From: Justler (justler@xxxxxxxxxxxxxx)
Date: Sat Jul 06 2002 - 04:19:47 GMT-3
The command does allow internal BGP sessions to use the specified
interface for TCP connections. So in your setup, you're telling Router
B that it needs to peer up to Router A's 172 address. Router B on the
is told to peer up with IP Address 172.16.1.1 (Router A), but that
update-source command on Router A actually tells Router B to peer up w/
loopback0 instead, which I believe it decides in the BGP session
setup. I think it is mainly used in iBGP, but there could be a use in
eBGP that I am not aware of. Someone PLEASE correct me if i'm wrong!
Cisco's documentation of this is not very good. I even checked the
Internet Routing Architectures book for a detailed explanation with no
luck.
Greg
> Group,
>
> I have a BGP question and the scenario is as follows.What am i doing
> wrong ? Scenario is:
>
> RTA -- RTB -- RTC
>
> They are all in the same AS 1.
>
> RTA's Serial: 10.1.1.1
> RTB's Serial: 10.1.1.2 (connecting to RTA)
> RTB's Serial: 10.1.2.1 (connecting to RTC)
> RTC's Serial: 10.1.2.2
>
> Each Router also has a Loopback Interface for IBGP connection.
>
> RTA's Loopback0: 172.16.1.1
> RTB's Loopback0: 172.16.1.3
> RTC's Loopback0: 172.16.1.2
>
>
> Ok - what I am confused is the BGP command "neighbor x.x.x.x
> update-source <interface x>"?? My understanding of the command is
> that the router who used this command can specify another interface
> to be used for IBGP Neighbor connections.
>
> So at here, RTA will tell RTB (172.16.1.3) to use RTA's Loopback
> interface for IBGP session.
>
> RTA:-
>
> router ospf 1
> log-adjacency-changes
> network 10.1.0.0 0.0.255.255 area 0
> network 172.16.1.1 0.0.0.0 area 1
> !
> router bgp 1
> bgp log-neighbor-changes
> network 1.0.0.0 mask 255.128.0.0
> neighbor 172.16.1.3 remote-as 1
> neighbor 172.16.1.3 update-source Loopback0
>
>
> RTB:-
>
> router ospf 1
> log-adjacency-changes
> network 10.1.0.0 0.0.255.255 area 0
> network 172.16.1.3 0.0.0.0 area 3
> !
> router bgp 1
> no synchronization
> bgp log-neighbor-changes
> neighbor 172.16.1.1 remote-as 1
> neighbor 172.16.1.1 route-reflector-client
> neighbor 172.16.1.2 remote-as 1
> neighbor 172.16.1.2 update-source Loopback0
> neighbor 172.16.1.2 route-reflector-client
>
>
> But at RTB, there is no neighbor update-source statement pointing
> back to RTA. Nevertheless, the BGP session still managed to get
> established. How can BGP still managed to establish the BGP
> connection??
>
>
> RouterB#sh ip bgp summary
> BGP router identifier 172.16.1.3, local AS number 1
> BGP table version is 3, main routing table version 3
> 2 network entries and 2 paths using 266 bytes of memory
> 1 BGP path attribute entries using 60 bytes of memory
> 0 BGP route-map cache entries using 0 bytes of memory
> 0 BGP filter-list cache entries using 0 bytes of memory
> BGP activity 4/4 prefixes, 4/2 paths, scan interval 15 secs
>
> Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down
> State/PfxRcd
> 172.16.1.1 4 1 15 13 3 0 0 00:03:52 1
> 172.16.1.2 4 1 14 14 3 0 0 00:04:17 1
> RouterB#
>
>
> Or is this command only required for EBGP sessions? Anyone with
> ideas.
>
> =====
> Thanks in advance for ur time and replies.
> Hunt
>
>
> http://www.sold.com.au - SOLD.com.au
> - Find yourself a bargain!
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