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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 133.33.33.33/32,,,,, R66 interface loopback 6 is 166.66.66.66/32

Requirment

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 166.66.66.0/24

!

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 (166.66.66.0/24,,,,the origin was 32 bits) ?(i.e why have not we choose /26, or /28, /30)

1 REPLY
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)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
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