![]() ![]() |
9.5 | ![]() |
The BGP Routing Process | |
9.5.3 | ![]() |
Implementing BGP routing policy |
Input and output policies generally are defined using route maps.
Route maps are used with BGP to control and modify routing
information. Route maps are used to define how the routes are
redistributed between routing domains.
Recall from Module 8, Route Optimization, that the route-map command is entered using the following syntax:
Notice 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 ten by default. For example, the route-map command might be used as follows, to define a route map named MYMAP:
When BGP applies MYMAP to routing updates, it applies the lowest instance first. In this case, instance ten (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. If the update does not match in any instance, the update is not redistributed or controlled. 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. The set command specifies an action that is to be taken if the routing update meets the conditions defined by the match command.Figure 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 or wildcard bit set given in an extended access list applies to the prefix. The second address or wildcard bit set applies to the subnet mask of the advertised route. Access list 1 identifies all routes of the form 1.1.1.x.
A route map can be applied on the incoming, using the keyword in, or the outgoing, using the keyword out, BGP updates. Figure![]()
|