How are you ending up with a greater number of AS hops for the route
on R6 learned via AS3?
> r6#sh ip bgp 10.0.0.0
> BGP routing table entry for 10.0.0.0/8, version 3
> Paths: (2 available, best #2, table Default-IP-Routing-Table)
> Advertised to non peer-group peers:
> 1.1.34.3
> 3 3 1
^^^
| Why does there seem to be an as-prepend here? --Alan
> 1.1.34.3 from 1.1.34.3 (1.1.34.3)
> Origin IGP, localpref 100, valid, external
> 2 1
> 1.1.24.2 from 2.2.2.1 (2.2.2.1)
> Origin IGP, localpref 100, valid, internal, best
Again, in this case, the iBGP learned route is preferred because it is
only two AS hops away... The externally learned route, from peer
1.1.34.3, shows AS3 twice in the path, making this route 3 AS hops
away.
----- Original Message -----
From: "Przemyslaw Karwasiecki"
To: "W. Alan Robertson"
Cc: "Peter van Oene" ; "Groupstudy - CCIELAB"
; "Groupstudy - Cisco Certification"
Sent: Tuesday, February 05, 2002 10:15 PM
Subject: Re: Undocumented iBGP Behavior (Confirmed by Cisco)
> Ok,
>
> Lab is done. I expected 20 minutes, it tooks 1 hour.
> Important lesson about time management learnt :)
>
> small legend:
> r5 and r6 are routers in AS4
>
> If anyone care I can send complete configs.
>
> Przemek
>
> r5#sh ip bgp summ
> BGP router identifier 2.2.2.1, local AS number 4
> BGP table version is 2, main routing table version 2
> 1 network entries and 1 paths using 133 bytes of memory
> 1 BGP path attribute entries using 60 bytes of memory
> 1 BGP AS-PATH entries using 24 bytes of memory
> 0 BGP route-map cache entries using 0 bytes of memory
> 0 BGP filter-list cache entries using 0 bytes of memory
> BGP activity 1/9 prefixes, 2/1 paths, scan interval 60 secs
>
> Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down
State/PfxRcd
> 1.1.24.2 4 2 23 22 2 0 0 00:18:14
1
> 2.2.2.2 4 4 27 26 2 0 0 00:21:53
0
> r5#sh ip bgp 10.0.0.0
> BGP routing table entry for 10.0.0.0/8, version 2
> Paths: (1 available, best #1, table Default-IP-Routing-Table)
> Advertised to non peer-group peers:
> 2.2.2.2
> 2 1
> 1.1.24.2 from 1.1.24.2 (1.1.1.1)
> Origin IGP, localpref 100, valid, external, best
> r5#
> telnet-server#6
> [Resuming connection 6 to r6 ... ]
>
> r6#sh ip bgp summ
> BGP router identifier 2.2.2.2, local AS number 4
> BGP table version is 3, main routing table version 3
> 1 network entries and 2 paths using 169 bytes of memory
> 2 BGP path attribute entries using 120 bytes of memory
> 2 BGP AS-PATH entries using 48 bytes of memory
> 0 BGP route-map cache entries using 0 bytes of memory
> 0 BGP filter-list cache entries using 0 bytes of memory
> BGP activity 1/6 prefixes, 2/0 paths, scan interval 60 secs
>
> Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down
State/PfxRcd
> 1.1.34.3 4 3 21 20 3 0 0 00:15:20
1
> 2.2.2.1 4 4 27 28 3 0 0 00:22:13
1
> r6#sh ip bgp 10.0.0.0
> BGP routing table entry for 10.0.0.0/8, version 3
> Paths: (2 available, best #2, table Default-IP-Routing-Table)
> Advertised to non peer-group peers:
> 1.1.34.3
> 3 3 1
> 1.1.34.3 from 1.1.34.3 (1.1.34.3)
> Origin IGP, localpref 100, valid, external
> 2 1
> 1.1.24.2 from 2.2.2.1 (2.2.2.1)
> Origin IGP, localpref 100, valid, internal, best
> r6#
>
>
> Once better route is selected in Local-RIB, the other,
> previously advertised is withdrawn.
>
>
>
> On Tue, 2002-02-05 at 21:15, W. Alan Robertson wrote:
>
> > If you can, build your test scenario to look like this:
> >
> >
> > [eBGP ]
> > ___[AS 1]___
> > | |
> > | |
> > [ eBGP ] [ eBGP ]
> > [ AS 2 ] [ AS 3 ]
> > | |
> > | |
> > | |
> > | |
> > [ BGP ] [ BGP ]
> > [ AS 4 ] [ AS 4 ]
> > | |
> > | |
> > _|___________|_
> >
> >
> > Originate a route (say the 10.0.0.0/8 route) in AS 1, and see what
> > happens in AS 4.
> >
> > You should see two entries for the 10/8 network in 'show ip bgp'
> > output, one of which is learned via the eBGP peer, and gets
installed
> > in your routing table, and the second, learned via the iBGP peer,
> > which does not get installed in your routing table.
> >
> > That is the time-honored behavior, the behavior we've all come to
know
> > and love since the dawn of time, etc, etc, amen. :)
> >
> > Now, in 12.0(20), this behavior is changed to they behavior I have
> > described in this thread. I don't know when this change occurred,
but
> > hope to have that question answered tomorrow. Most of my other
BGP
> > customers are running 12.1 stuff...
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=34582&t=34582
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