mohammedmahmoud Fri, 06/15/2007 - 12:15


By default all routing protocols (except BGP) load balances over equal cost links with a default maximum number of links of 4 which can be increased up to 6 (and 16 in the new codes), EIGRP supports also unequal cost path load-balancing (using the variance command), while BGP doesn't use the concept of load balancing as its logic is totally different than the other routing protocols.

I hope that i've briefly answered your question, please do elaborate on any point you require.


Mohammed Mahmoud.

Richard Burts Fri, 06/15/2007 - 12:17


There are 2 aspects to consider in understanding load balancing: how routes are put into the routing table (balancing only happens if there are at least 2 routes in the table for the same destination) and how packets are forwarded toward the destination.

Placing routes into the routing table is done by the routing protocol. Routing protocols include OSPF, EIGRP, RIP, BGP, and static routing. Each routing protocol has its own method of deciding which route is the best route and should be put into the routing table. With the exception of BGP all the other routing protocols when they find more than 1 route with the same best metric will put multiple routes for the same destination into the routing table.

Once we have multiple routes for a destination in the routing table then the second part of balancing takes place. When the router is forwarding packets toward a destination and when there is more than 1 route for that destination in the routing table than balancing happens. In balancing the router can either do per destination load balancing or per packet load balancing. It helps to understand that this part of balancing is done the router packet forwarding logic and is not done by the routing protocols.

So the routing protocols do the first part of balancing when they put more than 1 route to the destination into the routing table. And the router forwarding logic does the other part of balancing.



sasif Mon, 06/18/2007 - 21:25

I am not sure why it is said that BGp doesnot do load balancing. Under the BGP router config if you configure the

maxumum paths 2 = eBGP

maxumum path ibgp 2 = iBGP

maximun paths eibgp 2 =eiBGP

BGP does load balancing for all three cases. Am I missing something here ??

s.arunkumar Mon, 06/18/2007 - 21:56

my understanding....

eBGP by default will not load balance..we need to give "maximum-path <2-6>" to achive it,with a condition that all the as-path be exactly,the neighbors through which it load balances should be in same AS.

In ibgp no load balancing is possible,but load sharing is possible with a condition that its possible only through different neighbors(not with one neighbor with multiple link connection).

Experts,Pls clarify if i am right or something else..????

.... arun ... :)

aravindhs Tue, 06/19/2007 - 04:23

For load-balancing BGP with more than one link to the neighbour router, we can use Loopback interfaces as the peering points and route through both the physical links to reach the peer.

EBGP does load balancing using maximum-paths only to one remote-as and not when the ASes are different. But they can be on different remote routers in the same remote-as.

If it is different remote-ASes we can only do load-sharing based on IP prefixes, like .. IPs from 0.x.x.x to 127.x.x.x go via one path and the IPs from 128.x.x.x go through the other.

I came to know about the above bgp loadbalancing techniques from a cisco document. more posts would be nice.




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