When Attributes Are Modified; Default Drop Disposition - Cisco ASR 9000 Series Configuration Manual

Implementing Routing Policy
The result is a route with a MED of 8, unless the community list of the route matches both cs1 and cs2, in
which case the result is a route with a MED of 12.
In the case in which the attribute being modified can contain only one value, it is easy to think of this case as
the last statement wins. However, a few attributes can contain multiple values and the result of multiple actions
on the attribute is cumulative rather than as a replacement. The first of these cases is the use of the additive
keyword on community and extended community evaluation. Consider a policy of the form:
route-policy community-add
set community (10:23)
set community (10:24) additive
set community (10:25) additive
end-policy
This policy sets the community string on the route to contain all three community values: 10:23, 10:24, and
10:25.
The second of these cases is AS path prepending. Consider a policy of the form:
route-policy prepend-example
prepend as-path 2.5 3
prepend as-path 666.5 2
end-policy
This policy prepends 666.5 666.5 2.5 2.5 2.5 to the AS path. This prepending is a result of all actions being
taken and to the AS path being an attribute that contains an array of values rather than a simple scalar value.

When Attributes Are Modified

A policy does not modify route attribute values until all tests have been completed. In other words, comparison
operators always run on the initial data in the route. Intermediate modifications of the route attributes do not
have a cascading effect on the evaluation of the policy. Take the following example:
ifmed eq 12 then
set med 42
if med eq 42 then
drop
endif
endif
This policy never executes the drop statement because the second test (med eq 42) sees the original, unmodified
value of the MED in the route. Because the MED has to be 12 to get to the second test, the second test always
returns false.

Default Drop Disposition

All route policies have a default action to drop the route under evaluation unless the route has been modified
by a policy action or explicitly passed. Applied (nested) policies implement this disposition as though the
applied policy were pasted into the point where it is applied.
OL-30423-03
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
Semantics of Policy Application
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