Routing Information Protocol

RIP is one the oldest routing protocols in use today.

RIP is a distance vector protocol. Table 1-9 defines the characteristics of a distance vector protocol.

Table 1-9 Distance Vector Protocol Characteristics

Characteristic

Description

Periodic updates

Periodic updates are sent at a set interval; for IP RIP, this interval is 30 seconds.

Broadcast updates

Updates are sent to the broadcast address 255.255.255.255. Only devices running routing algorithms listen to these updates.

Full table updates

When an update is sent, the entire routing table is sent.

Triggered updates

Also known as Flash updates, these are sent when a change occurs outside the update interval.

Split horizon

This method stops routing loops. Updates are not sent out an outgoing interface from which the source network was received. This saves bandwidth, as well.

Count to infinity

Maximum hop count. For RIP, it is 15, and for IGRP, it is 255.

Algorithm

Example: Bellman-Ford for RIP.

Examples

RIP and IGRP.

RIP comes in two versions: RIPv1 (does not support VLSM) and RIPv2. Both versions of RIP automatically summarize at the network boundary (you can configure the classful routing protocol, RIPv2, to support VLSM).

The following list summarizes RIPv1 characteristics:

■ Distance vector protocol

■ Metric is hop count (maximum is 15; 16 is unreachable)

■ Periodic updates every 30 seconds

■ Up to 25 networks per RIP update

■ Implements split horizon

■ Implements triggered updates

■ No support for VLSM or authentication

■ Administrative distance is 120

■ Updates are sent to the broadcast address 255.255.255.255

NOTE Split horizon is a routing technique in which information about routes is prevented from exiting the router interface through which that information was received. Split horizon updates are useful in preventing routing loops. To enable split horizon, the Cisco IOS command is ip split-horizon (an interface command). Split horizon on Frame Relay subinterfaces is enabled by default. Always use the Cisco IOS command show ip interface to determine if split horizon is enabled or displayed.

A triggered update is a method by which a routing protocol sends an instant message as soon as a network failure is detected. If a triggered update were not used, the only way the update would be sent would be via the normal update every 30 seconds, causing a delay in network convergence times. Split horizon is a favorite topic in CCIE lab exams.

Poison Reverse updates explicitly indicate that a network is unreachable rather than implying that a remote network is unreachable by not sending that network in an update. Poison Reverse updates are intended to defeat routing loops in large IP networks.

Split horizon, Poison Reverse, and triggered updates are methods used by distance vector protocols to avoid routing loops

RIPv2 was developed to enable RIPv1 to support VLSM, so it is a classless routing protocol that also supports authentication. RIPv1 and RIPv2 use the same hop count as the metric.

The following list summarizes RIPv2 characteristics:

■ Distance vector protocol

■ Metric is hop count (maximum is 15; 16 is unreachable)

■ Periodic updates every 30 seconds

■ Up to 25 networks per RIP update

■ Implements split horizon

■ Implements triggered updates

■ Supports VLSM (subnet mask carried in updates)

■ Supports authentication

■ Administrative distance is 120

■ Updates sent to multicast address 224.0.0.9

■ Can set up neighbors to reduce broadcast traffic (send unicast updates)

To enable RIPv1 or RIPv2 on a Cisco router, the command required is router rip. By default, after you enable this command and install the network statements, RIPv1 sends and receives updates and RIPv2 only listens for updates.

Consider a two-router topology running VLSM and RIP. Figure 1-14 displays two routers, named R1 and R2, with a 30-bit network used across the WAN. Loopbacks are used to populate the IP routing tables.

To start, enable RIP on both routers with the commands in Example 1-10. Version 2 must be enabled because you are implementing VLSM across the WAN links between R1 and R2.

Figure 1-14 Practical Example of Routing RIP

131.108.1.1/24

131.108.3.0/30 Frame Relay

131.108.2.1/24

Figure 1-14 Practical Example of Routing RIP

131.108.3.0/30 Frame Relay

R1's Loopbacks Loopback0 131.108.4.1/24 Loopback1 131.108.5.1/24 Loopback2 131.108.6.1/24

R2's Loopbacks Loopback0 131.108.7.1/24 Loopback1 131.108.8.1/24 Loopback2 131.108.9.1/24

R1's Loopbacks Loopback0 131.108.4.1/24 Loopback1 131.108.5.1/24 Loopback2 131.108.6.1/24

R2's Loopbacks Loopback0 131.108.7.1/24 Loopback1 131.108.8.1/24 Loopback2 131.108.9.1/24

Example 1-10 displays the RIP configuration on R1. The same configuration commands are applied to R2.

Example 1-10 IP RIP Configuration on R1

router rip version 2

network 131.108.0.0

You can view the RIP forward database with the command show ip rip database. Example 1-11 displays the output when show ip rip database is executed on R1.

