[c-nsp] OSPF NSSA Question

Jay Hennigan jay at west.net
Sun Oct 3 19:11:07 EDT 2004

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
> backbone.
> 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
> good.
> 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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