Subject: [Idr] IGP upcall to BGP?

From: "András Császár \(ETH/RL\)" <[email-masked]>
Date: Wed, 23 Jun 2004 09:53:15 +0200

draft-ietf-idr-bgp4-24 writes:

The local speaker MUST determine the immediate next-hop address from the
NEXT_HOP attribute of the selected route (see Section 5.1.3). If either the
immediate next hop or the IGP cost to the NEXT_HOP (where the NEXT_HOP is
resolved through an IGP route) changes, Phase 2 Route Selection MUST be
performed again.
<<

Sure, yes, that makes sense since if you have multiple alternative
inter-domain routes to a given destination, than it might be possible that
the router decided for one of them based on the lowest IGP cost tie breaking
rule.

So far, so good. But, what happens if there is a fault inside the AS, and
the connectivity remains after IGP re-routing but the IGP cost changes.
According to the above citation, BGP could then, of course, select a new
route.

My question: how (and how fast) does BGP learn about IGP cost change? Sure,
IGP link state advertisements at some point reach the given BGP speaker, but
how does this info get to the BGP module?

Anyway, could you please suggest me good articles/papers about the interface
between IGP (IS-IS/OSPF) and BGP? What timers and processing delays are
involved in both directions? I mean what happens if a BGP speaker receives
an UPDATE of a newly reachable destination prefix in an AS which has
multiple providers. This info has to get into interior routers via IGP. (For
a moment, let's forget about default IGP routes...)

Thanks,
András

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---

From: Jennifer Rexford <[email-masked]>
Date: Thu, 24 Jun 2004 13:38:09 -0400 (EDT)

> y question: how (and how fast) does BGP learn about IGP cost change? Sure,
> involved in both directions? 

For a recent study of this issue, see

  Renata Teixeira, Aman Shaikh, Tim Griffin, and Jennifer Rexford,
  "Dynamics of hot-potato routing in IP networks," Proc. ACM 
  SIGMETRICS, June 2004.
    http://www.research.att.com/~jrex/papers/sigmetrics04.pdf
    http://www.research.att.com/~jrex/talks/ima04.ppt
    http://www.nanog.org/mtg-0402/teixeira.html

-- Jen