Asymmetrical routing occurs when a packet takes a certain path from source host A across the network to destination host B but then a return packet takes a separate path from the source host B to destination host A. For normal data, this is not an issue, but for some special cases, like data traveling through stateful-inspection firewalls, this routing behavior can cause some issues.
As far as VoIP is concerned, the path through the network isn't nearly as important as how the traffic is treated and the total delay of the path. However, occasionally you may run into an instance where one of the paths is congested or treats packets differently. If you get complaints where one party to a VoIP conversation receives poor sound quality while the other party receives acceptable quality, asymmetrical routing, or something like it, could be the culprit. One thing in particular to be wary of is when you're using RSVP.
Be careful when configuring routing protocols or such things as policy-based routing in your VoIP network that you pay attention to the return path and not just the outbound path. Failure to do so can result in a challenging troubleshooting experience where you search and search for something wrong in one path only to find that the problem is in a completely separate area of your network.
For more information on troubleshooting VoIP call quality, read Tom Lancaster's tip on VoIP echo problems.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.