This document shows how Open Shortest Path First (OSPF) injects a default route into a normal area. Default routes injected into a normal area can be originated by any OSPF router. The OSPF router does not, by default, generate a default route into the OSPF domain. In order for OSPF to generate a default route, you must use the default-information originate command.
There are two ways to advertise a default route into a normal area. The first is to advertise 0.0.0.0 into the OSPF domain, provided the advertising router already has a default route. The second is to advertise 0.0.0.0 regardless of whether the advertising router already has a default route. The second method can be accomplished by adding the keyword always to the default-information originate command.
There are a number of things. We can change the metric or metric type but the most important thing most people forget is the always keyword.
If you use the default-information originate you can advertise a default route in OSPF. OSPF won’t advertise a default route if you don’t already have it in your routing table. If you add the alwayskeyword it will advertise the default route even if you don’t have it in the routing table. Once you have advertised the default route it will look like this on other routers:
R2#show ip ospf database | begin Type-5
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
0.0.0.0 172.16.3.1 59 0x80000001 0x008D64 1
R2#show ip route ospf
O*E2 0.0.0.0/0 [110/1] via 192.168.12.1, 00:00:24, FastEthernet0/0