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| 9.9 |  | Redundancy, Symmetry, and Load Balancing |
| 9.9.4 |  | Symmetry |

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 by way of the same point or close to the same point that it left the AS.

To promote symmetry, 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 more complex to manipulate. Through appropriate planning and use of BGP attributes and route filters, an AS can control which paths the outside world finds most desirable.