An entire compound condition may be enclosed in parentheses:
(med eq 42 and next-hop in (10.0.2.2))
The parentheses may serve to make the grouping of subconditions more readable, or they may force the
evaluation of a subcondition as a unit.
In the following example, the highest-precedence not operator applies only to the destination test, the and
operator combines the result of the not expression with the community test, and the or operator combines
that result with the MED test.
med eq 10 or not destination in (10.1.3.0/24) and community matches-any ([12..34]:[56..78])
With a set of parentheses to express the precedence, the result is the following:
med eq 10 or ((not destination in (10.1.3.0/24)) and community matches-any
([12..34]:[56..78])
The following is another example of a complex expression:
(origin is igp or origin is incomplete or not med eq 42) and next-hop in (10.0.2.2)
The left conjunction is a compound condition enclosed in parentheses. The first simple condition of the inner
compound condition tests the value of the origin attribute; if it is Interior Gateway Protocol (IGP), then the
inner compound condition is true. Otherwise, the evaluation moves on to test the value of the origin attribute
again, and if it is incomplete, then the inner compound condition is true. Otherwise, the evaluation moves to
check the next component condition, which is a negation of a simple condition.
apply
As discussed in the sections on policy definitions and parameterization of policies, the apply command
executes another policy (either parameterized or unparameterized) from within another policy, which allows
for the reuse of common blocks of policy. When combined with the ability to parameterize common blocks
of policy, the apply command becomes a powerful tool for reducing repetitive configuration.
Attach Points
Policies do not become useful until they are applied to routes, and for policies to be applied to routes they
need to be made known to routing protocols. In BGP, for example, there are several situations where policies
can be used, the most common of these is defining import and export policy. The policy attach point is the
point in which an association is formed between a specific protocol entity, in this case a BGP neighbor, and
a specific named policy. It is important to note that a verification step happens at this point. Each time a policy
is attached, the given policy and any policies it may apply are checked to ensure that the policy can be validly
used at that attach point. For example, if a user defines a policy that sets the IS-IS level attribute and then
attempts to attach this policy as an inbound BGP policy, the attempt would be rejected because BGP routes
do not carry IS-IS attributes. Likewise, when policies are modified that are in use, the attempt to modify the
policy is verified against all current uses of the policy to ensure that the modification is compatible with the
current uses.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
567
Implementing Routing Policy
Attach Points
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