[c-nsp] OSPF NSSA Question

Jay Hennigan jay at west.net
Sun Oct 3 20:47:36 EDT 2004

On Sun, 3 Oct 2004, Dan Armstrong wrote:

> 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?

Because by definition OSPF routers within an area will all have the same
link-state database.  Route aggregation is expected to be done between areas.

You could try a distribute-list in on each access router limiting it
to placing the default and/or your major network(s) into its routing
table.  I'm not sure if this would break things, probably best to try
it in a lab scenario first.

> 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?

It depends.  How many access routers, and how many customer routes
per each?  The routing table of the distribution table will be the same
isize regardless, and it will still need to do the same housekeeping in
terms of hello packets, etc. if each access router is coming in on a
separate logical interface (VLAN?).  If all of the access routers are
coming into the distribution router on a single logical network, making
them all different areas can get messy.

How stable is the network?  If customer interfaces don't flap much
causing routes to be withdrawn and then redistributed, the extra overhead
of the routing table may not be a big deal.  On the other hand, different
areas shouldn't tax the distribution router excessively (again, you haven't
given us an indication about numbers of routes/routers and how you think
this will scale) and will result in a much cleaner routing table in each
access router.  Route flaps on one access router won't affect any others.

Multiple areas would allow lightweight access routers but might require
a beefier distribution router, but I don't think it will need to be much
beefier depending on what the numbers are.  Under 30 or so areas should
not be a big deal at all.  Hundreds might.  "Whack-O" doesn't show up
on my calculator.  :-)  More areas mean more configuration on the
distribution router, of course.  The load on the distribution router
might actually decrease with multiple areas, particularly if the network
isn't stable.  With a single area the distribution router is processing
a lot of LSAs to all of the access routers every time a cuatomer route

How you intend to scale it, whether your subnets are logically able to
be summarized, etc. can all influence the design.

> 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...

I think that in a totally stubby scenario, you might have issues with
redistribution of the static customer routes back to the backbone.
Are you extending OSPF to the customer premise router or statically
routing the customer subnet on the access router and redistributing?

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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