Dampening

This section covers dampening.

This section covers dampening.

Cisco .com

Cisco .com

Dampening Commands:

router(config)# bgp dampening router(config-router)# bgp dampening half-life reuse suppress max-suppress router(config-router)# bgp dampening route-map route-map-name routerft clear ip bgp dampening [prefix mask]

Penalty Half-life time Suppress limit Suppressed Reuse limit History entry

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Border Gateway Protocol (BGP) sends a WITHDRAWN message to its peers when a prefix transitions from up to down. BGP sends an UPDATE message when the prefix transitions from down to up.

This is commonly referred to as a route flap, and can cause high CPU utilization while the BGP routes are converging. Also, if you are redistributing BGP into your Interior Gateway Protocol (IGP), this flapping can cause it to become less stable. Route flap dampening was introduced in Internetwork Operating System (IOS) Release 11.0 as a mechanism for minimizing the instability caused by route flapping. The following terms are used to describe route flap dampening:

■ Penalty: A numeric value that is assigned to a route when it flaps. The default value is 1000.

■ Half-life time: A configurable numeric value that describes the time required to reduce the penalty by one half. The default is 15 minutes.

■ Suppress limit: A numeric value that is compared with the penalty. If the penalty is greater than the suppress limit, the route is suppressed. The default value is 2000.

■ Suppressed: A route that is not advertised even though it is up. A route is suppressed if the penalty is more than the suppressed limit.

■ Reuse limit: A configurable numeric value that is compared with the penalty. If the penalty is less than the reuse limit, a suppressed route that is up will no longer be suppressed. The default is 750.

■ History entry: An entry that is used to store flap information about a route that is down.

A route that is flapping receives a penalty of 1000 for each flap. When the accumulated penalty reaches the suppress limit (2000), BGP suppresses advertisement of the route, even if the route is up. The accumulated penalty is decremented in half for each half-life time interval (15 minutes). When the accumulated penalty is less than the reuse limit, the route is advertised again, if it is still up.

Dampening is not applied to routes that are learned via internal BGP (iBGP). This restriction avoids forwarding loops and prevents iBGP peers from having a higher penalty for routes that are external to the AS.

The following commands are used when configuring route flap dampening: bgp dampening bgp dampening half-life reuse suppress max-suppress bgp dampening route-map route-map-name clear ip bgp dampening [prefix mask]

By default bgp dampening is disabled. The command bgp dampening enables it.

To modify the damping timers, issue the bgp dampening half-life reuse suppress max-suppress command.

To enable bgp dampening and apply different dampening parameters to different prefixes based on IP-address or AS-path information use the bgp dampening route-map command.

To clear dampening information for a specific or all dampened routes (unsuppress suppressed routes) issue the clear ip bgp dampening command.

Route Dampening

In this scenario, the advertised network 2.2.2.0/24 on R2 is flapping. R3 has been configured for route dampening as such:

R3(config)# router bgp 300

R3(config-router)# bgp dampening

The command debug ip bgp dampening has been issued and you receive the following output. During this time the network 2.2.2.0/24 has been flapping approximately every 30-45 seconds.

03:03:25: BGP(0): charge penalty for 2.2.2.0/24 path 65500 with halflife-time 15

reuse/suppress 750/2000 03:03:25: BGP(0): flapped 1 times since 00:00:00. New penalty is 1000 R3#

03:04:18: BGP(0): charge penalty for 2.2.2.0/24 path 65500 with halflife-time 15

reuse/suppress 750/2000 03:04:18: BGP(0): flapped 2 times since 00:00:53. New penalty is 1961 R3#

03:05:11: BGP(0): charge penalty for 2.2.2.0/24 path 65500 with halflife-time 15

reuse/suppress 750/2000 03:05:11: BGP(0): flapped 3 times since 00:01:46. New penalty is 2886 R3#

03:06:16: BGP(0): suppress 2.2.2.0/24 path 65500 for 00:28:10 (penalty 2754) 03:06:16: halflife-time 15, reuse/suppress 750/2000

Now that the network 2.2.2.0/24 has been suppressed, you can view suppressed routes by issuing the following command:

R3# show ip bgp dampened-paths

BGP table version is 7, local router ID is 3.3.3.3

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal Origin codes: i - IGP, e - EGP, ? - incomplete

Network From Reuse Path

Notice that network 2.2.2.2 has been dampened and will not be used for 27 minutes and 40 seconds.

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