IP Routing

Fine-Tuning RIP

We saw in the section on RIP metrics that the preferred path between NewYork and Ames would be the two-hop path via Chicago rather than the one-hop 56-kbps path that RIP selects. The RIP metrics can be manipulated to disfavor the one-hop path through the use of offset lists:

hostname NewYork
...
router rip
network 172.16.0.0
offset-list 10 in 2 serial1
...
access-list 10 permit 172.16.100.0 0.0.0.0

hostname Chicago
...
router rip
network 172.16.0.0

Ames#config terminal
router rip
network 172.16.0.0
offset-list 20 in 2 serial1
...
access-list 20 permit 172.16.1.0 0.0.0.0

NewYork adds 2 to the metric for the routes specified in access list 10 when learned via serial1, and Ames adds 2 to the metric for the routes specified in access list 20 when learned via serial1. The direct route over the 56-kbps link thus has a metric of 3, and the route via Chicago has a metric of 2. The new routing tables look like this:

NewYork>sh ip route
...
Gateway of last resort is not set

C       192.168.1.0 is directly connected, Ethernet1
     172.16.0.0/16 is subnetted, 6 subnets
C       172.16.1.0 is directly connected, Ethernet0
C       172.16.250.0 is directly connected, Serial0
C       172.16.251.0 is directly connected, Serial1
R       172.16.50.0 [120/1] via 172.16.250.2, 0:00:11, Serial0
R       172.16.100.0 [120/2] via 172.16.250.2, 0:00:19, Serial0 R 172.16.252.0 [120/1] via 172.16.250.2, 0:00:11, Serial0 [120/1] via 172.16.251.2, 0:00:19, Serial1 Ames>sh ip route ... Gateway of last resort is not set 172.16.0.0/16 is ...