Troubleshooting Outbound IP Traffic Flow Issues Because of BGP Policies

BGP's real power is in managing IP traffic flows coming in and going out of the AS. BGP in general and Cisco IOS Software in particular offer a great deal of flexibility in manipulating BGP attributes LOCAL_PREFERENCE, MED, and so forth to control BGP best-path calculation. This best-path decision determines how traffic exits the AS. With the large size of BGP networks today, it is crucial that BGP operators understand how BGP attributes should be managed.

This section discusses what problems can arise while trying to manage traffic leaving the AS. Here is the list of the most common problems encountered in managing outbound traffic flow:

• Multiple exit points exist, but traffic goes out through one or a few exit routers.

• Traffic takes a different interface from what is shown in the routing table.

• A multiple BGP connection exists to the same BGP neighbor, but traffic goes out through only one connection.

• Asymmetrical routing occurs and it causes a problem especially when NAT and time-sensitive applications are used.

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