Cisco Systems IOS XR manual Default Drop Disposition, Control Flow, RC-217

Implementing Routing Policy on Cisco IOS XR Software

Information About Implementing Routing Policy

if med 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.

Consider a policy to allow all routes in the 10 network and set their local preference to 200 while dropping all other routes. You might write the policy as follows:

route-policy two

if destination in (10.0.0.0/8 ge 8 le 32) then set local-preference 200

endif

end-policy

route-policy one apply two

end-policy

It may appear that policy one drops all routes because it neither contains an explicit pass statement nor modifies a route attribute. However, the applied policy does set an attribute for some routes and this disposition is passed along to policy one. The result is that policy one passes routes with destinations in network 10, and drops all others.

Control Flow

Policy statements are processed sequentially in the order in which they appear in the configuration. Policies that hierarchically reference other policy blocks are processed as if the referenced policy blocks had been directly substituted inline. For example, if the following policies are defined:

route-policy one set weight 100

end-policy

route-policy two set med 200

end-policy

route-policy three apply two

set community (2:666) additive end-policy

route-policy four apply one apply three pass

end-policy

Cisco IOS XR Routing Configuration Guide

RC-217