There are several reasons why there might be assymetric routing. Probably the easiest to understand is a situation where you are running BGP to two different providers. You may have a policy which directs traffic to a particular destination through ISP-A. But the destination policy routes traffic to you through ISP-B.
There are several other situations which will result in assymetric routing. You might have a situation where two routers are connected by two serial links (perhaps Frame Relay). If rtrA is using default bandwidth and treats both serials as having T1 bandwidth it will see two equal paths and may send some particular traffic over serial 1. But if the other router has a configured bandwidth of 64K on one of its serials (to reflect CIR perhaps) and has T1 on the other, then rtrB will send all traffic over serial 2 and will result in assymetric routing.
Another situation where assymetric routing may result is where there are two different routing protocols being used - perhaps RIP around the source and EIGRP around the destination. RIP may choose one path because the hop count is smaller. EIGRP may choose the other path because the bandwidth is more favorable. Assymetric routing is the result.
In many cases there is assymetric routing and things work (to the extent that lots of the time no one is aware that assymetric routing is happening). In some cases it does cause problems, particularly in situations where some device is doing stateful inspection and needs to see responses come back the same way that the original traffic went out.
HTH
Rick
HTH
Rick
