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Dell EMC Configuration Guide for the S4048T–ON System 9.14.2.4

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Enabling Route Flap Dampening

When EBGP routes become unavailable, they “flap” and the router issues both WITHDRAWN and UPDATE notices.

A flap is when a route:
  • is withdrawn
  • is readvertised after being withdrawn
  • has an attribute change

The constant router reaction to the WITHDRAWN and UPDATE notices causes instability in the BGP process. To minimize this instability, you may configure penalties (a numeric value) for routes that flap. When that penalty value reaches a configured limit, the route is not advertised, even if the route is up. In Dell EMC Networking OS, that penalty value is 1024. As time passes and the route does not flap, the penalty value decrements or is decayed. However, if the route flaps again, it is assigned another penalty.

The penalty value is cumulative and penalty is added under following cases:
  • Withdraw
  • Readvertise
  • Attribute change
When dampening is applied to a route, its path is described by one of the following terms:
  • history entry — an entry that stores information on a downed route
  • dampened path — a path that is no longer advertised
  • penalized path — a path that is assigned a penalty

To configure route flap dampening parameters, set dampening parameters using a route map, clear information on route dampening and return suppressed routes to active state, view statistics on route flapping, or change the path selection from the default mode (deterministic) to non-deterministic, use the following commands.

  • Enable route dampening.
    CONFIG-ROUTER-BGP mode
    bgp dampening [half-life | reuse | suppress max-suppress-time] [route-map map-name]
    Enter the following optional parameters to configure route dampening parameters:
    • half-life: the range is from 1 to 45. Number of minutes after which the Penalty is decreased. After the router assigns a Penalty of 1024 to a route, the Penalty is decreased by half after the half-life period expires. The default is 15 minutes.
    • reuse: the range is from 1 to 20000. This number is compared to the flapping route’s Penalty value. If the Penalty value is less than the reuse value, the flapping route is once again advertised (or no longer suppressed). Withdrawn routes are removed from history state. The default is 750.
    • suppress: the range is from 1 to 20000. This number is compared to the flapping route’s Penalty value. If the Penalty value is greater than the suppress value, the flapping route is no longer advertised (that is, it is suppressed). The default is 2000.)
    • max-suppress-time: the range is from 1 to 255. The maximum number of minutes a route can be suppressed. The default is four times the half-life value. The default is 60 minutes.
    • route-map map-name: name of a configured route map. Only match commands in the configured route map are supported. Use this parameter to apply route dampening to selective routes.
  • Enter the following optional parameters to configure route dampening.
    CONFIG-ROUTE-MAP mode
    set dampening half-life reuse suppress max-suppress-time
    • half-life: the range is from 1 to 45. Number of minutes after which the Penalty is decreased. After the router assigns a Penalty of 1024 to a route, the Penalty is decreased by half after the half-life period expires. The default is 15 minutes.
    • reuse: the range is from 1 to 20000. This number is compared to the flapping route’s Penalty value. If the Penalty value is less than the reuse value, the flapping route is once again advertised (or no longer suppressed). The default is 750.
    • suppress: the range is from 1 to 20000. This number is compared to the flapping route’s Penalty value. If the Penalty value is greater than the suppress value, the flapping route is no longer advertised (that is, it is suppressed). The default is 2000.
    • max-suppress-time: the range is from 1 to 255. The maximum number of minutes a route can be suppressed. The default is four times the half-life value. The default is 60 minutes.
  • Clear all information or only information on a specific route.
    EXEC Privilege
    clear ip bgp [vrf vrf-name] dampening [ip-address mask]
  • View all flap statistics or for specific routes meeting the following criteria.
    EXEC or EXEC Privilege mode
    show ip bgp [vrf vrf-name] flap-statistics [ip-address [mask]] [filter-list as-path-name] [regexp regular-expression]
    • ip-address [mask]: enter the IP address and mask.
    • filter-list as-path-name: enter the name of an AS-PATH ACL.
    • regexp regular-expression: enter a regular express to match on.
    By default, the path selection in Dell EMC Networking OS is deterministic, that is, paths are compared irrespective of the order of their arrival. You can change the path selection method to non-deterministic, that is, paths are compared in the order in which they arrived (starting with the most recent). Furthermore, in non-deterministic mode, the software may not compare MED attributes though the paths are from the same AS.
  • Change the best path selection method to non-deterministic.
    Change the best path selection method to non-deterministic.
    CONFIG-ROUTER-BGP mode
    bgp non-deterministic-med
    NOTE When you change the best path selection method, path selection for existing paths remains unchanged until you reset it by entering the clear ip bgp command in EXEC Privilege mode.

Examples of Configuring a Route and Viewing the Number of Dampened Routes

To view the BGP configuration, use the show config command in CONFIGURATION ROUTER BGP mode or the show running-config bgp command in EXEC Privilege mode.

The following example shows how to configure values to reuse or restart a route. In the following example, default = 15 is the set time before the value decrements, bgp dampening 2 ? is the set re-advertise value, bgp dampening 2 2000 ? is the suppress value, and bgp dampening 2 2000 3000 ? is the time to suppress a route. Default values are also shown.

DellEMC(conf-router_bgp)#bgp dampening ?
<1-45> Half-life time for the penalty (default = 15)
route-map Route-map to specify criteria for dampening
<cr>
DellEMC(conf-router_bgp)#bgp dampening 2 ?
<1-20000>     Value to start reusing a route (default = 750)
DellEMC(conf-router_bgp)#bgp dampening 2 2000 ?
<1-20000>     Value to start suppressing a route (default = 2000)
DellEMC(conf-router_bgp)#bgp dampening 2 2000 3000 ?
<1-255>       Maximum duration to suppress a stable route (default = 60)
DellEMC(conf-router_bgp)#bgp dampening 2 2000 3000 10 ?
route-map     Route-map to specify criteria for dampening
<cr>

To view a count of dampened routes, history routes, and penalized routes when you enable route dampening, look at the seventh line of the show ip bgp summary command output, as shown in the following example (bold).

DellEMC>show ip bgp summary
BGP router identifier 10.114.8.131, local AS number 65515
BGP table version is 855562, main routing table version 780266
122836 network entrie(s) and 221664 paths using 29697640 bytes of memory
34298 BGP path attribute entrie(s) using 1920688 bytes of memory
29577 BGP AS-PATH entrie(s) using 1384403 bytes of memory
184 BGP community entrie(s) using 7616 bytes of memory
Dampening enabled. 0 history paths, 0 dampened paths, 0 penalized paths

Neighbor    AS    MsgRcvd MsgSent TblVer  InQ OutQ Up/Down State/PfxRcd
10.114.8.34 18508 82883   79977   780266  0   2 00:38:51   118904
10.114.8.33 18508 117265  25069   780266  0   20 00:38:50  102759
DellEMC>

To view which routes are dampened (non-active), use the show ip bgp dampened-routes command in EXEC Privilege mode.

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