BGP Scalability and Advanced Features
Route maps are used with BGP to control as well as modify routing table
information and to define when routes are redistributed between ASes.
Route maps can be defined as very complex access lists that allow some con-
ditions to be applied to identified routes. If the conditions find a match, then
an action you identify as the administrator using the set command takes
Route maps are used in communities, which we will discuss later in this chapter.
Unlike standard and extended access lists for filtering incoming and out-
going data on interfaces, the statements in route maps are sequentially num-
bered, allowing statements to be edited, inserted, and deleted. A collection of
route-map statements using an identical route-map name is considered a
single route map. One way that route maps are similar to access lists is that
you must specify the source and destination address as well as the subnet mask.
To configure route maps, you begin in the Global Configuration mode.
You then issue the route-map command followed by the name of the route
map. You must then identify a condition you would like to set for the routing
information. You have two choices: either deny or permit the routing infor-
mation. You can then optionally identify a sequence number. You then press
the Enter key, which will take you into a new command-line interface
mode called Route Map Configuration mode, which is indicated by the
prompt. This is a new mode that you prob-
ably have never seen on a router. Let's look at the command and the syn-
taxes, and then we'll demonstrate how to use this command on a router:
route-map map-tag [permit|deny] [sequence-number]
Now we will create a route map using 10 as the first sequence number
(which is the default first sequence number):
RouterA(config)#route-map routemap1 permit 10
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