18. July 2012 · Comments Off on OSPF Stub Areas · Categories: Cisco · Tags: , , , , ,

OSPF Stub Areas

OSPF stub areas limit the parts of the network where specific LSAs are allowed. The idea being that if an OSPF router receives an LSA it must process it, which takes a certain amount of processor and memory resources. By limiting the types of LSAs that can reach specific networks, the devices within these stub areas do not have to be as powerful but still retain reachability to the rest of the OSPF network.

There are three main types of OSPF stub areas:

  • Stub Areas
  • Totally Stubby Areas
  • Not So Stubby Areas

Stub Areas

An area that is configured as a stub is able to receive all types (as discussed above) of LSA except an LSA Type 5. Any routes that are destined for external networks are forwarded using a default route that is injected into the network in place of the LSA Type 5.

Totally Stubby Areas

Like a stub area, a totally stubby area is unable to receive LSA Type 5 packets. Along with this, the area is also unable to receive LSA Type 3 packets that include network advertisements (Not External) from other areas. Again, like a stub area, all traffic that is destined for these networks (both internal and external networks outside the area) is destined for a default router that is injected in place of both the LSA Type 3 and Type 5.

Not So Stubby Areas (NSSA)

A NSSA is almost exactly the same as a normal stub area but allows an ASBR (Autonomous System Boundary Router) to exist within the area. With a typical stub area, it is not possible to locate an ASBR inside the area as LSA type 5 packets are not allowed. A NSSA gets around this by using an LSA Type 7 packet in place of the LSA Type 5 packet within the NSSA; once this traffic from the ASBR exits the NSSA it is converted to an LSA Type 5 for transmission to the rest of the OSPF network.