Route Dampening

Two-router experiment

R1 originates a host route; R2 learns it over eBGP. The session uses the physical interface, so flapping Loopback10 withdraws only the test prefix, not the entire neighbor.

hostname R1
ip cef
interface Loopback0
 ip address 10.255.0.1 255.255.255.255
exit
interface Loopback10
 ip address 198.51.100.1 255.255.255.255
exit
interface GigabitEthernet1
 ip address 192.0.2.1 255.255.255.252
 no shutdown
exit
router bgp 65001
 bgp router-id 10.255.0.1
 no bgp default ipv4-unicast
 neighbor 192.0.2.2 remote-as 65002
 address-family ipv4 unicast
  no auto-summary
  neighbor 192.0.2.2 activate
  network 198.51.100.1 mask 255.255.255.255
 exit-address-family
end

hostname R2
ip cef
interface Loopback0
 ip address 10.255.0.2 255.255.255.255
exit
interface GigabitEthernet1
 ip address 192.0.2.2 255.255.255.252
 no shutdown
exit
router bgp 65002
 bgp router-id 10.255.0.2
 no bgp default ipv4-unicast
 neighbor 192.0.2.1 remote-as 65001
 address-family ipv4 unicast
  no auto-summary
  neighbor 192.0.2.1 activate
 exit-address-family
end

R2# show ip bgp
     Network          Next Hop        Metric LocPrf Weight Path
*>   198.51.100.1/32  192.0.2.1            0             0 65001 i

Read the penalty model

A withdrawal typically adds a penalty. The penalty decays continuously; half-life is not a timer that suddenly subtracts half at one instant. A path above the suppression threshold is temporarily unusable even after readvertisement. It becomes eligible again below the reuse threshold, provided it is still advertised.

Enable global or selective dampening

router bgp 65002
 address-family ipv4 unicast
  bgp dampening 15 750 2000 60
 exit-address-family

This applies the example parameters to eligible external routes. For a prefix-specific experiment, replace it with the following route-map configuration on R2. Unmatched routes do not receive this route-map dampening treatment.

ip prefix-list FLAP seq 5 permit 198.51.100.1/32
route-map DAMPEN permit 10
 match ip address prefix-list FLAP
 set dampening 15 750 2000 60
exit

router bgp 65002
 address-family ipv4 unicast
  no bgp dampening
  bgp dampening route-map DAMPEN
 exit-address-family

Withdraw and readvertise the prefix

On R1, repeat the shutdown/no shutdown pair on Loopback10, allowing the withdrawal and announcement to reach R2 each time. Do not use the BGP transport interface for this experiment.

interface Loopback10
 shutdown
exit
interface Loopback10
 no shutdown
exit

R2# show ip bgp dampening parameters
dampening 15 750 2000 60
  Half-life time: 15 mins
  Suppress penalty: 2000   Reuse penalty: 750

R2# show ip bgp dampening dampened-paths
     Network            From          Reuse       Path
*d   198.51.100.1/32     192.0.2.1     00:20:00    65001 i

A constructed rapid-flap example reaches penalty 3000. After 15 minutes it is 1500; after 30 minutes it reaches 750. Actual suppression/reuse checks and new flaps affect the observed times. A withdrawn route may retain history without being a usable path.

Recover and restore

First stop the instability. Then either wait for decay or clear the recorded penalty for the test prefix. Removing the configuration is the final lab cleanup.

R2# clear ip bgp dampening 198.51.100.1 255.255.255.255

R2# show ip bgp
     Network          Next Hop        Metric LocPrf Weight Path
*>   198.51.100.1/32  192.0.2.1            0             0 65001 i

router bgp 65002
 address-family ipv4 unicast
  no bgp dampening
 exit-address-family

Classic aggressive settings can delay legitimate reachability after ordinary routing events. Modern deployment guidance favors carefully selected parameters and scope, rather than enabling the historical defaults everywhere.