I’ve been doing some prep for BSCI lately and am back into OSPF. One thing I ran into was keeping the various types of areas straight, and remembering all the restrictions that come with them. It wasn’t helped by an unclear description I read in the book, which caused me to really get into the details of NSSA areas.
Stub areas exist to reduce the number of LSAs that are processed within an area. This is done by sacrificing some information. Practically, this means we filter out various routes that are coming into the area and replace it with a default route. Remember that as a link state protocol, all routers within the area have to have the same information, so most of the work is done on the ABR.
A stubby area ignores external routes (O E1, O E2), and permits both inter and intra area routes (O IA and O). The path to the external routes is replaced by a default route injected by the ABR.
Leaving a default route to the ABR to get to the external destinations leads you to say “If I can do that for external routes, why not do the same thing to the interarea routes?”. That would be a totally stubby area. Totally stubby areas have only intra area routes (O), with the ABRs injecting default routes. Because an ABR, by definition, is on the backbone, it knows all the routes.
From an LSA perspective, the area itself is flooding type 1 and 2 LSAs to represent the various router and link states. The ABR sorts through those and issues type 3 LSAs to the backbone, where it becomes an O IA route. External routes are type 5 LSAs.
So an ABR operating in stubby mode filters out type 5 LSAs. An ABR in totally stubby mode filters out both type 5 and 3. This is also why only the ABR needs the “no-summary” attribute (referring to the type 3 summary LSA, not a summary route), because only the ABR filters and injects a default route.
Anyone who’s been around a network for long knows that it’s hard to predict where you’re going to have a connection to another system where you might need to redistribute a route into your IGP. Any route redistributed into OSPF is automatically an external route, and makes the router an ASBR. However, ASBRs and external routes are not allowed in stubs, hence the NSSA. An NSSA behaves the same as a stubby area except that ASBRs are allowed, with some trickery happening to get around the rules.
In an NSSA, external routes are allowed if they originate inside. The ABRs still filter out the type 5 LSAs at the border. Any ASBR within an NSSA advertises external routes as type 7 routes instead of type 5. Thus, they show up in the routing table as O N1 instead of O E1. The ABR converts the type 7 LSA into a type 5 LSA before it advertises the LSA to the backbone.
Because of the restriction on the type 5 LSAs, and the need to understand type 7 LSAs, all routers in the area need to be configured as an NSSA.
The “no-summary” still exists, and is only needed on the ABR. So, in addition to the NSSA behaviour, you’re still allowed to filter out those interarea type 3 LSAs at the border.
NSSAs are goofy in that you need to originate a default route explicitly in the nssa command.
I have a labbed up example using dyna-gen that shows how it works using 3 routers. I’ll post all the configs and network descriptions if you want to follow along with real gear or simulated gear…. Stay tuned.
Content Copyright Sean Walberg
OSPF special areas: stubby, not so stubby, and totally stubby