ing false AS numbers. This is called AS path pre-pending. It is another way
to influence route selection.
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
flap 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
dampening
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.
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.
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: