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 checked 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.