Subject: [Isis-wg] BCP for building Integrated IS-IS Network

From: Michael Kim <[email-masked]>
Date: Thu, 13 Jan 2000 23:26:38 +0100

Hi!
I have som question about design issue of the Integrated IS-IS network.
 Network is 200-300  nodes.

Should this network be buildt with multiple level-1 IS-IS area and 1
level-2 IS-IS area or should this be buildt with just big one level-1-2
IS-IS area.

I prefer to build network with the first alternative because in the
first alternative will keep the database small on the routers. It will
take few ms to recalculate SPF.

I got recommendation from some vendor that network with this size  it
will be the best to build with just one big level-1-2 area. Reason  is
that  there will be just one database so it will take less time to
recalculate SPF. In the first case there will be some routers talking
both level-1 and level-2 so there will be 2 database on those router and
for those routers it will take longer time to recalculate SPF. But I
think this is wrong becase in second alternative all the router is
talking level-1 and level-2 so all of the router will have one level-1
and one level-2 database with bigger LSDB. It will increase
recalculation time.

Do I think right?
Is there any BCP document other recommendation document for build IS-IS
network?

Thank You in advances

/michael

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---

From: [email-masked]
Date: Thu, 13 Jan 2000 16:46:24 -0800 (PST)

> level-2 IS-IS area or should this be buildt with just big one level-1-2

  This all depends on the strength of the IS-IS implementation 
 of your vendor. And on the type of routers that you are using.
 I know at least one vendor where you can build areas with a 1000+
 routers, if you buy their high end boxes.

> take few ms to recalculate SPF.

  Yes, the SPFs will be cheaper. But you run two of them.
 Also the mathemetical part of the SPF is usually not that expensive.
 Route installation (and FIB updates) is typically more expensive.
 Also when you use a lot of inter-area routes (L1->L2), it doesn't
 matter much if you deal with complexity of inter-area routes, or
 the complexity of SPF and intra-area routes.
  All this is just IMHO, and from experience with one implementation.

  The benefits of using multiple areas are:
 1) a little more protection against link flaps
 2) ability to extend your network easier. once your network grows so
    much that you finally really need multipkle areas, you have already
    layed them out
 3) you can do summarization and filtering between area/level boundaries

  Drawbacks:
 1) added configuration and troubleshooting complexity
 2) if you do BGP, the fact that L1 areas are stub areas can give you
    troubles. (Giving MED to customers, doing shortest-exit-routing,
    etc). BTW, in some implementations L1 areas are more like NSSA.
    Also you can now do route-leaking with the old TLVs, or with
    the newer TLVs. See the recent drafts.

> for those routers it will take longer time to recalculate SPF.

  I would say the main reason to run 200-300 routers in one area
 is simplicity. Why bother with multiple areas if you can get away
 with just one.

> But I

  I think the duration of SPF computation is not really relevant
 to the design of your network.

> Is there any BCP document other recommendation document for build IS-IS

  Don't think so.

> Thank You in advances

  Hope this helps,

      Henk.