9.3
Issues of Redundancy, Symmetry and Load Balancing
9.3.7 Symmetry
Symmetry refers to the fact that traffic leaving the AS from an exit point comes back through the same point. This is easy to achieve if a single exit and entrance point exists. But, given the mandates of redundancy and the presence of multiple connections, traffic tends to be asymmetrical. When it is, customers and providers notice a lack of control over how traffic flows in and out of their ASs. Traffic leaving the AS from the East Coast might end up taking the "scenic route," coming back from the West Coast and traveling inside the AS multiple hops before returning to its origin.

Actually this is not as bad as it sounds, and in some situations asymmetrical traffic is acceptable, depending on the overall physical topology with regard to the speed of the links and the number of hops between locations. In general, customers and providers would like to see their traffic come back close to or at the same point it left the AS to minimize potential delays that could be incurred otherwise.

To accommodate symmetry, a primary link should be chosen, and a best effort should be made to enable most traffic to flow on this link. Redundancy would be accommodated by enabling other links to be backup links to be used if the primary link is problematic.