Cisco ASR 9000 Series Routing Configuration Guide

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Alternative routes must exist in the network, otherwise advertising a lower preference has no effect. For
example, there is no advantage in configuring Graceful Maintenance for a singly-homed customer router
which does not have alternate routes.
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If time consuming policies exist, either at the output of the sending router or at the input of the receiving
router, the Graceful Maintenance operation can take a long time.
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Configuring an eBGP ASBR neighbor results in advertising an implicit null label for directly connected
routes via BGP. If a user shuts down an eBGP neighbor, the label is not reprogrammed as the system
withdraws rewrites on any neighbor state changes. Implicit null label feature support helps avoid churn
in terms of adding or removing rewrites for neighbor flaps.
Graceful Maintenance Operation
When Graceful Maintenance is activated, the affected routes are advertised again with a reduced preference.
This causes neighboring routers to choose alternative routes. You can use any of the following methods to a
signal reduced route preference:
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Add GSHUT community: Use this method to allow remote routers the freedom to set a preference.
Receiving routers must match this community in a policy and set their own preference.
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Reduce LOCAL_PREF value: This works for internal BGP neighbors. Use this method if remote
routers do not match the GSHUT community.
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Prepend AS Path: This works for both internal and external BGP neighbors. Use this method if remote
routers do not match the GSHUT community.
When Graceful Maintenance is activated on a BGP connection, the following two operations happen:
1
All routes received from the connection are re-advertised to other neighbors with a lower preference. Note,
this happens to only those routes that have actually been advertised to other neighbors. It is possible that
a received route was not selected as the best path and therefore not advertised. In that case, it will not be
re-advertised.
2
All routes that were advertised to the connection is re-advertised with a lower preference.
In order for the first operation to happen, all routes received from the connection are tagged with an internal
attribute called graceful-shut. This attribute is stored internal to only the router; it is not advertised by BGP.
This attribute can be seen when the route is displayed with the show bgp command. It is different from the
GSHUT community. The GSHUT community is advertised by BGP and can be seen in the community list
when the route is displayed with the show bgp command.
All routes that have the graceful-shut attribute are given the lowest preference during route-selection. Any
new route updates that are sent or received on a BGP session under Graceful Maintenance are also treated as
described above.
Inter Autonomous System
Advertising a lower preference to another AS in the public Internet may cause unnecessary routing
advertisements in distant networks, which may not be desirable. An additional configuration under the neighbor
address family, send-community-gshut-ebgp, is necessary for the router to originate the GSHUT community
to the eBGP neighbor.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
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Implementing BGP
BGP Graceful Maintenance

Cisco ASR 9000 Series Specifications

General

Brand	Cisco
Model	ASR 9000 Series
Category	Network Router
Language	English