Lab 32 - OSPF Conditional Default Routing

Prerequisites: CCNP level skills.

Topology

Pic. 1 - OSPF Topology.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

Task 1

R1 does NOT have a default route but must become a default gateway for R2 (it is assumed that BGP is running between R1 and R3). The design stipulates that R1 should advertise this default route in OSPF only if the link between R1 and R3 is up.

Lab Solution

Task 1

R1 does NOT have a default route but must become a default gateway for R2 (it is assumed that BGP is running between R1 and R3). The design stipulates that R1 should advertise this default route in OSPF only if the link between R1 and R3 is up.

R1 Configuration:

!

ip access-list standard LINK_R1_R3
 permit 10.1.13.0 0.0.0.255
!
route-map CHECK_LINK permit 10
 match ip address LINK_R1_R3
!
router ospf 1
 router-id 1.1.1.1
 log-adjacency-changes
 network 10.1.12.1 0.0.0.0 area 0
 network 172.16.101.1 0.0.0.0 area 0
 default-information originate always route-map CHECK_LINK

!

Note!
The command 'default-information originate always' allows to advertise default route in OSPF without actually having one in the routing table. However, this 'always' keyword here will be dependent on the presence of 10.1.13.0/24 in the routing table (link between R1 and R3 must be up). In case the link is down, 10.1.13.0/24 is withdrawn from R1's routing table and default route is no longer advertised towards R2 (withdrawn). This way we accomplish the conditional router advertisement with the 'always' keyword used in the configuration.

Verification:

Pic. 2 - R2's Routing Table - R1_R3 Link Up.

Shutting down interface between R1 and R3 results in withdrawing this default route.