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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.
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