Cisco CCNA / CCNP Certification: OSPF ASBRs Explained And Illustrated

Cisco CCNA / CCNP Certification: OSPF ASBRs Explained And Illustrated

When I first started studying for my CCNP, some of the concepts of OSPF really confused me. This was especially true for the ASBR, stub areas, and total stub areas. Sure, I could memorize the LSA types associated with these devices and area types, but there just weren't any illustrations that explained exactly what was going on.

This CCNP tutorial shows an ASBR in operation, and also introduces you to a basic example of route redistribution. Don't worry, it gets more complicated. :)

Here's the network we'll be working with in this tutorial:

R2 R3

Networks:

R1 - R5 Ethernet Segment: 10.1.1.0 /24

R1 - R2 - R3 Serial Segment: 172.16.123.0 /24 (Preconfigured with OSPF)

R1 and R5 are running RIP over their common Ethernet segment, 10.1.1.0 /24. R5 has three loopbacks it will be advertising into the RIP domain.

R1 is also running OSPF, with R2 and R3 as neighbors. Even though R1 knows about the loopbacks on R5, its OSPF neighbors do not. R1 has these routes in its RIP routing table, and for OSPF neighbors to learn of these routes, route redistribution must be manually configured.

R5#conf t
R5(config)#router rip
R5(config-router)#version 2
R5(config-router)#no auto-summary
R5(config-router)#network 5.0.0.0
R5(config-router)#network 10.0.0.0

R1#conf t
R1(config)#router rip
R1(config-router)#version 2
R1(config-router)#no auto-summary
R1(config-router)#network 10.0.0.0

R1#show ip route rip
5.0.0.0/24 is subnetted, 3 subnets
R 5.1.1.0 [120/1] via 10.1.1.5, 00:00:10, Ethernet0
R 5.2.1.0 [120/1] via 10.1.1.5, 00:00:10, Ethernet0
R 5.3.1.0 [120/1] via 10.1.1.5, 00:00:10, Ethernet0

R1 has a route for all three of R5