In the next display, the same session is being cleared by using the soft configuration feature. Note how the metric 5000 was sent without killing the BGP session, and the overhead is much smaller.

    RTA#clear ip bgp 172.16.20.1 soft out
    BGP: start outbound soft reconfiguration for 172.16.20.1
    BGP: 172.16.20.1 computing updates, neighbor version 0,
    table version 24, starting at 0.0.0.0
    BGP: 172.16.20.1 send UPDATE 172.16.25.0/24, next 172.16.20.2,
    metric 5000, path 3
    BGP: 172.16.20.1 send UPDATE 172.16.30.0/24, next 172.16.20.2,
    metric 5000, path 3
    BGP: 172.16.20.1 send UPDATE 172.16.50.0/24, next 172.16.20.2,
    metric 5000, path 3
    BGP: 172.16.20.1 send UPDATE 172.16.60.0/24, next 172.16.20.2,
    metric 5000, path 3
    BGP: 172.16.20.1 send UPDATE 172.16.70.0/24, next 172.16.20.2,
    metric 5000, path 3
    BGP: 172.16.20.1 send UPDATE 172.16.90.0/24, next 172.16.20.2,
    metric 5000, path 3
    BGP: 172.16.20.1 send UPDATE 172.16.65.0/26, next 172.16.20.2,
    metric 5000, path 3
    BGP: 172.16.20.1 send UPDATE 172.16.112.0/24, next 172.16.20.2,
    metric 5000, path 3
    BGP: 172.16.20.1 send UPDATE 172.16.220.0/24, next 172.16.20.2,
    metric 5000, path 3
    BGP: 172.16.20.1 send UPDATE 192.68.11.0/24 —
    unreachableBGP: 172.16.20.1 send UPDATE 192.68.222.0/24,
    next 172.16.20.2, metric 5000, path 3 2
    BGP: 172.16.20.1 5 updates enqueued (average=52, maximum=68)
    BGP: 172.16.20.1 update run completed, ran for 24ms,
    neighbor version 0, start version 24, throttled to 24,
    check point net 0.0.0.0
    BGP: scanning routing tables

Route Dampening

Route dampening is a mechanism to minimize the instability caused by route flapping and oscillation over the network. The following are the commands used to control route dampening:

    bgp dampening [[route-map map-name] [half-life-time reuse-value
    suppress-value maximum-suppress-time]]

•  half-life-time—range is 1-45 minutes; current default is 15 minutes.

•  reuse-value—range is 1-20000; default is 750.

•  suppress-value— range is 1-20000; default is 2000.

•  max-suppress-time—maximum duration a route can be suppressed. Range is 1-255; default is four times half-life-time.

A route map can be associated with bgp dampening to selectively apply the dampening parameters if certain criteria are found. Example criteria include matching on a specific IP route or AS_path or community.

Figure 11-15 shows two ASs, AS3 and AS1. RTA in AS3 is running IBGP with RTG in AS3 and EBGP with RTC in AS1. Information coming via EBGP from AS3 is injected into OSPF in AS1. RTC has noticed lots of fluctuations in network 172.16.220.0/24 coming from AS3, causing oscillation in its BGP and in turn in OSPF. The 172.16.220.0/24 keeps showing up and disappearing from RTH's routing table. To rectify the problem, RTC will apply dampening to the BGP by using a route map to selectively dampen route 172.16.220.0/24 only.

Figure 11-15  Route dampening.

RTG configuration:

   router bgp 3
    no synchronization
    network 172.16.112.0 mask 255.255.255.0
    neighbor 172.16.70.1 remote-as 3
    no auto-summary

RTA configuration:

   router bgp 3
    no synchronization
    network 172.16.220.0 mask 255.255.255.0
    network 172.16.70.0 mask 255.255.255.0
    neighbor 172.16.20.1 remote-as 1
    neighbor 172.16.70.2 remote-as 3
    neighbor 172.16.70.2 next-hop-self
    no auto-summary

RTC is EBGP peered with RTA, and IBGP peered with RTH. RTC is injecting the BGP routes it receives into OSPF, which is running in AS1. RTC is applying BGP dampening with a route map SELECTIVE_DAMPENING, which applies the dampening parameters to network 172.16.220.0/24 only. All other routes such as 172.16.112.0/24 will not be dampened.

RTC has specified the dampening parameters in the following manner. The half-life-time is 20 minutes, the reuse limit for the penalty is 950, routes will be suppressed if the cumulative penalty exceeds 2500, and the maximum time a route could be suppressed is 80 minutes.

RTC configuration:

   router ospf 10
    redistribute bgp 1 subnets
    network 192.68.0.0 0.0.255.255 area 0

   router bgp 1
    bgp dampening route-map SELECTIVE_DAMPENING
    network 192.68.11.0
    neighbor 172.16.20.2 remote-as 3
    neighbor 192.68.6.1 remote-as 1
    no auto-summary

   access-list 1 permit 172.16.220.0 0.0.0.255

   route-map SELECTIVE_DAMPENING permit 10
    match ip address 1
    set dampening 20 950 2500 80

   route-map SELECTIVE_DAMPENING permit 20

The following output shows how RTC treats the flapping route 172.16.220.0/24. A flap is counted any time the path information changes for a route. The first output shows the route before any flaps have occurred.

    RTC#show ip bgp 172.16.220.0
    BGP routing table entry for 172.16.220.0/24, version 326
    Paths: (1 available, best #1, advertised over IBGP)
    3
     172.16.20.2 from 172.16.20.2 (172.16.220.1)
      Origin IGP, metric 0, valid, external, best

The following output shows the route after one flap. The route is down and is put in "history" state. The route was given the default penalty of 1000, which has already decayed to 997.

    RTC#show ip bgp 172.16.220.0
    BGP routing table entry for 172.16.220.0/24, version 327
    Paths: (1 available, no best path, advertised over IBGP)
    3 (history entry)
     172.16.20.2 from 172.16.20.2 (172.16.220.1)
      Origin IGP, metric 0, external
      Dampinfo: penalty 997, flapped 1 times in 00:00:06

The following shows the route after a second flap (it has come back up again). Another penalty of 1000 has been added, and the cumulative penalty after decay has reached 1454.

    RTC#show ip bgp 172.16.220.0
    BGP routing table entry for 172.16.220.0/24, version 328
    Paths: (1 available, best #1, advertised over IBGP)
    3
     172.16.20.2 from 172.16.20.2 (172.16.220.1)
      Origin IGP, metric 0, valid, external, best
      Dampinfo: penalty 1454, flapped 2 times in 00:01:20