Ospf nssa

danb83

Can anyone provide an example real world scenario where OSPF NSSA would be or has been used?


My understanding of the theory is that a NSSA can contain an ASBR (unlike Stub areas), and this ASBR can redistribute external routes as Type 7 LSAs (basically type 5 LSAs in disguise).


So a NSSA allows us to connect an ASBR to another network, and then redistribute the external routes via the NSSA into the rest of the network.


Can this 'additional' network be running any routing protocol including OSPF? The examples I have seen always show the additional network using EIGRP.


thanks

d4nz1g

They often use EIGRP only as an example, but I could imagine a branch site redistributing static and connected routes into OSPF, and this site does not need to know the full ospf topology. It would only receive type 3 lsa.

Btw, never used it, never seen it in a real network.

fredrikjj

You use it when these two conditions are met:

1) You have a router that, due to low memory, can't handle all the external routing information in your network and therefore must be stub (or totally stub)
2 That router must also inject external routing information.

That should make it a very rare thing to have to use in a modern network. Keep in mind that OSPF was design in a different era when routers were much much less powerful. Another thing to remember is that NSSA was an extension to OSPF that was designed to work with existing routers that did not support NSSA, explaining some odd things about the feature, like the translation from type 7 to type 5 LSA when external routes leave the NSSA area.

nice343

Stub area = allows LSA types 1,2,3
Total stub area = allows LSA types 1 ,2

If you are faced with a situation where redistributed routes(type 5 LSA's) have to pass through a stub or a total stub area, that's when you consider an NSSA.

What an NSSA does is to "convert" (for lack of a better word), type 5 LSA's into type 7 LSA and after the routes leave that area it's converted back into type 5 LSA.

The NSSA area connects an ASBR to an external network which is running any routing protocol other than OSPF (static, eigrp, BGP etc...)