Example 1-11 show ip rip database Command on R1

R1#show ip rip database

131

108

.0.0/16

auto-summary

131

108

.1.0/24

directly connected, Ethernet0/0

131

108

.2.0/24

[1]

via 131

1

38.3.2, 00:00:12, Serial0/0

131

108

.3.0/30

directly connected, Serial0/0

131

108

.4.0/24

directly connected, Loopback0

131

108

.5.0/24

directly connected, Loopback1

131

108

.6.0/24

directly connected, Loopback2

131

108

.7.0/24

[1]

via 131

1

58.3.2, 00:00:12, Serial0/0

131

108

.8.0/24

[1]

via 131

1

58.3.2, 00:00:12, Serial0/0

131

108

.9.0/24

[1]

via 131

1

98.3.2, 00:00:12, Serial0/0

Example 1-11 displays the directly connected routes and the four dynamically discovered routes via Serial0/0 to R2. To confirm that the entries are reachable, display the IP routing table on R1 and perform a few ping requests across the Frame Relay cloud.

Example 1-12 displays the IP routing table and the successful ping requests to the four remote networks.

Example 1-12 show ip route and ping to R2

R1#show ip route

RIP,

5.0/16 is variably subnetted, 9 subnets, 2 masks

5.0/16 is variably subnetted, 9 subnets, 2 masks

R

131.1

38.9

0/24

[120/1] via

131.108.3

2, 00:00:00,

Serial0/0

R

131.1

38.8

0/24

[120/1] via

131.108.3

2, 00:00:00,

Serial0/0

R

131.1

08.7

0/24

[120/1] via

131.108.3

2, 00:00:00,

Serial0/0

C

131.1

08.6

0/24

is directly

connected,

Loopback2

C

131.1

38.5

0/24

is directly

connected,

Loopback1

C

131.1

08.4

0/24

is directly

connected,

Loopback0

C

131.1

58.3

0/30

is directly

connected,

Serial0/0

R

131.1

08.2

0/24

[120/1] via

131.108.3

2, 00:00:01,

Serial0/0

C

131.1

08.1

0/24

is directly

connected,

Ethernet0/0

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 131.108.2.1, timeout is 2 seconds: !!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 4/6/8 ms

R1#ping 131.108.7.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 131.108.7.1, timeout is 2 seconds:

Example 1-12 show ip route and ping to R2 (Continued)

Success rate is 100 percent (5/5), round-trip min/avg/max = 4/6/8 ms

R1#ping 131.108.8.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 131.108.8.1, timeout is 2 seconds: !!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 4/5/8 ms

R1#ping 131.108.9.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 131.108.9.1, timeout is 2 seconds: !!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 4/5/8 ms R1#

Example 1-12 displays the four remote networks reachable by the Serial 0/0 and four successful ping requests (five replies from each remote network) to those interfaces on R2.

Stop R2 from sending R1 any updates via the Frame Relay cloud to demonstrate the passive-interface command, passive-interface serial0/0. New Cisco IOS revision levels also permit the administrator to set a default state (routing or just listening-passive) for all interfaces. The Cisco IOS command passive-interface default ensures that all interfaces, unless specified otherwise, are passive. To take an interface from passive into active, for example, you would use the command no passive-interface serial0/0 in conjunction with the passive-interface default command. This new command can be helpful in large IP networks.

Example 1-13 displays the passive interface configuration on R2 Serial0/0.

Example 1-13 Passive Interface Configuration on R2

R2(config)#router rip

R2(config-router)#passive-interface serial 0/0

R1's routing table now contains no remote entries from R2, which will still receive updates because the command affects only outbound updates. Example 1-14 confirms the missing routing RIP entries in R1's IP routing table.

Example 1-14 show ip route on R1

R1#show ip

route

Codes: C -

connected,

131.10

8.0

.0/16 is

variably subnetted, 5

subnets, 2 masks

C 131

.10

8.6.0/24

is directly connected

Loopback2

C 131

.10

8.5.0/24

is directly connected

Loopback1

C 131

.10

8.4.0/24

is directly connected,

Loopback0

C 131

.10

8.3.0/30

is directly connected,

Serial0/0

C 131

.10

8.1.0/24

is directly connected,

Ethernet0/0

NOTE RIPv2 also offers MD5 authentication as an optional authentication mode added by Cisco to the original RFC 1723-defined plain-text authentication. The configuration is identical to that for plain-text authentication, except for the use of the additional command ip rip authentication mode md5. You must configure router interfaces on both sides of the link for the MD5 authentication method, making sure the key number and key string match on both sides.

Was this article helpful?

0 0

Post a comment