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Symmetry is achieved when traffic leaving the
AS from one exit point comes back through the same point. Symmetry always
exists if an AS maintains a single connection to outside networks. However, the
need for redundancy often results in multihoming an AS. If an AS has many
different links to the outside world, traffic tends to flow
asymmetrically. An asymmetrical traffic flow can result in increased delay
and other routing problems. In general, customers and providers would like
to see their traffic come back via the same, or close to the same, point
that it left the AS.
To promote symmetry, you should choose a
primary path and configure routing policies that force traffic to flow
along this path. A default route with a low administrative distance or a
high Local Preference might serve to control the flow of outbound traffic,
but inbound traffic can be trickier to manipulate. Through appropriate
planning and use of BGP attributes (such as the Community attribute and
route filters), an AS can control which paths the outside world finds most
desirable. Thus, you can control how the outside world reaches networks
within your AS by controlling your routers' advertisements.
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