When a route lter is changed (created, modied or deleted) by a user, the lter change notication will be sent to all relevant protocols, so
that protocols can take appropriate actions. For example if BGP4 is using a route-map (say MapX) to control the routes advertised to a
particular peer, the change of route-map (MapX) will cause BGP4 to re-evaluate the advertised routes, and make the appropriate
advertisements or withdrawals according to the new route-map policy.
A route lter change action can happen in three ways.
1. A new lter is dened (created).
This lter name may be already referenced by an application. The application needs to be notied of the addition of the new
lter, and will bind to and use the new lter. In general, if a lter name is referenced by an application, but is not actually dened,
the application assumes the default deny action for the lter.
2. An existing lter is undened (removed).
If the deleted lter is already used and referenced by an application, the application will unbind itself from the deleted lter.
3. An existing lter is modied (updated).
If the lter is already used and referenced by an application, the application will be notied.
Protocols are automatically notied when a route lter is created, deleted or modied. In addition, when a protocol is notied of a
lter change, appropriate steps are taken to apply the new or updated lter to existing routes.
Filter update delay and BGP
The
lter-changes-update-delay command applies (remove only) to changes of lters that are already used or referenced by
applications. If the content of a lter is changed, the new lter action takes eect after lter-changes-update-delay for existing routes.
The notication delay also applies to situations where the usage or reference of a lter is changed in BGP.
For example, the following BGP neighbor command sets or changes the route-map lter on a neighbor:
device(config-bgp)# neighbor x.x.x.x route-map map_abc out
In this case, the device applies the route-map "map_abc" to the peer, and updates the neighbor out-bound routes after a delay.
If the delay-time is 0, BGP does not start peer out-bound policy updates immediately.
Use the clear lter-change-update or clear ip bgp neighbor soft-out commands to trigger BGP policy updates.
Similarly, the lter-changes-update-delay command also applies to the neighbor in-bound policy change.
NOTE
The auto-update action for a BGP peer lter is newly introduced in release 08.0.01. In previous releases, a user needs to
manually issue the clear ip bgp neighbor soft out command to cause the device to apply the new route-map retroactively to
existing routes.
The general guideline is to dene a policy rst , then apply it to a BGP peer.
BGP4 policy processing order
The order of application of policies when processing inbound and outbound route advertisements on the device is:
1. lp
prex-list
2. Outbound Ip prex-list ORF, if negotiated
3. Filter-list (using As-path access-list)
4. Distribute list (using IP ACL - IPv4 unicast only)
5. Route-map
Conguring a static BGP4 network
FastIron Ethernet Switch Layer 3 Routing
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