From: alsontra@hotmail.com
Date: Sat Jan 01 2005 - 14:14:30 GMT-3
All,
While revisiting the ole BGP next-hop-self command with respect to NBMA and
Multi-access networks I'm trying to determine what the preferred behavior is
for next hop values on Multi-access segments. I am referring to the
following example under BGP Next Hop (Multiaccess Networks):
http://www.cisco.com/warp/public/459/bgp-toc.html#bgpnexthop
Take the following example for instance:
Topology is Ethernet:
R2(AS500).2-----------.6-R6(AS400)-11.11.11.0/24
-
-
-.7
R7(AS400)
BGP Peering
R2(AS500)----R7(AS400)-----R6(400)
When R6 originates a route and that prefix is in turn advertised to R2 in
AS500, the bgp behavior is as described in the case study. That is, R7
advertises R6 as the next hop to prefixes generated by R6.
<snip>
SW1#sh ip bgp nei 140.1.1.2 advertised-routes
BGP table version is 10, local router ID is 150.1.7.7
Status codes: s suppressed, d damped, h history, * valid, > best, i -
internal
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*>i11.11.11.0/24 140.1.1.6 0 100 0 i
</snip>
However, when the bgp table is displayed on R2 the next hop value is
modified to R7. Typically this is the case for eBGP connectivity, but
multi-access networks are supposed to forward traffic in a more
direct/efficient manner. (no extra hops)
<snip>
R2#sh ip bgp
BGP table version is 4, local router ID is 150.1.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i -
internal
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 11.11.11.0/24 140.1.1.7 0 400 i
</snip>
*********************************
R2#traceroute 11.11.11.11
Type escape sequence to abort.
Tracing the route to 11.11.11.11
1 140.1.1.7 4 msec 4 msec 4 msec
2 140.1.1.6 4 msec * 4 msec
R2#
*************notice extra hop****
Shouldn't R2 perfer R6 as the next hop? Isn't this the default behavior of
BGP on Multi-access media?
Al
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