together in the network to
allow for aggregation.
• The use of Classless InterDomain Routing
(CIDR) and VLSM not only prevents address
waste, but also promotes route aggregation, or
summarization.
Route Summarization
Route Summarization
Route Summarization (Super network)
Route Summarization Rules
• A router must know in detail the subnet
numbers attached to it.
• A router does not need to tell other routers
about each individual subnet if the router can
send one aggregate route for a set of routers.
• A router using aggregate routes would have
fewer entries in its routing table.
RIP version 2
RIPv1 Limitations
• It does not send subnet mask information in its updates.
• It sends updates as broadcasts on 255.255.255.255.
• It does not support authentication.
• It is not able to support VLSM or classless interdomain routing

(CIDR).
RIPv2
• RIP v2 is an improved version of RIP v1 and
shares the following features:
– It is a distance vector protocol that uses a hop count
metric.
– It uses holddown timers to prevent routing loops – default
is 180 seconds.
– It uses split horizon to prevent routing loops.
– It uses 16 hops as a metric for infinite distance.
RIPv2 vs. RIPv1
Configuring RIP v2
• The router command starts the routing process.
The network command causes the
implementation of the following three functions:
– The routing updates are multicast out an interface.
– The routing updates are processed if they enter that same
interface.
– The subnet that is directly connected to that interface is
advertised.
Configuring RIP v2
Verifying RIP Configuration
IP Routing Table
Troubleshooting RIP v2
Debug ip RIP output
Summary
• VLSM and the reasons for its use
• Subnetting networks of different sizes using
VLSM
• Route aggregation and summarization as they

relate to VLSM
• Key features of RIP v1 and RIP v2
• Configuration of RIP v2
• Verifying and troubleshooting RIP v2 operation
Module 2
Single Area OSPF
Objectives
• Link State Routing Protocol
• Single Area OSPF Concepts
• Single Area OSPF Configuration
Link State Routing Protocol
Link State and Distance Vector Routing
Link-state routing protocol
• Collect routing information from all other
routers in the network or within a defined area
of the network.
• Each router independently calculates its best
paths to all destinations in the network.
• It is less likely to propagate incorrect
information provided by any of its neighboring
routers
Link-state routing protocol functions
• Link-state routing protocols perform the
following functions:
– Respond quickly to network changes
– Send triggered updates only when a network change has
occurred
– Send periodic updates known as link-state refreshes
–Use a hello mechanism to determine the reachability of
neighbors

Link State Routing
Link-state routing concepts
• Link-state routing uses the
following features:
– Link-state advertisements (LSAs)
– A topological database
– The shortest path first (SPF)
algorithm
– The resulting SPF tree
– A routing table of paths and ports
to each network to determine the
best paths for packets
Advantages of link-state routing
• Fast convergence
• Use cost metric
• No routing loop
• The awareness of the entire network greatly assist
troubleshooting.
• Support CIDR and VLSM
Disadvantages of link-state routing
• Require more memory and processing power than
distance vector routers.
• Require strict hierarchical network design.
• They require good understanding of link-state routing.
• Initial discovery process by flooding LSAs can
significantly decrease the capability of the network to
transport data.
Single Area OSPF Concepts
Single Area OSPF
• Open Shortest Path First

(OSPF) is a link-state
routing protocol based on
open standards.
• OSPF proves to be a
robust and scalable
routing protocol.
• OSPF can be used and
configured as a single
area for small networks.
Large OSPF network
• Large OSPF networks use a hierarchical design.
• Multiple areas connect to a distribution area, area 0, also called the
backbone.
• Defining areas reduces routing overhead, speeds up convergence,
confines network instability to an area and improves performance.
OSPF terminology
OSPF terminology
OSPF terminology
OSPF terminology
OSPF terminology
OSPF terminology
OSPF terminology
OSPF terminology
Problem: Link state Updates
OSPF Features
• OSPF addresses the following issues:
– Speed of convergence
– Support for Variable Length Subnet Mask (VLSM)
– Network size
– Path selection

– Grouping of members
Shortest path algorithm
• Each node has a complete database of all the links so complete
information about the physical topology is known.
• All router link-state databases are identical.
• The shortest path algorithm then calculates a loop-free topology