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CCNA Semester 3

Chapter 01Chap

ter 01

CLASSLESSCLASSLESSROUTINGROUTING

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ObjectivesObjectives

Define VLSM and briefly describe the reasons for itsuse

Divide a major network into subnets of different sizesusing VLSM

Define route aggregation and summarization as theyrelate to VLSM

Configure a router using VLSM

Identify the key features of RIP v1 and RIP v2

Identify the important differences between RIP v1and RIP v2

Configure RIP v2

Verify and troubleshoot RIP v2 operation

Configure default routes using the ip route and ipdefault-network commands

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Table of ContentTable of Content

1VLSM

2 CIDR

3 Classless routing

4RIP version 2

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VLSMVLSM

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Issues with IP AddressingIssues with IP Addressing

IP addressing crisis

As early as 1992, the IETF identified twospecific concerns:

IP address exhaustion

Routing table growth

U N I V E R S I T YU N I V E R S I T Y

Internet

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IP Addressing SolutionsIP Addressing Solutions

Short term to extension to IPv4 Subneting masking: RFCs 950, 1812

Variable-length subnet masks: RFC 1812

Classless interdomain routing: RFCs 1518,1519, 2050

Address allocation for private Internets:RFC 1918

Network AddressTranslation: RFC 1631

Route summarization: RFC 1518

Ultimate solution: IPv6 128-bit address

space

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172.16.14.32/27

172.16.14. 64/27

172.16.14.96/27

What is VLSM ?What is VLSM ?

Subnet 172.16.14.0/24 is divided into smallersubnets (Sub-subnet):

Subnet with one mask at first (/27)

Further subnet one of these subnets notused elsewhere (/30)

CC

BB

AA

HQHQ

172.16.1.0/24

172.16.2.0/24

HQHQ172.16.0.0/16

172.16.14.136/30

172.16.14.132/30

172.16.14.140/30

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A waste of spaceA waste of space

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When to use VLSM?When to use VLSM?

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Calculating subnets with VLSMCalculating subnets with VLSM

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Calculating subnets with VLSM (2)Calculating subnets with VLSM (2)

Derived from the

172.16.32.0/20Subnet 172.16.33.0/3

0

172.16.32.0/26

172.16.32.64/26

172.16.32.128/2

6

172.16.32.192/26

26-Bit Mask (62 Hosts)

Derived from the

172.16.33.0/26 Subnet

30-Bit Mask (2 Hosts)

172.16.33.12/30

172.16.33.8/30

172.16.33.4/30

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Exercise Calculating subnets with VLSMExercise Calculating subnets with VLSM

60 Hosts 12 Hosts 12 Hosts

Using VLSMs, define appropriate subnets for addressing thenetworks using 192.168.10.0/24.

28 HostsKL

SydneyPerth Singapore

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Exercise Calculating subnets with VLSMExercise Calculating subnets with VLSM


192.168.10.0/24

KL


192.168.10.64/27

192.168.10.128/30192.168.10.132/30 192.168.10.136/30

28 Hosts

192.168.10.0/26 192.168.10.96/28 192.168.10.112/2

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Classless Interdomain RoutingClassless Interdomain Routing

CIDR is a mechanism developed to alleviateexhaustion of addresses and reduce routing tablesize.

Block addresses can be summarized into singleentries without regard to the classful boundary ofthe network number.

Summarized blocks are installed in routing tables.

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Route aggregation with VLSMRoute aggregation with VLSM

KLSydney

Perth

Singapore

192.168.48.0/24192.168.49.0/24192.168.50.0/24192.168.51.0/2

4

192.168.52.0/2

4192.168.53.0/24192.168.54.0/24192.168.55.0/24192.168.56.0/2

4192.168.57.0/24192.168.63.0/24

19

2.168.

56.0/21

192.168.52.0/22

192.168.48.0/20

192.168.48.0/22

Upstream

provider

Upstream

provider

Route summarization reduces routing table sizeby aggregating routes to multiple networks intoone supernet

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Route summarization exampleRoute summarization example

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Configuring VLSMConfiguring VLSM


192.168.10.0/24

KL


192.168.10.64/27

192.168.10.128/30192.168.10.132/30

192.168.10.136/30

28 Hosts

192.168.10.0/26 192.168.10.96/28 192.168.10.112/2

S010.137

S0 10.138

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CLASSLESS ROUTING

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Network Summarization in Classful Routing

Summary routes are exchanged betweenforeign networks

Summary routes are automatically created atClass A, B, and C network boundaries

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Classful Subnet Issues

All router interfaces within the same networkmust have the same subnet mask.

This approach may not fully use availableallocation of host addresses.

All subnets of the same major network must

be contiguous.

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IP Routing TableIP Routing Table with a Classful Protocolwith a Classful Protocol

p1r3# show ip route

Gateway of last resort is 0.0.0.0 to network 0.0.0.0

10.0.0.0/24 is subnetted, 3 subnets,

R 10.1.1.0/24 [120/1] via 10.1.2.2, 00:00:05, Ethernet0

C 10.1.2.0/24 is directly connected, Ethernet0

R 10.1.3.0/24 [120/2] via 10.1.2.2, 00:00:05, Ethernet0R 192.168.24.0/24 [120/2] via 10.1.2.2, 00:00:16, Ethernet0

R 172.16.0.0/16 [120/3] via 10.1.2.2, 00:00:16, Ethernet0

R* 0.0.0.0/0 [120/3] via 10.1.2.2, 00:00:05, Ethernet0

Where will the router send traffic bound for the

following destinations?

