[c-nsp] BGP Route selection

[Pete Templin]

Gert Doering wrote:

> On Fri, May 23, 2008 at 03:08:54PM +0100, Gary Roberton wrote:
>> Router A BGP table entry is shown here;
>>
>> *  90.0.0.0         10.40.1.6               50             0 64604 1000 i
>>
>> *>                  10.40.1.2                              0 64603 1000 i
> 
> Ah.  Different next-hop ASes.
> 
> You need to configure "bgp always-compare-med" or "bgp deterministic-med"
> to take the MED into account if the neighbour AS is not the same.
> 
> Otherwise MED is ignored (because by original design, it's "multi exit
> discriminator", to be used for multiple paths to the *same* neighbour AS).

"bgp deterministic-med" will NOT help in this situation - the neighbor 
AS is different.  Deterministic MED only helps if the default comparison 
sequence (newest and next-newest compared first, best-path-so-far and 
subsequently-older paths on each sequential comparison) won't discover 
the otherwise-best exit point.

See 
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094925.shtml 
for the Cisco scoop.

pt