[nsp] eBGP routes not balancing
bergum at cisco.com
Mon Nov 17 18:46:17 EST 2003
>>>>> On Mon, 17 Nov 2003 14:59:02 -0800, Steve Francis <steve at expertcity.com> said:
Steve> Jeff Chan wrote:
>> After enabling "bgp bestpath compare-routerid" and resetting the
>> sessions, most of the traffic went to the peer with the lower
>> numerical address (ID), which again is unbalanced. So I've
>> disabled compare-routerid and will probably reset all the
>> sessions late at night with "clear ip bgp all" in the hopes
>> that the ages of the routes will then be close enough, since
>> they're all cleared and re-learned at the same time, to
>> result in traffic that is closer to balanced.
>> Both providers are large, our pipes to them and routing tables
>> learned similar sized, so balanced traffic should be a
>> reasonable goal. Any other suggestions are welcomed. Aside
>> from external route servers, how is anyone else dealing with
>> this? Balancing fully multi-homed traffic must be an issue
>> for lots of folks.... Why can't IOS just do the right thing?
Steve> If both providers are presenting you with the same number of AS hops to
Steve> the destination networks, then oldest path will win. (subject to usual
Steve> caveats about other selection mechanims: origin type, etc)
Steve> One way I've made the load balancing more equal in such situations is to
Steve> apply a route map to one of the peers, that says if the route learned is
Steve> for an IP in the range 0.0.0.0 through 22.214.171.124, it has a
Steve> local_pref of 90; everything else from that peer has a local_pref of 110.
Steve> The local_pref of the other peer will have the default of 100.
Steve> Thus everything for the top half of the Internet will prefer the peer
Steve> with the route map; everything else will prefer the peer w/o it (for
Steve> You can tweak the IP where the route map changes its local_pref to get
Steve> the traffic as balanced as you like.
I did a similar thing back when, but I used the origin attribute, since
tweaking that will still allow the shortest AS path to win, where tweaking
local pref could send traffic to a longer AS path.
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