From: Edmund Roche-Kelly (edr9007@xxxxxxx)
Date: Mon Jun 24 2002 - 15:54:06 GMT-3
I don't think that applies in this case. The multiaccess
case is when the ebgp session is over a multiaccess link.
If A and D are in AS 2, B and C are in AS 1, and A is
peering with B and C in this diagram, and B and
C are both advertising 172.16.101.0, C will advertise it
to A with next hop set to B. A should still advertise it
to D with next hop B.
+-B--| 172.16.101.0
|
D----A---+
|
+-C
The problem below is caused by maximum paths -- see
http://www.cisco.com/warp/public/459/25.shtml
"If maximum-paths n is enabled, and there are multiple external or
confederation-external paths from the same neighboring AS or sub-AS,
BGP inserts up to n most recently received paths in the IP routing
table.
This allows eBGP multipath load sharing. The maximum value of n is
currently 6. The default value, when this option is disabled, is 1. The
oldest received path is marked as the best path in the output of show ip
bgp <longer-prefixes>, and the equivalent of next-hop-self is performed
before forwarding this best path to internal peers."
"Bezverkhi, Serguei" wrote:
>
> Could not find it fast in Doyle, but here is the explanation from Cisco
> Web site:
>
> Q. In eBGP sessions, which IP address is sent as the next hop?
> A. In eBGP peering, the next hop is the IP address of the neighbor that
> announced the route. However, when the route is advertised on a
> multi-access media (such as Ethernet or Frame Relay), the next hop is
> usually the IP address of the router interface connected to that media,
> which originated the route. For a more detailed explanation, please
> refer to BGP Nexthop Attribute.
>
> Hope this helps
>
> Serguei
>
> -----Original Message-----
> From: Carlos G Mendioroz [mailto:tron@huapi.ba.ar]
> Sent: June 24, 2002 1:37 PM
> To: Bezverkhi, Serguei
> Cc: Hunt Lee; ccielab@groupstudy.com
> Subject: Re: BGP Next-Hop attribute
>
> How ?
> Serguei, for those of us without Doyle, would you please copy the
> details ?
>
> I think this has more to do with the "maximum-path" than with the
> multiaccess media between RTD & RTA.
>
> Somehow, RTA wants to "use" many paths to the 172.17.1.0/24, and the
> only way it has is to say "let me do it" by doing something like
> "next-hop-self". If you take the maximum-path away, you'll revert to
> standard 10.1.1.2 nexh hop.
>
> HTH,
>
> "Bezverkhi, Serguei" wrote:
> >
> > 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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