Friday 23 July 2010

BSCI - OSPF - Area Types - Not-So-Stubby-Area (Dude!*)

(*sorry, can't help it - I just keep hearing some surf dude trying to think up a clever name to describe these area types and not succeeding very well...) 

Not-So-Stubby-Area (NSSA) - special area type for when you end up adding a way out of the network via a Stub Area. For example you take over another company and you want to extend their RIPv2 network in to yours by making a router that was in your stub area an ASBR.

ISP~~~~(ASBR1) ----AREA0----(RouterB)---Area1-NSSA---(RouterA)~~~~New RIPv 2 Network
  • In the (crude) example above, RouterA was within the stub area attached to RouterB. It now has a path to external networks attached to it.
  • To Address the fact that RouterA is in a stub area, this area is configured to be a NSSA and in turn RouterA directs Type7 LSA packets to the ABR, in this case RouterB.
  • RouterB in turn takes this Type7 LSA and converts it in to a Type5, External Link, LSA and propagates into Area0.
Configure a NSSA
1) On all the routers in the NSSA do:
RouterA(config)#router ospf 1
RouterA(config-router)#no area 1 stub
   *remove the stub configuration
RouterA(config-router)#area 1 nssa

2) On the new ASBR within the NSSA redistribute your required RIPv2 netrworks:
RouterA(config)#router ospf 1
RouterA(config-router)#redistribute rip subnets

3) On the NSSA ABR, which is RouterB in the example above, do:
RouterB(config)#router ospf 1
RouterB(config-router)#area 1 nssa default-information originate
  *This causes RouterB to generate a O* N2 default route of and propagate in to the NSSA

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