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North American Network Operators Group

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RE: AT&T NYC

  • From: Derek Samford
  • Date: Thu Aug 29 16:59:09 2002

Ralph,
	Okay, no one ever said an IBGP mesh was bad. We were all upset
by the mention of an IGP distributed into an EGP. Let's do a little math
here. The formula for IBGP sessions goes as follows.

n*(n-1)/2

2=1
3=3
4=6
5=10

So you've only got 4 routers? That's fine, 6 sessions is not too hard to
maintain. However, one more router, annd10 can get to be cumbersome and
10 Routers in your network, you have to maintain 45 BGP sessions. My
personal favorite approach (And this may, or may not, start a religious
war.) is confederations. The great part is, if your IBGP mesh inside a
Sub-as gets to large, you can add a route-reflector, and have a hybrid
RR/Confederation approach. This is very scalable, although there are
some issues with being able to follow shortest path out of a
confederation, so you need to have a little skill at traffic
engineering. Building networks is easy Ralph, building SCALEABLE
networks, is not.

Derek

> -----Original Message-----
> From: Ralph Doncaster [mailto:ralph@istop.com]
> Sent: Thursday, August 29, 2002 4:44 PM
> To: Derek Samford
> Cc: 'Robert A. Hayden'; 'Michael Hallgren'; 'Peter van Dijk';
> nanog@merit.edu
> Subject: RE: AT&T NYC
> 
> On Thu, 29 Aug 2002, Derek Samford wrote:
> 
> > I personally prefer using IS-IS for loopback/infrastructure routes,
and
> > I use confederations for my IBGP. If a confederation ever gets to
large,
> > I can always add a route-reflector inside the confederation. Ralph,
you
> > have never failed to amaze me with your love for WCP (Worst Current
> > Practices.)
> 
> OK, then hand me a clue and explain why ruing an iBGP mesh with 3-4
> routers is so bad (seeing as Bassam Halabi didn't in his book).
> 
> -Ralph
> 






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