I've got a router with 2 T1s going to different ISPs. Each ISP advertises a default route to this router with the same weight via BGP.
What is the normal behavior in this situation? I was hoping to get the router load sharing the outbound traffic equally between the 2 T1s. What I get is all outbound traffic going out the T1 where the destination IP address is the lowest. How can I equalize the outgoing traffic?
Default routes are processed the same way any other routes are, by comparing attributes like AS path length, weight, local preference, etc. If all the BGP attributes are the same, the final tie-breaker is the age of the route (older routes are preferred).
BGP is very poor at load balancing. Search the newsgroup archive for "bgp load balance" to find past discussions and suggestions.
Try with bgp multipath...
B.R. Igor
I thought , it was the lowest "router-id". Is it the oldest route in age ?
I've added 'maximum-paths 2' to my bgp stanza. I can't seem to tell any difference. What's the point of bgp multipath?
I'm just trying to setup more equal utilization for our 2 T1s. Can BGP not really get close to that? What's our alternatives?
Generally speaking, load balancing using BGP in a dual ISP setup is considered "good" if you can get within a two-to-one ratio between the two links, and that assumes significant numbers of internal users and external destinations. Typically, a single user with a single TCP connection to a single Internet server will see higher performance if all the traffic goes over a single T1, and typical load sharing set ups are configured to ensure that that is the case.
As Barry stated in his reply, Google Groups is your friend, as this topic has been hashed to death multiple times over the past few years, making old timers (the ones who actually know the answers) reluctant to waste their time doing it yet again.
The tie-breaker used to be lowest router-id. Several years ago Cisco changed it to age.
I think maximum-paths will only insert multiple routes learned from the same ASN. So the point of it is when you have multiple T1's to the same ISP.
There are some tricks you can play with route-maps that set weights based on as-path access lists. Like I said before, search the archives.
Thanks Barry
I'm not quiet experienced in BGP, but If you can get two routes to the same destination learned from BGP in your IP routing table, then try to play around with cef load balancing. It requires two or more prefixes to the same destination in it's FIB to do proper load balancing (CEF load balancing is not, AFAIK, enabled by default).
B.R. Igor
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