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| 5.9 |  | Troubleshooting BGP |
| 5.9.6 |  | Troubleshooting outbound and inbound BGP policy issues |

The real power of BGP 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 BP 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.

Some 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.

Just as in managing outbound IP traffic from an AS, Cisco IOS Software offers BGP operators configuration options to manage inbound traffic in an AS. It is important that inbound traffic from other autonomous systems be managed as well. If this does not happen, capacity of the network will not be fully utilized. This causes congestion in one part of the network while other parts are underutilized. The end result of this mismanagement of inbound traffic flow is sluggish throughput, slow round-trip times, and delays in IP traffic. Therefore, it is essential that all inbound BGP policies are check and configured correctly.

Some of the most common problems in managing inbound IP traffic in an AS using BGP are:

- Multiple connections exist to an AS, but all the traffic comes in through one BGP neighbor, in the same AS.
- A BGP neighbor in an AS should just be a backup provider, but some traffic from the Internet still comes through that AS.
- Asymmetrical routing occurs.
- Traffic to a certain subnet should come through a particular connection, but it is coming from somewhere else.

Each of these problems, both with outbound and inbound policy issues, could require extensive troubleshooting. In the next section, the problem of traffic taking a different interface from what is shown in the routing table will be examined more thoroughly to show an example of troubleshooting policy issues.