Subject: [Isis-wg] BGP in L1 routers

From: Henrik Villför <[email-masked]>
Date: Fri, 17 Mar 2000 13:35:27 +0100

Hi.
   Reading <draft-ietf-isis-domain-wide-02.txt> I am a bit surprised to find
the following.

"  When a router learns multiple possible paths to external destinations
   via BGP, it will select only one of those routes to be installed in
   the forwarding table.  One of the factors in the BGP route selection
   is the IGP cost to the BGP next hop address.  Many ISP networks
   depend on this technique, which is known as "shortest exit routing".
   If a L1 router does not know the exact IGP metric to all BGP speakers
   in other L1 areas, it cannot do effective shortest exit routing. "

   What confuses me is that I was under the very strong impression that
destinations could not be installed from BGP unless a route to the BGP next
hop existed in the RIB and FIB and that a default route (as would be what is
available in an L1 router for an external route) would not do. 

   I would much appreciate if one of you wise people would spare the time to
enlighten me.

Thank's in advance,

   Henrik Villfor
   Telia Research

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---

From: Danny McPherson <[email-masked]>
Date: Fri, 17 Mar 2000 10:43:28 -0700

Extracted from RFC 1771, Section 9.1.2:

[...]

   If the NEXT_HOP attribute of a BGP route depicts an address to which
   the local BGP speaker doesn't have a route in its Loc-RIB, the BGP
   route SHOULD be excluded from the Phase 2 decision function.

[...]

   The local speaker MUST determine the immediate next hop to the address 
   depicted by the NEXT_HOP attribute of the selected route by performing 
   a lookup in the IGP and selecting one of the possible paths in the IGP.  
   This immediate next hop MUST be used when installing the selected route 
   in the Loc-RIB.  

I see no reason why "one of the possible paths in the IGP" could not be the 
"default" route, so long as the immediate next hop address can be determined.

-danny