From: kym blair (kymblair@xxxxxxxxxxx)
Date: Sat Jul 06 2002 - 06:46:30 GMT-3
Hunt,
Interesting that A and B peered up. You should have to include "nei
172.16.1.1 update-source loop 0" on RouterB, or use 10.1.1.2 in the neighbor
statement on RouterA. I hope someone will explain how this was possible
without either of those.
Incidentally, Greg is right, peering to the loopback interfaces is easily
done with iBGP because the routers already know of the peer's loopback
address via IGP. But it is also common to use the loopback address for eBGP
peering. It's a good idea if you have two paths between the remote AS. You
would add two more steps. First, the remote AS would need a route to your
loopback network in their ip routing table (probably via static route or
default route) and your router would need a route to their loopback network
in your routing table in order to make the tcp connection. Second, you
would need "neighbor X.X.X.X ebgp-multihop 2".
An unrelated suggestion: recommend you always add "bgp router-id X.X.X.X"
under the bgp routing process, and likewise "router-id X.X.X.X" under the
ospf routing process since those two must match up. Nothing gets messed up
if you add another loopback in the future.
HTH, Kym
>From: "Justler" <justler@subnetzero.net>
>Reply-To: "Justler" <justler@subnetzero.net>
>To: ccielab@groupstudy.com
>Subject: Re: BGP Command Q
>Date: Sat, 6 Jul 2002 02:19:47 -0500 (CDT)
>
>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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