Multiple Modifications Of The Same Attribute - Cisco ASR 9000 Series Routing Configuration Manual

Implementing Routing Policy
then the expression, fully parenthesized, would look like the following:
((not med eq 10) and destination in (10.1.3.0/24)) or community matches-any ([10..25]:35)

Multiple Modifications of the Same Attribute

When a policy replaces the value of an attribute multiple times, the last assignment wins because all actions
are executed. Because the MED attribute in BGP is one unique value, the last value to which it gets set to
wins. Therefore, the following policy results in a route with a MED value of 12:
set med 9
set med 10
set med 11
set med 12
This example is trivial, but the feature is not. It is possible to write a policy that effectively changes the value
for an attribute. For example:
set med 8
if community matches-any cs1 then
set local-preference 122
if community matches-any cs2 then
set med 12
endif
endif
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.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
Semantics of Policy Application
559