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prefix is suppressed for a calculated period (a penalty) which is further incremented with every
subsequent flap. The penalty is then decremented by a half-life value until the penalty is below a
reuse threshold. So, if stable for a certain period, the hold-down is released from the prefix and the
route is reused and re-advertised. You can reset dampening defaults with the
[half-life | reuse | suppress | suppress-max][route-map route-map-#]
Dampening should only be used for EBGP peering at network boundaries, so that flapping can be
suppressed as close to the unstable source as possible.
Recommendations for Route Flap Dampening
We recommend you configure the following dampening values with harsher treatment for /24
and longer prefixes:
•
Do not start dampening before the fourth straight flap: set the suppress value to 3000
•
Figure prefixes of /24 and longer as follows: maximum = minimum outage of 60 minutes
•
Calculate prefixes /22 and /23 as follows: maximum outage of 45 minutes but potential for
less because of half-life value - minimum of 30 minutes outage
•
Set all other prefixes as follows: maximum outage - 30 minutes, minimum outage - 10 minutes
If a specific dampening implementation does not allow configuration of prefix-dependent values,
use the softest set as follows:
•
Do not start dampening before the fourth flap in a row with a maximum outage of 30 minutes,
and minimum outage of 10 minutes
After dampening a route, you can display related data, including the interval before it will be
unsuppressed with the
Capability Advertisement
BGP4 requires that when an OPEN message is received with one or more unrecognized optional
parameters, BGP peering terminate. To address this issue, capability advertisement allows the
graceful introduction of new capabilities without requiring that peering be terminated. This is
achieved through a new optional parameter in the OPEN message - capabilities - which lists the
capabilities supported by the speaker.
Route Refresh
When an inbound routing policy for a peer changes, all prefixes from that peer must be made
available and then re-examined against that new policy. This can be performed by:
•
Resetting the BGP session with the neighbor. This invalidates the cache, adversely affecting
network operation.
•
Performing soft reconfiguration which stores an unmodified copy of all routes from a particular
peer at all times by using the
reconfiguration activates policies without actually clearing the BGP policy but imposes a
higher memory cost.
The XSR's route refresh capability offers an alternative by introducing the dynamic exchange of a
route refresh request message between BGP peers. Receiving this request from a peer causes the
subsequent re-advertisement of all outbound prefixes that satisfy outbound policy constraints to
that peer.
show ip bgp dampened-paths
neighbor soft-reconfiguration inbound
bgp dampening
command.
XSR User's Guide 6-17
Overview
command.
command. This
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