BGP Persistence Configuration: Example
This example sets long lived graceful restart (LLGR) stale-time of 16777215 on BGP neighbor 3.3.3.3.
router bgp 100
neighbor 3.3.3.3
remote-as 30813
update-source Loopback0
graceful-restart stalepath-time 150
address-family vpnv4 unicast
long-lived-graceful-restart capable
long-lived-graceful-restart stale-time send 16777215 accept 16777215
!
address-family vpnv6 unicast
long-lived-graceful-restart capable
long-lived-graceful-restart stale-time send 16777215 accept 16777215
BGP Graceful Maintenance
When a BGP link or router is taken down, other routers in the network find alternative paths for the traffic
that was flowing through the failed router or link, if such alternative paths exist. The time required before all
routers involved can reach a consensus about an alternate path is called convergence time. During convergence
time, traffic that is directed to the router or link that is down is dropped. The BGP Graceful Maintenance
feature allows the network to perform convergence before the router or link is taken out of service. The router
or link remains in service while the network reroutes traffic to alternative paths. Any traffic that is yet on its
way to the affected router or link is still delivered as before. After all traffic has been rerouted, the router or
link can safely be taken out of service.
The Graceful Maintenance feature is helpful when alternate paths exist and these alternate paths are not known
to routers at the time that the primary paths are withdrawn. The feature provides these alternate paths before
the primary paths are withdrawn. The feature is most helpful in networks where convergence time is long.
Several factors, such as large routing tables and presence of route reflectors, can result in longer convergence
time.
When a BGP router or link is brought into service, the possibility of traffic loss during convergence also exists,
although it is less than when a router or link is taken out of service. The BGP Graceful Maintenance feature
can also be used in this scenario.
Restrictions for BGP Graceful Maintenance
The following restrictions apply for BGP Graceful Maintenance:
•
If the affected router is configured to send the GSHUT community attribute, then other routers in the
network that receive it must be configured to interpret it. You must match the community with a routing
policy and set a lower preference.
•
The LOCAL_PREF attribute is not sent to another AS. Therefore, the LOCAL_PREF option cannot be
used on an eBGP link.
This restriction does not apply to eBGP links between member-ASs of an AS
confederation.
Note
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
123
Implementing BGP
BGP Graceful Maintenance
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