What you are seeing is normal.

First, when your Router1 has two routes to a destination
in its bgp table, it will prefer one to Transit1 over one
to Router2, all else equal.  Similarly Router2 will prefer
the route to Transit2.

Next, Router1 will not pass to Router2 all the routes from
Transit1.  Suppose, for a particular net, Router1 receives
a route from Transit1 and Router2, and supposed Router1
decides that Router2's route is the best.  Then Router1
will not pass *any* route for that net to Router2.  You
should expect this for nets that are originated by Transit2
and most of their direct customers.
--
Bob Tinkelman <bob@...>
ISPnet, Inc.  718.464.4747



> Hi,

> We recently added a second transit link to our network and
> set up BGP on the link.

> The current situation is this :


>                     PC
>                     |
> Switch1 -------- Switch2
>    |                |
> Router1 -------- Router2
>    |                |
> Transit1         Transit2


> Router1 has a BGP link to Transit1, Router2 to Transit2, both
> get the full routing table.
> Router1 and Router2 have an iBGP link.

> The 'loop' at Router1-Router2-Switch1-Switch2 uses STP, so no
> problems there. The default gateway for PC is Switch2, the
> default for Switch2 is Router2.

> So far, so good.

> When we connected Transit2 to Router2, we noticed that the
> routing table in both Router1 and Router2 changed quite a bit,
> which was to be expected. Most routes went via Transit2, which
> was normal, because the AS path length was shorter.

> However, when the path length is the same and we perform a
> traceroute on both Router1 and Router2 to the same host, they
> both go out via a different link... their own. I don't think
> that's normal, right ?


> Second problem : when I get a summary of the BGP, this is what
> I get :

> Router 1 :
> Core1# bgp show summary
> Neighbor        V    AS MsgRcvd MsgSent      Up/Down Prefixes
> Rcvd/Sent
> --------        -    -- ------- -------      ------- ---------
> ---------
> [Group Id: transit1]
> 212.23.45.137   4  8928   81494    9792   1d3h11m26s
> 120248/1
> [Group Id: ibgp]
> 62.213.192.18   4 28707  364891  169743   1d23h32m2s
> 115210/20896
> BGP summary, 2 groups, 2 peers
> Core1#


> Router 2:
> core2# bgp show summary
> Neighbor        V    AS MsgRcvd MsgSent      Up/Down Prefixes
> Rcvd/Sent
> --------        -    -- ------- -------      ------- ---------
> ---------
> [Group Id: transit2]
> 64.212.16.113   4  3549  235153    3328    1d3h42m9s
> 120464/1
> [Group Id: ibgp]
> 62.213.192.17   4 28707  128012  329835   1d23h34m7s
> 20898/115223
> BGP summary, 2 groups, 2 peers
> core2#


> If I read this correctly, Router1 only sent 20896 prefixes to
> Router2, although it received 120248 from Transit1. On the
> other hand, Router2 sent 115223 to Router1 and it received
> 120464 from Transit2... where does that difference come from ?
> Why doesn't Router1 send all of its routes to Router2 ? Is
> that the reason why most routes go via Transit2 ?


> Any suggestions would be very nice.


> Kind greetings,

> Wim



> ---

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