From: mcaplan.cs@xxxxxxxxxxxxxxx
Date: Mon May 21 2001 - 10:40:05 GMT-3
Darren,
Yes, thats right. When the link via R1 goes down, R1 will inform R2 via IBGP
about the lost route; ie it will send a WITHDRAWN route update. R2 will then
remove this route from its BGP table, and then advertise its best route to
R1. R1 will then pick up the backup route via R2.
Although to get some certainty on it, I would try it out and watch what
happens via 'debug ip bgp update'
With regard to the messages one sees via debug, I think its reported
differently at each end. One end reports seeing the WITHDRAWN updates coming
in, the other end reports sending a 'route unreachable' update.
eg in a non related network here are some debugs's from opposite ends of an
IBGP session
R1
1d04h: BGP: 192.68.6.1 rcv UPDATE about 172.16.112.0/24 -- withdrawn
R2
2w0d: BGP: 192.68.6.2 send UPDATE 172.16.112.0/24 -- unreachable
Cheers
Mark
> ----------
> From: Darren Hosking[SMTP:dhosking@commander.com.au]
> Sent: Montag, 21. Mai 2001 15:25
> To: 'mcaplan.cs@clearstream.com'; ccielab@groupstudy.com
> Subject: RE: should all IBGP neighbours have all EBGP paths?
>
> I cleared the IBGP AS and I did see WITHDRAWN updates coming through on
> the
> router with higher local preference (didn't see the other router sending
> them - used debug ip bgp update). So if the link to the ISP via r1 with
> higher local preference goes down, IBGP will update this router (r1) with
> the routes from r2? Are these routes requested by one router or sent by
> the
> other?
>
> Thanks, Darren
>
> Configs are as follows:
>
> r1 (primary link):
>
> router bgp 65000
> no synchronization
> bgp log-neighbor-changes
> neighbor 10.0.1.1 remote-as 1221
> neighbor 10.0.1.1 ebgp-multihop 255
> neighbor 10.0.1.1 route-map SET_OUTBOUND_TRAFFIC in
> neighbor 10.0.1.1 filter-list 10 out
> neighbor 172.24.96.1 remote-as 65000
> neighbor 172.24.96.1 next-hop-self
>
> ip as-path access-list 10 permit ^$
>
> route-map SET_OUTBOUND_TRAFFIC permit 10
> set local-preference 300
>
> r2 (backup link):
>
> router bgp 65000
> no synchronization
> bgp log-neighbor-changes
> neighbor 10.0.2.1 remote-as 1221
> neighbor 10.0.2.1 route-map SET_OUTBOUND_TRAFFIC in
> neighbor 10.0.2.1 filter-list 10 out
> neighbor 172.24.24.1 remote-as 65000
> neighbor 172.24.24.1 next-hop-self
>
> ip as-path access-list 10 permit ^$
>
> route-map SET_OUTBOUND_TRAFFIC permit 20
> set local-preference 250
>
>
>
> -----Original Message-----
> From: mcaplan.cs@clearstream.com [mailto:mcaplan.cs@clearstream.com]
> Sent: Monday, 21 May 2001 7:07:PM
> To: dhosking@commander.com.au; ccielab@groupstudy.com;
> zhutong@ronghai.com.cn
> Subject: RE: should all IBGP neighbours have all EBGP paths?
>
>
> Darren,
>
> I cant tell without diagrams and configs, but I suspect its a case of BGP
> only advertises the best path.
> Consider the following example
>
> r3----------------r4 AS2
> | |
> ++++++++++++++++++++++
> | |
> | |
> r1-----------------r2 AS1
>
>
>
> If R2 has the better local preference to AS2, it will tell R1 about it. R1
> will then decide that R2 is the best route to AS2. Any route that R1 had
> advertised to R2 as a route to AS2 will be then withdrawn. If you use
> 'debug
> ip bgp updates' you can actually see this WITHDRAWN message being sent.
> After that R2 will only have a single route to AS2 in the BGP table. R1
> however will probably have 2 routes in its BGP table - the best one via R2
> and the other via R3.
>
> Hope this helps
>
> Mark
>
> > ----------
> > From: zhutong[SMTP:zhutong@ronghai.com.cn]
> > Reply To: zhutong
> > Sent: Montag, 21. Mai 2001 08:02
> > To: Darren Hosking; ccielab@groupstudy.com
> > Subject: Re: should all IBGP neighbours have all EBGP paths?
> >
> > Give your config pls.
> >
> > ----- Original Message -----
> > From: "Darren Hosking" <dhosking@commander.com.au>
> > To: <ccielab@groupstudy.com>
> > Sent: Monday, May 21, 2001 11:44 AM
> > Subject: should all IBGP neighbours have all EBGP paths?
> >
> >
> > > I have 2 BGP routers connected externally to the same AS and
> internally
> > to
> > > each other using IBGP. The local preference is set to prefer one link
> > over
> > > the other. All paths are visible on the non-preferred router (shows
> both
> > > paths out with the other as preferred), however, the router with the
> > > preferred path to the ISP only shows the paths it knows about and not
> > the
> > > paths via the other link.
> > >
> > > Is this correct? Don't both need all paths or are they exchanged when
> > the
> > > preferred path is no longer available (could be a lot of traffic)?
> > >
> > > I'm using Halabi 1st edition as reference, is the 2nd edition much
> > better
> > > and in what areas (ie should I buy the newer edition).
> > >
> > > Thanks, Darren
> > >
> > > The output of sh ip bgp summ and sh ip bgp for one network is shown
> > below.
> > >
> > > Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down
> > > State/PfxRcd
> > > 10.0.1.1 4 1221 6221 3628 43815 0 0 2d12h
> > 6606
> > > 172.24.24.1 4 65000 6626 6313 43815 0 0 13:10:59
> > 6606
> > >
> > > BGP table version is 43789, local router ID is 172.24.97.1
> > > 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
> > > *>i9.3.4.0/24 172.24.24.1 300 0 1221 ?
> > > * 10.0.1.1 250 0 1221 ?
> > >
> > > Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down
> > > State/PfxRcd
> > > 10.0.2.1 4 1221 43026 26728 37714 0 0 4d01h
> > 6605
> > > 172.24.96.1 4 65000 11860 12348 37714 0 0 13:10:26
> > 0
> > > <---?????
> > >
> > > BGP table version is 37689, local router ID is 172.24.23.1
> > > 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
> > > *> 9.3.4.0/24 10.0.2.1 300 0 1221 ?
> > > **Please read:http://www.groupstudy.com/list/posting.html
> > **Please read:http://www.groupstudy.com/list/posting.html
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