BGP Route Dampening

BGP route dampening controls the effects of route flapping between E-BGP peers. Route dampening is generally used to help service providers prevent one customer's router or circuit problems from affecting the stability of the provider's network by withdrawing problem BGP routes. There are two ways to enable route dampening: The first is to globally enable route dampening for all BGP peers using the bgp dampening command; the second is to use a route map to specify certain routes that are to be dampened and the parameters that are to be applied to the dampened networks. The following syntax shows the bgp dampening command and its optional parameters.

bgp dampening [[route-map route-map-name] | [half-life] | reuse-limit start-
suppress suppress-duration]]

Using the bgp dampening command, route dampening can be configured three ways:

  • Global route dampening using default parameters
  • Global route dampening using custom parameters
  • Specific route dampening using custom parameters

Table 9-12 shows the optional bgp dampening command parameters and their descriptions.

Table 9-12. BGP Route Dampening Parameters

Dampening Command

Description

half-life

The amount of time to wait before decrementing the dampening penalty, ranging from 1 to 45 minutes. The default half-life is 15 minutes.

reuse-limit

The value between 1 and 20,000 that is compared to the penalty value to determine route reusability. If the penalty is greater than the suppress limit, the route will be suppressed; if not, it will be reused. The default suppress limit is 750.

start-suppress

This value between 1 and 20,000 specifies the penalty that will be used if a route is suppressed. The default route suppression penalty is 2000 for each route flap.

suppress-duration

This value specifies the maximum duration that a route will be suppressed. The range for the suppress duration is from 1 to 255 minutes. The default suppress duration is 4 times the half-life, or in other words 60 minutes.

route-map route-map-name

Specifies that a route map will be used to specify the route dampening parameters. Route maps are used to specify the routes that dampening policies should apply to. The same route dampening parameters apply when a route map is used.

After route dampening has been activated, a route flap penalty of 1000 points is assessed to the affected route. The router maintains a history for each route that has flapped, and that history stores the dampening information on a route-by-route basis. The half-life value is used to decrease the time suppression penalty by one-half after a route flaps. Therefore, if a route ceases to flap, it will not be dampened and the history will eventually be cleared. If the route flaps again, another penalty is imposed, and after the suppress-limit has been reached, the route is dampened. When a route has been dampened, it will not be advertised to other BGP peers until the suppress-duration has expired.

Note

The BGP route dampening penalty is initially set to 1000 points and cannot be changed; however, all other parameters are user configurable. You can accept the default values or create your own custom dampening policy based on the particular network requirements.

Look at the network shown in Figure 9-19. In this figure, the Service_Provider router in AS 18,901 is configured with a route dampening policy that dampens routes using the default dampening parameters with the exception of the half-life. In this case, the half-life is changed to 5 minutes, as shown in Example 9-93.

Example 9-93. BGP Configuration for the Service_Provider Router

Service_Provider# show run | begin bgp
router bgp 18901
 no synchronization
 bgp log-neighbor-changes
 bgp bestpath dampening 5
bgp dampening 5
 network 143.68.5.200 mask 255.255.255.252
 network 143.68.5.204 mask 255.255.255.252
 neighbor 143.68.5.202 remote-as 60001
 neighbor 143.68.5.206 remote-as 20148
 no auto-summary
vt320919.gif

Figure 9-19 Service Provider to Customer Network

By default, the bgp best path dampening command is automatically entered after the bgp dampening command has been issued in later 12.2 releases of Cisco IOS Software. This command is also used to enable and disable route dampening. There are several ways to verify and track the BGP route dampening configuration, the most detailed of which is the show ip bgp dampened parameters command. Example 9-94 uses the show ip bgp dampening parameters command to show the BGP route dampening parameters for the Service_Provider router.

Example 9-94. show ip bgp dampening parameters Command

Service_Provider# show ip bgp dampening parameters
 dampening 5 750 2000 20
  Half-life time      : 5  mins       Decay Time       : 775 secs
  Max suppress penalty: 12000         Max suppress time: 20 mins
  Suppress penalty    :  2000         Reuse penalty    : 750

This command displays all the parameters for the local BGP route dampening policy, and in this case, it shows that the Service_Provider router has been configured with a 5-minute half-life. The alteration of the half-life parameter changed the maximum suppress time, so suppressed routes will not be as harshly penalized. Example 9-95 shows the default BGP route dampening parameters.

Example 9-95. Default BGP Route Dampening Parameters

Service_Provider# show ip bgp dampening parameters
 dampening 15 750 2000 60 (DEFAULT)
  Half-life time      : 15 mins       Decay Time       : 2320 secs
  Max suppress penalty: 12000         Max suppress time: 60 mins
  Suppress penalty    :  2000         Reuse penalty    : 750

The show ip bgp dampening flap-statistics command displays detailed information for all dampened routes; in this case, the Service_Provider router has dampened the route to the 186.241.70.0/23 network because it flapped four times. Example 9-96 uses the show ip bgp dampening flap-statistics command to show that the route has been dampened for 3 minutes and 34 seconds and will be eligible for reuse in 10 minutes and 20 seconds.

Example 9-96. show ip bgp dampening flap-statistics Command

Service_Provider# show ip bgp dampening flap-statistics
BGP RIB version is 13, local router ID is 1.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
Origin codes: i - IGP, e - EGP, ? - incomplete
   Network          From            Flaps Duration Reuse    Path
*d 186.241.70.0/23  143.68.5.202    4     00:03:34 00:10:20 60001

The clear ip bgp dampening command enables you to clear the dampened route and the flap statistics associated with the route.

Many steps can be taken to prevent BGP route dampening from occurring using the techniques covered earlier in this chapter; for instance, the Customer_B router could have been configured using some of the following BGP features:

  • Multiple links and loopback interfaces to prevent network outages
  • Aggregating routes into smaller, more stable prefixes so that one network failure will not affect all routes advertised to the upstream neighbor
  • Creating static routes to Null0 to nail down unstable IGP routes

It will always be in your service provider's best interest to protect the stability of their network using BGP route dampening. Their policies might adversely affect a poorly configured network, so you should always try to configure your network with the most stable, redundant BGP configuration possible.

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