From: ANDY NWEBUBE (wizdata@xxxxxxxxxxx)
Date: Tue May 22 2001 - 12:38:45 GMT-3
Hi,
I have come across this situation before and the reason this happens is
this:
Your primary router (R1) is made the preferred router. it receives updates
from AS1221: prefix 9.3.4.0/24. It applies the local_pref attrib. to it and
because it works out that degree of preference is itself it installs the
route as the preferable route.
Your backup router (R2) also receives updates from AS1221: prefix
9.3.4.0/24. It applies applies the Local_pref attrib. to it and works out
the degree of preference. which works out that the preferred route would be
via R1.
Now the tricky bit. They have both received External updates and the next
bit is to do the internal updates. (mind you, which ever update is done
first or last, the final result would be reflected in Adj-RIB).
R1
Ext UpD receive 9.3.4.0/24 via 10.0.1.1 preferred route
Int Upd Recevie 9.3.4.0/24 via 172.24.96.1
R2
Ext Upd receive 9.3.4.0/24 via 10.0.2.1
Int Upd receive 9.3.4.0/24 vai 172.24.24.1 preferred route
preferred route for R2 is learnt via R1 and therefore it would not be
advertised back to R1 so it is deleted.
Hence the outcome:
R1
Ext UpD receive 9.3.4.0/24 via 10.0.1.1 preferred route
----Delete--Int Upd Recevie 9.3.4.0/24 via 172.24.96.1--Deleted (Assuming
that it was first installed and then R2 starts using it as Pref Route so it
stops advertising it back to R1. Thats when you have your withdrawn updates)
R2
Ext Upd receive 9.3.4.0/24 via 10.0.2.1
Int Upd receive 9.3.4.0/24 vai 172.24.24.1 preferred route
Regards,
Andy
andy@ipbackbone.co.uk
www.ipbackbone.co.uk
>From: Darren Hosking <dhosking@commander.com.au>
>Reply-To: Darren Hosking <dhosking@commander.com.au>
>To: "'mcaplan.cs@clearstream.com'" <mcaplan.cs@clearstream.com>,
>ccielab@groupstudy.com
>Subject: RE: should all IBGP neighbours have all EBGP paths?
>Date: Mon, 21 May 2001 23:25:57 +1000
>
>I cleared the IBGP AS and I did see WITHDRAWN updates coming through on the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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