Two distinct steps are involved in manipulating a route or a set of
routes:
- Identify the network number and subnet mask of the route to
which you would like the policies to be applied. For BGP, this
information is called the network-layer reachability information
(NLRI). Recall from Chapter 8 that the NLRI consists of a prefix
and prefix-length pair. Throughout this section, the NLRI is
referred to simply as the prefix.
- Implement the policies (which can be filtering prefixes out
altogether or manipulating the attributes of a prefix to
influence the routing decision).
The identification process typically relies on a route map.
Prefixes can be selected by their destination network number, the AS
from which the prefix originated, the AS_Path, or another specific
attribute value. Prefixes are identified using a route map's
match
statement. After a route map matches a given prefix, the actions
specified by the route map will be executed, and processing will be
considered complete. In other words, when a prefix matches, it will not
be passed through any remaining clauses in the route map.
What actions can the route map take after it has identified a
match? The simplest actions are either to permit the route to pass
through or to filter it out by denying it. Actions that are more
complex tweak the attributes of a prefix to influence the routing
process in some way.
Note that route map can match a prefix based on several
criteria, such as network number or AS_Path information. Also note
that once a route matches there are no further comparisons, the
order in which the matches are configured in the route map is
important. If a route map clause that permits all routes is put at
the beginning of the list, it overrides all the other policies
configured.
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