- Cisco Networking: OSPF conditional default route injection

On Cisco IOS, the default-information originate command is used to generate a default route within an OSPF process. There are two options when configuring origination of a default route: a persistent route (specified by appending the always keyword) will always be advertised, whereas a conditional route will only be advertised when certain other routes are present in the router's table. Consider the following topology:

R3 attaches the OSPF cloud to the Internet by two independent connections: a 100 Mbps metro Ethernet link to R1, and a backup 1.5 Mbps frame relay circuit to R2. We want R3 to inject a default route to the Internet into the OSPF domain. Simple enough, right?

R3(config)# router ospf 1 R3(config-router)# default-information originate always 

As predicted, a default route appears within the OSPF domain:

OSPF_Router# show ip route ospf O*E2 0.0.0.0/0 [110/1] via 172.16.0.3, 00:00:36, FastEthernet0/0 

But, here we encounter a design issue: if both of R3's connections to the Internet go down, what point is there in advertising the default? It would only black-hole Internet-bound traffic when other routers in the OSPF domain might have alternate connections out to the Internet. Fortunately, we can configure R3 to advertise a default route only when at least one of its two uplinks is functioning.

First, we create a prefix list or access list to match the subnet of either uplink. The example here uses a prefix list, but an access list would work just as well.

R3(config)# ip prefix-list Uplinks permit seq 5 10.0.1.0/24 R3(config)# ip prefix-list Uplinks permit seq 10 10.0.2.0/24 

Next, we create a route map that can be referenced by the OSPF default-originate command. Note that this route map merely acts as a "wrapper" around our prefix list.

R3(config)# route-map Routes_to_Inet R3(config-route-map)# match ip add prefix-list Uplinks 

Finally, we reconfigure OSPF to inject a default route only when at least one route matched by our route-map is present:

R3(config)# router ospf 1 R3(config-router)# default-information originate route-map Routes_to_Inet 

At this point, R3 should still be advertising a default route, as both Internet uplinks are functioning:

OSPF_Router# show ip route ospf O*E2 0.0.0.0/0 [110/1] via 172.16.0.3, 00:12:46, FastEthernet0/0 

If we disable one of R3's uplinks, the default route remains, because at least one of our prefixes is still in R3's routing table:

R3(config-router)# int f0/0 R3(config-if)# shutdown 

OSPF_Router# show ip route ospf O*E2 0.0.0.0/0 [110/1] via 172.16.0.3, 00:13:07, FastEthernet0/0 

However, if we disable the remaining uplink, the route-map referenced by our OSPF configuration no longer matches any routes in the table, and the default route is withdrawn:

R3(config-if)# int s1/0 R3(config-if)# shutdown 

OSPF_Router# show ip route ospf 

Following this same logic, the default route is restored as soon as one of the uplinks is revived:

R3(config-if)# int s1/0 R3(config-if)# no shut 

OSPF_Router# show ip route ospf O*E2 0.0.0.0/0 [110/1] via 172.16.0.3, 00:00:03, FastEthernet0/0