Subject: [Idr] multiple-next-hops vs add-paths : AS confed example

From: Curtis Villamizar <[email-masked]>
Date: Thu, 31 Aug 2006 21:18:07 -0400

There was no response to my AS confed example probably since it was at
the end of a long message and changed the topic.  So I'm resending the
example as a separate message.

Curtis

A similar situation exists with confederations.  A route may be passed
through different private AS but with BGP confederations the next hop
is generally not changed, relying on the next hop advertised by the
router that borders the external AS and relying on the IGP rather than
the set of EIBGP advertisers.  In this case the AS Paths would be
different *until* the confederation AS were dropped before advertising
the route externally.

Consider this case.

  ASe1 - R1/NH1 -----  R3/AS65001
                 \  /             \
                   X               R5/AS65003 - R6/AS65004
                 /  \             /
  ASe2 - R2/NH2 ----  R4/AS65002

R5 gets the following routes:

   R3:  AS65001 ASexternal1  NH1
        AS65001 ASexternal2  NH2

   R4:  AS65002 ASexternal1  NH1
        AS65001 ASexternal2  NH2

It doesn't seem that based on NH there is a good way to disambiguate
the components of this multipath.

A good question is exactly what R5 should advertise to R6 such that
the AS path is correct.  It seems that the <peering,prefix,path-id>
tuple adds an ability to disambiguate this case.

My understanding of add-path is that the advertisement would be:

  R5 -> R6:

    prefix X path-id 1  AS65003 AS65001 ASexternal1  NH1
    prefix X path-id 2  AS65003 AS65001 ASexternal2  NH2
    prefix X path-id 3  AS65003 AS65002 ASexternal1  NH1
    prefix X path-id 4  AS65003 AS65001 ASexternal2  NH2

Maybe the multiple-next-hops and add-path advocates can kick this
example around for a while.

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