From: alsontra@hotmail.com
Date: Sat Jan 31 2004 - 16:07:45 GMT-3
I think your correct Doug. For example.
RouterA--------------RouterB
If router A and B are both running bgp and both using interface addresses in
their respective neighbor statements, the router ID could possibly only
affect the way that the neighbor is displayed in the neighboring bgp table.
I say possibly because if the neighbors have loopback addresses configured
they will use those addresses for router-id's by default, otherwise they
uses the numerically highest address on the box. The update-source command
allows you establish a connection to a neighbors ip address , loopback or
otherwise, that is not a directly connected ip address.
A few things to remember: If you use the update-source command to specify an
interface that is not directly connected for an EBGP peering relationship,
you will have to specify the ebgp-multihop command. If you use the
update-source on an IBGP peer, you do not need to specify multi-hop command
because BGP assume by default that IBGP peers maybe multiple hops away.
Hope this helps. Search the GS archives on the topic.... there's alot of
good stuff out there.
$0.02
Alsontra
----- Original Message -----
From: "Calton, Doug" <Doug.Calton@getronics.com>
To: <ccielab@groupstudy.com>
Sent: Saturday, January 31, 2004 6:43 AM
Subject: RE: BGP Router-ID vs Update-source
> IF I understand correctly, I believe that Mr. Hays is correct. I think
> that bgp router-id is used to identify the router source in the bgp
> tables. The bgp update-source specifies what interface (IP addr) will
> be used to establish the tcp session between the peers. Without
> specifying that, bgp defaults to use the ip address of the interface
> providing a route to the remote peer ID.
>
> Does that make sense? It seems to be the way it works for me, at least.
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
> Jonathan Hays
> Sent: Saturday, January 31, 2004 8:54 AM
> To: ccielab@groupstudy.com
> Subject: RE: BGP Router-ID vs Update-source
>
>
> you wrote:
> >-----Original Message-----
> >From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On
> >Behalf Of Stuart Reabow
> >Sent: Saturday, January 31, 2004 5:30 AM
> >To: 'CCIE GroupStudy'
> >Subject: BGP Router-ID vs Update-source
> >
> >
> >Hi all,
> >
> >It appears that if you configure a bgp router-id as the
> >loopback0 add, then
> >there's no need to use the update-source lo0 command. The
> >router-id will be
> >used to source all comms. Does anybody see any problems with this?
> >
> >Stuart
> = = =
>
> Perhaps I am not understanding your statement, but I can't see that
> configuring the BGP router-ID alone will change the source address of
> BGP communications. The router-ID only indicates which router sourced
> the update. But by default the router will use the interface of the
> shared network with its BGP neighbor to source all communications. The
> only way to change this (that I know about) is to configure the "bgp
> neighbor update-source" command.
>
> The iBGP example below has Loopback0 (139.10.6.6) configured as the BGP
> router ID and advertises Loopback 6 (6.6.6.6) to R9, in the same AS. The
> "sh ip bgp" output on R9 indicates that sees the 6.6.6.0 network
> advertised by 139.10.69.6 (the IP address of the egress interface of R6
> connected to R9). The BGP router ID for R6 (139.10.6.6) is shown in
> parentheses in the output, but this is just a label.
>
> R6#sh ip int brief | i Loopback
> Loopback0 139.10.6.6 YES NVRAM up
> up
> Loopback6 6.6.6.6 YES manual up
> up
> R6#sh run | begin bgp
> router bgp 100
> bgp router-id 139.10.6.6
> bgp default local-preference 75
> bgp log-neighbor-changes
> network 6.6.6.0 mask 255.255.255.0
> neighbor 139.10.69.9 remote-as 100
> !
> ---
> R9_FRSw#sh ip bgp 6.6.6.0
> BGP routing table entry for 6.6.6.0/24, version 0
> Paths: (1 available, no best path)
> Not advertised to any peer
> Local
> 139.10.69.6 from 139.10.69.6 (139.10.6.6)
> Origin IGP, metric 0, localpref 75, valid, internal, not
> synchronized R9_FRSw#
>
> I changed the connection to eBGP (I put R6 and R9 in different AS's) but
> got the same results.
>
> Also, I ran debug for several minutes and saw no evidence that the
> source address of any communication was the router-ID. Perhaps I do not
> understand what you are asserting and if so, I apologize.
>
> Please clarify,
>
> Jonathan
>
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