BGP Traffic Engineering question

Joe Maimon jmaimon at
Tue Nov 10 20:46:07 UTC 2009

Aaron Hopkins wrote:
> On Tue, 10 Nov 2009, Drew Weaver wrote:
>> If you have several transit providers connected to your network and much
>> of your traffic is generally directed by the "BGP tiebreaker" (i.e.
>> lowest
>> IP address) is there a way, without specifying on a per-prefix basis to
>> prefer the "tie breaker winner" slightly less often?
> Assuming Cisco, set "bgp always-compare-med", "bgp deterministic-med", and
> in your route-map in, "set origin igp" and "set metric X". You can then
> vary X as you see fit as an alternate tie-breaker. As long as you never set
> the metric the same on two different paths for the same prefix, it'll never
> fall back to router-id.
> Depending on the transit provider, you can often match bgp communities to
> determine which are customer routes or the region where the announcement
> was
> heard, which you can then use as a tie-breaker when setting the metric.
> Barring that, as-path access-lists matching specific path fragments can do
> the same thing, but seems to take more work to maintain as relationships
> change over time.
> -- Aaron

Tutorial: Effective BGP Load Balancing Using "The Metric System"
Dani Roisman, Peak Web Consulting

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