Bgp Policy Attach Points; Additional-Path; Dampening - Cisco ASR 9000 Series Routing Configuration Manual

Attach Points
Each protocol has a distinct definition of the set of attributes (commands) that compose a route. For example,
BGP routes may have a community attribute, which is undefined in OSPF. Routes in IS-IS have a level
attribute, which is unknown to BGP. Routes carried internally in the RIB may have a tag attribute.
When a policy is attached to a protocol, the protocol checks the policy to ensure the policy operates using
route attributes known to the protocol. If the protocol uses unknown attributes, then the protocol rejects the
attachment. For example, OSPF rejects attachment of a policy that tests the values of BGP communities.
The situation is made more complex by the fact that each protocol has access to at least two distinct route
types. In addition to native protocol routes, for example BGP or IS-IS, some protocol policy attach points
operate on RIB routes, which is the common central representation. Using BGP as an example, the protocol
provides an attach point to apply policy to routes redistributed from the RIB to BGP. An attach point dealing
with two different kinds of routes permits a mix of operations: RIB attribute operations for matching and BGP
attribute operations for setting.
The protocol configuration rejects attempts to attach policies that perform unsupported operations.
Note
The following sections describe the protocol attach points, including information on the attributes (commands)
and operations that are valid for each attach point.
See Cisco ASR 9000 Series Aggregation Services Router Routing Command Reference for more information
on the attributes and operations.
New para for test

BGP Policy Attach Points

This section describes each of the BGP policy attach points and provides a summary of the BGP attributes
and operators.

Additional-Path

The additional-path attach point provides increased control based on various attribute match operations. This
attach point is used to decide whether a route-policy should be used to select additional-paths for a BGP
speaker to be able to send multiple paths for the prefix.
The add path enables BGP prefix independent convergence (PIC) at the edge routers.
This example shows how to set a route-policy "add-path-policy" to be used for enabling selection of additional
paths:
router bgp 100
address-family ipv4 unicast
additional-paths selection route-policy add-path-policy

Dampening

The dampening attach point controls the default route-dampening behavior within BGP. Unless overridden
by a more specific policy on the associate peer, all routes in BGP apply the associated policy to set their
dampening attributes.
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Implementing Routing Policy