Attach Points - Cisco NCS 6000 Series Configuration Manual

Implementing Routing Policy

Attach Points

Policies do not become useful until they are applied to routes, and for policies to be applied to routes they
need to be made known to routing protocols. In BGP, for example, there are several situations where policies
can be used, the most common of these is defining import and export policy. The policy attach point is the
point in which an association is formed between a specific protocol entity, in this case a BGP neighbor, and
a specific named policy. It is important to note that a verification step happens at this point. Each time a policy
is attached, the given policy and any policies it may apply are checked to ensure that the policy can be validly
used at that attach point. For example, if a user defines a policy that sets the IS-IS level attribute and then
attempts to attach this policy as an inbound BGP policy, the attempt would be rejected because BGP routes
do not carry IS-IS attributes. Likewise, when policies are modified that are in use, the attempt to modify the
policy is verified against all current uses of the policy to ensure that the modification is compatible with the
current uses.
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.
Note The protocol configuration rejects attempts to attach policies that perform unsupported operations.
The following sections describe the protocol attach points, including information on the attributes (commands)
and operations that are valid for each attach point.
See Routing Command Reference for Cisco NCS 6000 Series Routers 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.
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
Attach Points
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