Cisco Systems IOS XR manual Disposition, RC-220

Implementing Routing Policy on Cisco IOS XR Software

Information About Implementing Routing Policy

Unlike traditional !-comments in the CLI, RPL remarks persist through reboots and when configurations are saved to disk or a TFTP server and then loaded back onto the router.

Disposition

By default, a route is dropped at the end of policy processing unless either the policy modifies a route attribute or it passes the route by means of an explicit pass statement. For example, the following policy drops all routes because it neither modifies the attribute of any route nor explicitly passes it.

route-policy EMPTY

end-policy

Whereas the following policies pass all routes that they evaluate.

route-policy PASS-ALL pass

end-policy

route-policy SET-LPREF

set local-preference 200 end-policy

In addition to being implicitly dropped, a route may be dropped by an explicit drop statement. Drop statements cause a route to be dropped immediately so that no further policy processing is done. Note also that a drop statement overrides any previously processed pass statements or attribute modifications. For example, the following policy drops all routes. The first pass statement is executed, but is then immediately overridden by the drop statement. The second pass statement never gets executed.

route-policy DROP-EXAMPLE pass

drop

pass

end-policy

When one policy applies another, it is as if the applied policy were copied into the right place in the applying policy, and then the same drop-and-pass semantics are put into effect. For example, policies ONE and TWO are equivalent to policy ONE-PRIME:

route-policy ONE

apply route-policy two

if as-path neighbor-is '123' then pass

endif

end-policy

route-policy TWO

if destination in (10.0.0.0/16 le 32) then drop

endif

end-policy

route-policy ONE-PRIME

if destination in (10.0.0.0/16 le 32) then drop

endif

if as-path neighbor-is '123' then pass

endif

end-policy

Cisco IOS XR Routing Configuration Guide

RC-220