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3.24.2014

OSPF Areas (Part 4) - Totally Stubby

Links:
RFC 2328 – OSPF Version 2
RFC 2740 – OSPF for IPv6
Cisco.com - What are OSPF Areas and Virtual Links

 Breakdown of the Stub Area:
  • Blocks Type-5 External, Type-4 ASBR and Type-3 Summary LSAs.
  • ABRs generate a default route (0.0.0.0/0) for the area (Type-3 Summary)
  • Only ABRs for an area are configured for totally stubby areas. 
  • Cannot be backbone area (area 0)
  • Cannot redistribute into totally stubby areas (no ASBRs)
  • Virtual links are not allowed to transit totally stubby areas
  • E-Flag is set to 0 in Hello packet (same as stub)

In the OSPF Stub area post we saw stub areas filtered out all Type-4 and Type-5 LSAs from an area. The area ABRs created a default Type-3 Summary and advertised them into the area. Inter-area routes (Type-3) were still advertised into the area. 

Totally stubby areas take stub areas one step further and filter out the Type-3 LSAs as well. The logic is, if there is only one way out to other areas then why not just have a default route pointing traffic towards your ABR. 

Since the totally stubby filtering is done at the ABRs, configuration only needs to be applied to the ABRs. All routers in the area must still be configured as stub routers and the ABRs will be the only routers knowledgeable of the area being a totally stubby area.

Let’s hit the lab and see what is going on.

We are using the same topology as last time with the same configuration as we left it. As a reminder here is the LSDB and route table for R5.




And OSPFv3 for IPv6




As we can see R5 knows about all inter-area routes and also has default routes pointing back to R2 and R3 for all filtered external routes.

Configure R2 and R3 as totally stub areas with the area [area#] stub no-summary command and see what changes. Keep in mind this change will cause the neighbor adjacencies to drop.



We can verify area 3 is now a totally stubby area by checking both R2 and R3 with show ip ospf. Both IPv4 and IPv6 have the same output.



Notice the second and third lines under Area 3, They specify the area is both a stub area and this ABR advertises a default route into the area and filters all other summary LSAs.

NOTE: It is possible to have multiple ABRs with some configured as stub and some as totally stubby within an area. In this situation you will have OSPF inter-area LSAs leaked into the area by the stub ABRs. 

Now lets see what R5 shows




And OSPFv3 for IPv6




Just as expected, everything has been condensed into a single summary LSA from both ABRs. The routing table also shows that it is load balancing traffic across both ABRs.

The only thing you have to keep in mind when using a totally stubby area is the area routers lose visibility into the inter-area routes and fully rely on the ABRs to handle how to get outside the area.

Up next we have Not So Stubby Areas (NSSA).