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 http://tinyurl.com/yzlmmo8