7.8 OSPF Stub Areas

Configure a stub area on the ABR:

… note that each router within the stub area needs to have this command set, since the hello packets contain a stub area flag that must match between neighbors. The adjacency will remain down until all OSPF neighbors have this configured, as shown below.

The stub command will both inject a Type 3 default route, as well as continue to flood Type 3 LSAs from other areas.

Validation of the R2 routing table:

Note: R2 receives a default route via Type 3 LSA, but cannot reach the network 192.0.2.0/24 because R1 has not route to that IP. The solution would be to advertise default-information originate from the ASBR, thereby providing R1 with a default route to reach it.

To inject a default-route using default-information originate, the ASBR must already have a default route in its table. Otherwise, the ‘always’ keyword must be used.

Next, area 2 is configured as totally stubby areas via R1 and R3.

The “stub no-summary” command will both inject a Type 3 default route, but restrict flooding of Type 3 LSAs from other areas.

Note that on the leaf node within the totally stubby area, you do not need to indicate no-summary since it isn’t an ABR and doesn’t forward inter-area routes anyway. The stub command itself is sufficient to meet the hello packet flag requirement.

Here’s a look at R3’s routing table when operating in a totally stubby area: