Changing RIP Metric

We can change the metric learned by default (hop count) using the Offset-lists in RIP.


In above topology, on router R1, we see the route for prefix 192.9.1.5/32 learned via 3 paths, i.e. via R2, R3, and R4 with equal metrics (hop count 2).



Important Note:  In RIP, Hop count or the Metric gets increased to 1 when the update enters the interface (inwards) but not when it exits the interface. On R1, we can see the route of prefix 192.9.1.5/32 is learned with metric/hop count 2 from all three paths as per topology shown above. While R5 advertises the prefix 192.9.1.5/32, hop count will be zero because the update is exiting the router. As the update is received by router R3 on interface f1/1, metric/hop count gets increased from zero to 1. When R3 advertises the prefix 192.9.1.5/32 to R1, metric/hop count remains 1 as the update is exiting the router. When R1 receives this update on f1/1, metric/hop count again gets increased to 1 making the metric/hop count total to 2.

Let's try changing the metric/hop count on R3 by configuring an offset-list where R3 will advertise the prefix 192.9.1.5/32 to R1 with additional metric offset of 2. When R1 receives the update on f1/1 from R3, the total metric we will see for the prefix 192.9.1.5/32 learned via R2 is 4 (Kindly note that the original metric was 2 and then we will add an offset of 2 making the total metric/hop count to 4).

Step1: Create an access-list.

R3(config)#ip access-list standard rip_offset
R3(config-std-nacl)#permit any


Step2: Under RIP config mode, configure an offset-list to change the metric. Metric can be changed while advertising or while an update is received.

To change metric while the update is advertised by R3 through f1/0  to R1. In our topology we have use this method.

R3(config-std-nacl)#router rip
R3(config-router)#offset-list rip_offset out 2 f1/0  


To change metric while the update is received by R3 through f1/1 from R5.

R3(config-std-nacl)#router rip
R3(config-router)#offset-list rip_offset in 2 F1/1 

After we configure the offset list to change RIP metric on R3 while it advertises the prefix 192.9.1.5/32 to R1, below are debug outputs.

R1# debug ip rip
*Apr 16 18:58:36.831: RIP: received v2 update from 10.10.13.3 on FastEthernet1/1
*Apr 16 18:58:36.839:      192.9.1.5/32 via 0.0.0.0 in 4 hops


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