192.168.24.3

172.16.5.1

10.1.2.7

200.100.50.0

10.2.2.2

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The IP Classless CommandThe IP Classless Command

Router(config)# ip classlessRouter(config)# ip classless

Replaces the default behavior of classful routingprotocols to match against only known subnets of

a major network Changes default behavior of classful routingprotocol for unknown subnets

On by default in Cisco IOS version 12.0 and later

Has no effect on most classless routing protocolsbecause they use the longest-match criteriaby default

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WithoutWithout ip classlessip classless commandcommand

BHMGAD

S0/0

192.168.13.2

To10.2.2.4

192.168.1.4/30

10.1.1.0

10.3.3.0

Fa0/1

Fa0/0

Destination network Outbound interface

10.3.3.0 Fa0/1

10.1.1.0 Fa0/0

0.0.0.0 S0/0

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WithWith ip classlessip classless commandcommand

BHMGAD

S0/0

192.168.13.2

To10.2.2.4

192.168.1.4/30

10.1.1.0

10.3.3.0

Fa0/1

Fa0/0

Destination network Outbound interface

10.3.3.0 Fa0/1

10.1.1.0 Fa0/0

0.0.0.0 S0/0

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Classless Routing OverviewClassless Routing Overview

Classless routing protocols include the subnet mask withthe route advertisement.

Classless routing protocols support VLSM.

Summary routes can be manually controlled within thenetwork.

Example of classless routing protocols are as follows: OSPF

EIGRP

RIPv2 IS-IS

BGPv4

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Classless Subnetting Requirements

Router interfaces within the same networkcan have different subnet masks:

VLSM is supported

This approach maximizes allocation ofavailable host addresses.

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Automatic Network-Boundary Summarization

EIGRP on both Router A and Router B

advertises a summarized route to172.16.0.0/16.

Router C receives two routes to 172.16.0.0/16.

Router A or B, or both, should be configured

to not summarize.

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Effect of the auto-summary Command

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Effect of the no auto-summary Command

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RIP Version 2RIP Version 2

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RIP v1:RIP v1: CharacteristicsCharacteristics

IGP protocol, is classful routing. It is a distance vector protocol that uses a

hop count metric.

The maximum number of hops is 15. By default, routing updates are broadcast

every 30 seconds.

The router applies the one subnet mask thatis configured on the receiving interface toreceive network information.

Load balancing over as many as six equal-

cost paths, with four paths as the default.

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RIP v1:RIP v1: LimitationsLimitations

It does not send subnet mask information inits updates.

It sends updates as broadcasts on255.255.255.255.

It does not support authentication.

It is not able to support VLSM or classlessinterdomain routing (CIDR).

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RIP v1:RIP v1: ConfigurationConfiguration

NOTE:

Subnets will be understood as major network

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RIP v2 featuresRIP v2 features

Send out subnet mask information with theroute update.

Support VLSM or classless interdomain routing

(CIDR). Provides for authentication in its updates.

Provides Multicast routing updates, using the

Class D address 224.0.0.9. Use external route tags

Routing update is includes a next-hop routeip address.

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Comparing RIP v1 and v2Comparing RIP v1 and v2

RIP v1

Easy to configure.

Only supports classfulrouting protocol

No subnet informationwith routing update.

Does not support prefixrouting all the devices

in the same networkmust use same subnetmask.

No authentication in

updates

RIP v2

Easy to configure.

Supports use classless routing

Send subnet information withrouting updates.

Support prefix routing different subnets within thesame network can have

different subnet masks. Provides for authentication in

its updates.

Provides multicast routingupdates

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RIP v2:RIP v2: ConfigurationConfiguration

router rip

version 2

network 150.100.0.0

router rip

version 2

network 150.100.0.0

Kuala Lumpur Bangkok

150.100.4.0/24

150.100.2.0/24

150.100.3.0/24

NOTE:

Assigning IP network numbers without specifyingsubnet values

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Discontiguous subnets and classless routingDiscontiguous subnets and classless routing

RIP v1 always uses automatic summarization.

The default behavior of RIP v2 is to summarize at

network boundaries the same as RIP v1.

router ripversion 2no auto-summary

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Verifying RIP v2 :Verifying RIP v2 : SHOW IP PROTOCOLSSHOW IP PROTOCOLS

BA

172.16.1.1

192.168.1.0172.16.1.0

10.1.1.1 10.1.1.2

10.2.2.2

10.2.2.3

192.168.1.1

B

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Verifying RIP v2:Verifying RIP v2: SHOW IP ROUTESHOW IP ROUTE

BA

172.16.1.1

192.168.1.0172.16.1.0

10.1.1.1 10.1.1.2

10.2.2.2

10.2.2.3

192.168.1.1

B

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Troubleshooting RIP v2 :Troubleshooting RIP v2 : DEBUG IP RIPDEBUG IP RIP

BA

172.16.1.1

192.168.1.0172.16.1.0

10.1.1.1 10.1.1.2

10.2.2.2

10.2.2.3

192.168.1.1

B

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Other RIPv2 Commands (EXTRA)Other RIPv2 Commands (EXTRA)

Router(config-router)# neighbor ip-addressDefines a neighboring router with which to exchange

unicast routing information. (RIPv1 or RIPv2)

Router(config-if)# ip rip send|receive version 1 | 2 | 1

2

Configures an interface to send/receive RIP Version 1 and/orVersion 2 packets

Router(config-if)# ip summary-address rip ip_address

ip_network_mask

Specifies the IP address and network mask that identify theroutes to be summarized.

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