[nsp] eBGP routes not balancing

David Bergum 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?
  >> :P
  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 through, 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> outbound.)

  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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