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OSPF should have a longer match and interface loopback

R33 is ASBR between OSPF (S0/0, S0/1, S0/2) ) and RIP ( E0/1).

R66 is connected to E 0/1 of R33.

R33 interface loopback 3 is,,,,, R66 interface loopback 6 is


1- If All routes in the OSPF domain should have a longer match for R66’s interface loopback 6

2- No other routers should be advertised from RIP to OSPF.

3- Redistribute between OSPF and RIP on R33.

R33:-----part of the configuration

router ospf 1

redistribute rip subnets route-map FILTER_ROUTE


ip prefix-list ONLY_LOOPBACK seq 15 permit


route-map FILTER_ROUTE permit 10

match ip address prefix-list ONLY_LOOPBACK

I numbered loopbacks with number 3 and 6 to distinguish between them,

I am familiar with longer match, but what is the criteria for us to choose which prefix we have to use for our example here (,,,,the origin was 32 bits) ?(i.e why have not we choose /26, or /28, /30)

Cisco Employee

Re: OSPF should have a longer match and interface loopback

The requirement for a longer match tells me that we should advertise r6 loopback address as a /32.

I'm not sure why they redistribute this address as a /24. Bear in mind that if you don't use ripv2 the prefix mask that you will get for r6 loopback address is dependant on how the receiving interface (e0/1) is addressed.

With ripv2 you can receive r6 loopback address as a /32.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
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