[c-nsp] OSPF NSSA Question

Dan Armstrong dan at beanfield.com
Sun Oct 3 19:37:03 EDT 2004

Thank you to all that have replied...

Yeah, I guess that's why I am confused.... why do the routers in the 
stub/nssa area need any information other than a default route? 

The access routers are all in 1, non zero area.  It seems redundant to 
me that they need to learn routes from other routers in their area, 
since they all point back to the same place as the default route (the 
dist router).... If I setup each access router in a different area, I 
get exactly the behaviour I want, but that seems ridiculous to have an 
area for each access router - I would blow up my dist routers pretty 
quickly with too many areas, no?

I was looking into the stub feature in EIGRP, and it apperas to behave 
more like I want - but there is no way to prevent routers neighbouring 
in EIGRP that I don't want to neighbour, so for a bunch of other reasons 
that will send this off topic, I was hoping not to use EIGRP....

Even if the area is setup totall stubby (no summary) or not, the same 
effect occurs.  The intra area routes still get propagated...

Jay Hennigan wrote:

>On Sun, 3 Oct 2004, Dan Armstrong wrote:
>>I am tearing my hair out here with an NSSA problem, any help is appreciated:
>>Say I have a distribution layer router, that is connected to it's backbone
>>with OSPF in area 0.
>>I have a whack-O-Access routers, each access router in a different VLAN,
>>connecting to a different interface on the distribution layer router.
>Are the access routers part of area 0, each in a different area, or all
>in the same non-zero area?
>>Customer's IP blocks get anchored on the access routers, and my purpose for
>>using a routing protocol here is to get the IP blocks (which are C routes on
>>the access routers)  advertised up into the dist router, and on up into the
>>The only way anything can leave the access router is via the distribution
>>router, so I decided to make a stub area (nssa actually) for all the access
>>routers.  The access router only needs a default advertised, and life is
>>For some reason, I am missing, OSPF is advertising every little stupid block
>>on every access router to every other access router, via the distribution
>>router.  All I need is the default route, why does OSPF insist on advertising
>>all these blocks?  Isn't the whole point of a stub area to be able to prevent
>>a million specific prefixes from being advertised into it and add a simple
>>default route if a router only has 1 exit point?
>It depends.  If each access router is in a different area, then you can do
>this with OSPF using appropriate redistribution and default information
>originate.  The distribution router becomes an ABR for each access router.
>If the access routers are all in the same area, then OSPF will, by nature,
>cause all routers to propagate link state information to all other routers
>within that area, and maintain the same database on all routers within that
>Jay Hennigan - CCIE #7880 - Network Administration - jay at west.net
>WestNet:  Connecting you to the planet.  805 884-6323      WB6RDV
>NetLojix Communications, Inc.  -  http://www.netlojix.com/

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