BGP Confederation config problem...

Sat Mar 7 09:02:26 UTC 1998

Andy,

> We have a 4-memberAS confederation, each with two IBGP peers.  The
> arrangement looks sort of like an octagon.  the "r" is AS3 is an
> internal hop, not a BGP peer.
> 
>    _______     ______
>   |      R-----R---r |
>   |     / |   |    | |
>   |AS2 R  |   |AS3 R |
>    ----|--     ----|-
>        |           |
>    ____|__     ____|_
>   |AS4 R  |   |AS1 R |
>   |     \ |   |   /  |
>   |      R-------R   |
>    -------     ------
> 
> The problem is this:  How do you get BGP to choose the shortest
> "AS PATH", since internal AS paths are ignored in selecting BGP
> routes?  Right now, to top router in AS4 will always choose a
> route through (2 3 1) instead of (1), because it prefers
> "external" routes (even external confederation routes) over
> internal routes.
> 
> So, when given a choice, the router on the distant side of the AS
> will ALWAYS prefer the three-AS-hop path, because it is external. 
> Is there a way around this?!?

Perhaps I'm misunderstanding your question, but I thought BGP
considered AS_path length before considering whether the path was
an external path or an internal path.  According to the Cisco
documentation, BGP uses the following criteria to select a path
for a destination:

 1. If the path specifies a next hop that is inaccessible, drop
    the update.
 2. Prefer the path with the largest weight.
 3. If the weights are the same, prefer the path with the largest
    local preference.
 4. If the local preferences are the same, prefer the path that
    was originated by BGP running on this router.
 5. If no route was originated, prefer the route that has the
    shortest AS_path.
 6. If all paths have the same AS_path length, prefer the path
    with the lowest origin type (where IGP is lower than EGP, and
    EGP is lower than Incomplete).
 7. If the origin codes are the same, prefer the path with the
    lowest MED attribute.
 8. If the paths have the same MED, prefer the external path over
    the internal path.
 9. If the paths are still the same, prefer the path through the
    closest IGP neighbor.
10. Prefer the path with the lowest IP address, as specified by
    the BGP router ID.

So, I'm going to make the following assumptions:

 A. Your BGP configuration is correct, i.e. the routes to AS1 via
    AS2 don't have a larger weight or local preference.
 B. You don't have any connectivity problems, i.e. the BGP table
    in the top router in AS4 contains routes that follow both the
    (2 3 1) path and the (1) path to the networks in AS1.  
 C. AS4 only originates routes to networks in AS4, not the entire
    BGP confederation, i.e. no explicit "network" statements for
    the networks in AS1 and no redistribution of IGP routes to
    AS1's networks into AS4's BGP table.

If this is all true, then perhaps your problem is a "next hop"
problem.  A next hop problem can occur when the next hop for a BGP
entry is unreachable via the IGP.

In your example the direct routes to AS1 in the top router in AS4
would have as their next hop the external interface of the bottom
router in AS4.  So, are you running an IGP on the external
interface of the bottom router in AS4?

If you are not running an IGP on the external interface, then BGP
is not selecting the direct routes to AS1 because they're being
rejected due to rule 1 in the above table.  If this is indeed the
problem, make sure that when you go to fix the problem that you
run the IGP on the external interface in "passive" mode so that
the peer doesn't hear your IGP updates.

-Richard

------------------------------------------------------------
Richard Parker                      Third Point Systems
<richard_parker at thirdpoint.com>     2400 Broadway Ave. #D230
+1 (310) 829-5949                   Santa Monica, CA 90404