- Identify the
prefixes you would like the policies to apply to
- Implementing the
policies (which can be filtering prefixes out altogether or
manipulating the attributes of a prefix to influence the routing
decision).
Identifying Routes
Prefixes can be selected by their destination
network number, the AS from which the prefix originated, a list of
ASs the prefix has passed through, a specific attribute value, and
so on. Prefixes are identified using match
commands within the various clauses of a route map; after a given
prefix has matched,
the actions specified within that clause of the route map will be
executed, and processing will be considered complete. In other
words, once a prefix matches,
it won't be passed through any remaining clauses in the route map
for consideration.
Permitting or Denying the Routes
Once a prefix has been identified it can be
permitted or denied. The criteria for permitting or denying routes
depend on the policies an AS is setting; if the route is denied, it
is discarded.
Manipulating Attributes
The attributes of a prefix can also be changed
to affect the BGP best path decision process. The figure illustrates
how multiple route map clauses can be applied on a set of routes to
find a match. Note that each clause can have a match on one or more
criteria: for example, a prefix could be checked based on its network number
(address) as well as its AS_path information..
Also note that after a route matches, it is
not compared to any more clauses in the route map-so the order in
which the matches are configured is important. A route map clause
that permits all routes, for example, if put at the beginning of the
list, will override all the other policies configured.