| Input and output policies generally
are defined using route maps. Route maps are used with BGP to
control and modify routing information and to define the conditions routes are redistributed between routing domains.
Recall from Chapter 7, Route
Optimization, that the route-map
command is entered using the following syntax:
Router(config)#route-map map-tag
[permit | deny] [sequence-number]
Note that map-tag is a name
that identifies the route map; the sequence-number indicates the
position that an instance of the route map is to have in relation to
other instances of the same route map. Instances are ordered
sequentially, starting with the number 10 by default.
For example, the route-map
command might be used to define a route map named MYMAP:
route-map MYMAP permit 10
! First set of conditions goes here.
route-map MYMAP permit 20
! Second set of conditions goes here.
When BGP applies MYMAP to routing
updates, it applies the lowest instance first (in this case,
instance 10). If the first set of conditions is not met, the second
instance is applied, and so on, until either a set of conditions has
been met or there are no more sets of conditions to apply.
The condition portion
of a route map is set by using the match
and set
commands. The match command
specifies what criteria must be matched, and the set
command specifies an action
that is to be taken if the routing update meets the conditions
defined by the match command.
Figure
shows the commands needed to create a simple route map. Access list
1 is used here as a way to specify routes.
You may recall that there
are two types of access lists, standard and extended; the main
difference is that a standard access list is applied to the source
IP address, whereas an extended access list is normally applied to
the source and destination of a packet. However, when used to filter
routes within BGP, the first address/wildcard bit set given in an
extended access list applies to the prefix, and the second
address/wildcard bit set applies to the subnet mask of the
advertised route.
In Figure ,
access list 1 identifies all routes of the form 1.1.1.x. A routing
update of the form 1.1.1.x will match the access list and will be
propagated (because of the permit
keyword in the access list) with a metric set to 5.
When an update does not meet the criteria of a route map instance,
BGP applies the next instance, and so on, until an action is taken
or there are no more route map instances to apply. If the update
does not match in any instance, the update is not redistributed or
controlled.
The route map can be applied on the
incoming (using the keyword in)
or the outgoing (using the keyword out)
BGP updates. Figure
shows the commands needed to apply the route map MYMAP on the
outgoing updates toward BGP neighbor 172.16.20.2.
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