Advanced BGP Features: Route Dampening
From Cisco Docs: "Route dampening is a BGP feature designed to minimize the propagation of flapping routes across an internetwork.
When you check the BGP prefixes using the “show ip bgp”, besides the arguments that appeared so far (*, >, r) there
is another “Tag” that can appear, and it’s a letter “d”, which stends for DAMPENING.
#show ip bgpBGP table version is 5, local router ID is 192.168.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal <- CHECK THIS LINE
r RIB-failure, S Stale
From Cisco Docs: “Route dampening is a BGP feature designed to minimize the propagation of flapping routes across
an internetwork. A route is considered to be flapping when its availability alternates repeatedly”
If you’re configuring it without any parameter tuning, there is an enable command under the BGP process:
(config-router)#bgp dampeningIf you want to use this feature - make sure you understand the concept of PENALTIES being “rewarded” to a route
every time it FLAPS, and make sure you’re familiar with the PARAMETERS of BGP DAMPENING:
#sh 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
1. HALF-TIME (default 15 minutes)
When the penalty is assigned to a route, the accumulated penalty is decreased every 5 seconds. When the half-time
expires, accumulated penalties are reduced by half. Default HALF-TIME is 15 minutes, and range 1-45 minutes.
2. REUSE (default 750)
The route can be REUSABLE if the penalties for flapping route go BELOW THIS VALUE. By default it’s 750,
and the range is 1 to 20000
3. SUPRESS
The route is SUPRESSED when the penalties REACH THIS VALUE. Default is 2000, and the range is 1-20000
4. MAX-SUPRESS-TIME
Max time that the route can STAY SUPRESSED. Default is 4 times Half-Time value (60 minutes), range is 1-255
If you need to configure the BGP DAMPENING for a certain routes, use the ROUTE-MAP:
(config-router)#route-map DAMPEN_1
(config-route-map)#match ip add 15 <- CONFIGURE THE ROUTES YOU ARE DAMPENING IN AN ACL
(config-route-map)#set dampening 15 700 2000 60 <- SET DESIRED DAMPENING PARAMETERSThen apply it within the BGP configuration process:
(config-router)#bgp dampening route-map DAMPEN_1!!!This configuration can get quite complicated, so you might need to MATCH THE AS-PATH,
for this you need to be quite comfortable with META CHARACTERS, so for example match prefixes originated in AS 300:
(config)#ip as-path access-list 15 permit ^300$And then MATCH it in the route-map and SET the dampening parameters:
(config-router)#route-map DAMPEN_2
(config-route-map)#match as-path 15
(config-route-map)#set dampening 15 700 2000 60