Verification Of Attribute Comparisons And Actions; Policy Statements; Remark; Disposition - Cisco ASR 9000 Series Configuration Manual

Policy Statements

Verification of Attribute Comparisons and Actions

The policy repository knows which attributes, actions, and comparisons are valid at each attach point. When
a policy is attached, these actions and comparisons are verified against the capabilities of that particular attach
point. Take, for example, the following policy definition:
route-policy bad
set med 100
set level level-1-2
set ospf-metric 200
end-policy
This policy attempts to perform actions to set the BGP attribute med, IS-IS attribute level, and OSPF attribute
cost. The system allows you to define such a policy, but it does not allow you to attach such a policy. If you
had defined the policy bad and then attempted to attach it as an inbound BGP policy using the BGP
configuration statement neighbor 1.2.3.4 address-family ipv4 unicast route-policy bad in the system would
reject this configuration attempt. This rejection results from the verification process checking the policy and
realizing that while BGP could set the MED, it has no way of setting the level or cost as the level and cost
are attributes of IS-IS and OSPF, respectively. Instead of silently omitting the actions that cannot be done,
the system generates an error to the user. Likewise, a valid policy in use at an attach point cannot be modified
in such a way as to introduce an attempt to modify a nonexistent attribute or to compare against a nonexistent
attribute. The verifiers test for nonexistent attributes and reject such a configuration attempt.
Policy Statements
Four types of policy statements exist: remark, disposition (drop and pass), action (set), and if (comparator).

Remark

A remark is text attached to policy configuration but otherwise ignored by the policy language parser. Remarks
are useful for documenting parts of a policy. The syntax for a remark is text that has each line prepended with
a pound sign (#):
# This is a simple one-line remark.
# This
# is a remark
# comprising multiple
# lines.
In general, remarks are used between complete statements or elements of a set. Remarks are not supported in
the middle of statements or within an inline set definition.
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

If a policy modifies a route, by default the policy accepts the route. RPL provides a statement to force the
opposite—the drop statement. If a policy matches a route and executes a drop, the policy does not accept the
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
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Implementing Routing Policy
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