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4.03.2014

OSPF Areas (Part 6) - Totally NSSA

Links:
RFC 2328 – OSPF Version 2
RFC 3101 - The OSPF Not-So-Stubby Area (NSSA) Option
RFC 2740 – OSPF for IPv6
Cisco - OSPF Not-So-Stubby Area (NSSA) 

Breakdown of the Totally NSSA Area:

  • Redistribution allowed by ASBRs
  • NSSA ASBRs generate Type-7 LSAs for external routes
    • Appear as N1 or N2 external
    • ABRs convert Type-7 LSAs to Type-5
  • Blocks Type-5 External, Type-4 ASBR and Type-3 Summary LSAs
  • Only ABRs are configured for totally NSSA 
    • Internal routers configured as NSSA to form an adjacency
    • N-Bit is set to 0 in Hello packet
  • Cannot be backbone area (area 0)
  • Virtual links are not allowed to transit
At this point you should already have a good idea of what a Totally NSSA area is. Basically a totally NSSA area is the combination of, wait for it... wait...
Totally NSSA is a combination of a Totally-Stubby area and an NSSA area. Who would have thunk!?!

As we saw in the NSSA post, external routes were allowed into the area via ASBRs. The area was treated just like a Stub area and allowed Type-3 LSAs in from other areas. 

A Totally-NSSA area will do the same thing as a Totally-Stubby area and filter out all Type-3 LSAs and reduce the LSDB to a default route generated by the ABRs. 

So design-wise anything inside a totally-NSSA area is either going to know specific routes to external networks or its going to push everything to the ABRs via a default LSA.

Lets lab it up and see what we get.


Same lab as the NSSA post. Configuration of a totally-NSSA is done the same as with a stub area using the no-summary option with the area command.
Once again remember your adjacencies are going to bounce so do this during, say... lunch or an all-hands meeting. Seriously do it during a window.



Using show ip ospf lets verify the configuration.



Alright, hmm looks the exact same as an NSSA. What about the LSDB?



Alright all the Type-3 Summary Net Link (IPv4) states and Inter Area Prefix Link States (IPv6) have been replaced by default routes as expected.

Jumping over to R1 lets see if the external routes from area 3 are still being advertised into OSPF.



As expected, E2 routes coming from both ABRs.

Just like a totally-stubby area all internal routers lose visibility outside of their area and rely on the ABRs to get traffic out. This could be problematic in specific failure scenarios which would either cause an ABR to route traffic sub-optimally or worse, black-hole the traffic.