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hen configuring BGP, the AS path length is considered in selecting
a route. With the use of route maps, the AS path may be lengthened by add-
ing false AS numbers. This is called AS path pre-pending. It is another way
to influence route selection.
In addition to manipulating route selection, BGP has features that allow
network advertisements to be aggregated before they are advertised to neigh-
boring autonomous systems. There are many reasons to influence the routes
that are advertised. You will mainly want to control the route-selection pro-
cess to stop unnecessary advertisements in order to eliminate router confu-
sion and the high CPU utilization that can occur when routes flap. A route
is defined as a change in the state of the route. Once a route is established
and then removed from the BGP table, one flap has occurred. You can pre-
vent routing problems by using the bgp dampening command. The bgp
command maintains a threshold for route flaps. This means that
when the threshold is exceeded, the route is put into a hold-down. Hold-
downs implement a timing mechanism, and during the hold-down time, BGP
uses internal processes to monitor the route's status to see if the route comes
back up. If the route stops flapping for a given period of time, the route is
allowed back into the BGP table and can be advertised.
One of the most important items to define is the type of AS you are admin-
istering. When multiple autonomous systems interconnect, one or all of the
ASes can become a transit AS, which we discussed in Chapter 7, "BGP's
Basic Components." Depending on your network policy, this can be a good
thing or a bad thing.
One of the biggest problems occurs when you connect to another ISP and
the ISP uses your circuits, equipment, and bandwidth to connect to a neigh-
boring AS instead of using their own resources. You can eliminate this situ-
ation by using AS path filters. Using regular expressions, you can compare
AS path information and then either permit or deny it. Let's look at a sample
configuration detailing how to implement AS filters:
router bgp 200
no synchronization
bgp dampening
neighbor remote-as 100
neighbor filter-list 10 in
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