From: Bezverkhi, Serguei (Serguei.Bezverkhi@xxxxxx)
Date: Sat Jun 22 2002 - 23:09:39 GMT-3
On multi-access media BGP next hop behavior is different. It is well
explained in Doyle's book.
Serguei
-----Original Message-----
From: Hunt Lee [mailto:ciscoforme3@yahoo.com.au]
Sent: June 22, 2002 9:07 PM
To: ccielab@groupstudy.com
Subject: BGP Next-Hop attribute
Sorry, the bgp output stuffs up before, so here's a repost ;)
Hey all, I'm probably missing some fairly simple concept here. I have
setup 4 routers as follows:-
RTB
/
RTD --- RTA
\
RTC
Both RTB & RTC are connected (via Eth) to a network called
172.17.1.0/24. RTA & RTD are both in AS2 RTB & RTC are both in AS1
The connection between RTA & RTD is via Ethernet
RouterA#sh ip bgp
BGP table version is 9, local router ID is 25.25.25.2
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
*> 10.1.1.0/30 10.1.1.2 0 0 1 i
* 10.1.2.2 0 1 i
*> 10.1.2.0/30 10.1.1.2 0 1 i
* 10.1.2.2 0 0 1 i
*> 25.25.25.0/24 0.0.0.0 0 32768 i
*> 172.17.1.0/24 10.1.1.2 0 0 1 i
* 10.1.2.2 0 0 1 i
RouterA#sh running-config
router bgp 2
network 25.25.25.0 mask 255.255.255.0
neighbor 10.1.1.2 remote-as 1
neighbor 10.1.2.2 remote-as 1
neighbor 25.25.25.1 remote-as 2
maximum-paths 6
When I lookup RTD's BGP table (shown below), for the route
172.17.1.0/24,
instead of via 10.1.1.2 or 10.1.2.2, it is 25.25.25.2. Ok - here's
what I'm
confused about. Since I thought that in EBGP peering, the next hop is
usually
the IP address of the neighbor that announced the route, but IBGP
peering
preserve the Next-hop attribute learned from EBGP peers. So in the
case, if I
haven't use next-hop-self on RTA, I would think the EBGP next hop in
RTD's BGP
table would be either 10.1.1.2 or 10.1.2.2, but why is the EBGP
next-hop is
25.25.25.2?
RouterD#sh ip bgp
BGP table version is 6, local router ID is 172.16.0.2
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
*>i10.1.1.0/30 25.25.25.2 0 100 0 1
i*>i10.1.2.0/30 25.25.25.2 100 0 1
i*>i25.25.25.0/24 25.25.25.2 0 100 0 1 i
*>i172.17.1.0/24 25.25.25.2 0 100 0 1 i
Thanks,
H.
http://www.sold.com.au - SOLD.com.au
- Find yourself a bargain!
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