Attached Policy Modification; Verification Of Attribute Comparisons And Actions; Policy Statements - Cisco ASR 9000 Series Routing Configuration Manual

Policy Statements

Likewise, you cannot remove a route policy or set that is currently in use at an attach point because this
removal would result in an undefined reference. An attempt to remove a route policy or set that is currently
in use results in an error message to the user.
A condition exists that is referred to as a null policy in which the policy bar exists but has no statements,
actions, or dispositions in it. In other words, the policy bar does exist as follows:
route-policy bar
end-policy
This is a valid policy block. It effectively forces all routes to be dropped because it is a policy block that never
modifies a route, nor does it include the pass statement. Thus, the default action of drop for the policy block
is followed.

Attached Policy Modification

Policies that are in use do, on occasion, need to be modified. Traditionally, configuration changes are done
by completely removing the relevant configuration and then re-entering it. However, this allows for a window
of time in which no policy is attached and the default action takes place. RPL provides a mechanism for an
atomic change so that if a policy is redeclared, or edited using a text editor, the new configuration is applied
immediately—which allows for policies that are in use to be changed without having a window of time in
which no policy is applied at the given attach point.

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).
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Implementing Routing Policy