RIP – Using Offset List

Taking the following LAB:

All 4 routers are using RIPv2, R3 and R4 are advertising Loopback interface 1-3 with the corresponding networks.

On a normal state, R3 will have two equal cost paths to networks 192.168.4.0/24, 192.168.40.0/24 and 192.168.44.0/24.

If we want to influence on R3 in a way that he will prefer network 192.168.4.0 only through R1 we can use Offset-list on R4 and advertise network 192.168.4.0 with higher hop-count to R2. Due to the fact that R4 connected through multi-access network topology to R1 and R2 we will have to use GRE tunnels before we can use offset-list.

I have configured 2 tunnel interfaces on R4, one for each router (R1, R2):

interface Tunnel1

ip address 201.0.0.4 255.255.255.0

tunnel source 10.1.124.4

tunnel destination 10.1.124.1

!

interface Tunnel2

ip address 202.0.0.4 255.255.255.0

tunnel source 10.1.124.4

tunnel destination 10.1.124.2

and the same on R1:

interface Tunnel1

ip address 201.0.0.1 255.255.255.0

tunnel source 10.1.124.1

tunnel destination 10.1.124.4

And R2:

interface Tunnel2

ip address 202.0.0.2 255.255.255.0

tunnel source 10.1.124.2

tunnel destination 10.1.124.4

Next configure passive-interface for FastEthernet 0/0 on all 3 routers (R4, R1, R1) in order to prevent RIP route exchange on the main interface (which is multi-access) and configure tunnel interface on the RIP process to allow exchange of RIP route information through the tunnels:

[R4]

router rip

version 2

passive-interface FastEthernet0/0

network 10.0.0.0

network 192.168.4.0

network 192.168.40.0

network 192.168.44.0

network 201.0.0.0

network 202.0.0.0

no auto-summary

[R1]

router rip

version 2

passive-interface FastEthernet0/0

network 10.0.0.0

network 201.0.0.0

no auto-summary

[R2]

router rip

version 2

passive-interface FastEthernet0/0

network 10.0.0.0

network 202.0.0.0

no auto-summary

Now looking on R3 routing table we can see that network 192.168.4.0/24 is advertised from R1 and R2:

R3#sh ip route rip

R 192.168.44.0/24 [120/2] via 10.1.123.2, 00:00:00, Serial0/0

[120/2] via 10.1.123.1, 00:00:01, Serial0/0

R 201.0.0.0/24 [120/1] via 10.1.123.1, 00:00:01, Serial0/0

R 192.168.40.0/24 [120/2] via 10.1.123.2, 00:00:00, Serial0/0

[120/2] via 10.1.123.1, 00:00:01, Serial0/0

R 202.0.0.0/24 [120/1] via 10.1.123.2, 00:00:00, Serial0/0

R 192.168.4.0/24 [120/2] via 10.1.123.2, 00:00:00, Serial0/0

[120/2] via 10.1.123.1, 00:00:01, Serial0/0

10.0.0.0/24 is subnetted, 2 subnets

R 10.1.124.0 [120/1] via 10.1.123.2, 00:00:00, Serial0/0

[120/1] via 10.1.123.1, 00:00:01, Serial0/0

Configuring standard access-list on R4 to identify network 192.168.4.0/24:

R4(config)#access-list 1 permit 192.168.4.0

Then configure offset-list under router level for tunnel 2 which leads to R2:

R4(config-router)#offset-list 1 out 10 tunnel 2

Now looking on R3 routing table will reveal:

R3#sh ip route rip

R 192.168.44.0/24 [120/2] via 10.1.123.2, 00:00:10, Serial0/0

[120/2] via 10.1.123.1, 00:00:03, Serial0/0

R 201.0.0.0/24 [120/1] via 10.1.123.1, 00:00:03, Serial0/0

R 192.168.40.0/24 [120/2] via 10.1.123.2, 00:00:10, Serial0/0

[120/2] via 10.1.123.1, 00:00:03, Serial0/0

R 202.0.0.0/24 [120/1] via 10.1.123.2, 00:00:10, Serial0/0

R 192.168.4.0/24 [120/2] via 10.1.123.1, 00:00:03, Serial0/0

10.0.0.0/24 is subnetted, 2 subnets

R 10.1.124.0 [120/1] via 10.1.123.2, 00:00:10, Serial0/0

[120/1] via 10.1.123.1, 00:00:03, Serial0/0

Network 192.168.4.0/24 is now preferred only through R1 although in case of R1 failure, R2 will advertise 192.168.4.0/24 but with higher hop-count.