Case Study: Route Dampening
One thing that causes major problems in truly large-scale networks is a destination that flaps regularly, or goes up and down several times in succession within a short period of time. BGP allows a network administrator to stop accepting a route from an external neighbor for a certain period of time through dampening. Note that dampening works for eBGP routes only.
The configuration for this capability is very simple—it's just a single extra configuration command (see Figure 8-12).
Figure 8-12. Simple Dampening Example
For example, if you wanted to dampen the routes from Router B in Figure 8-12, you would configure:
router bgp 100
bgp dampening
Now, assume the link 192.168.1.0/24 flaps several times in a row. Router A will add a penalty to the route each time it flaps, which will eventually dampen the route. On Router A, this looks like the following:
A#show ip bgp flap
BGP table version is 7, local router ID is 10.1.1.1
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
h 192.168.1.0 172.28.1.2 3 00:02:10 100
A#show ip bgp
BGP table version is 7, local router ID is 10.1.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
h 192.168.1.0 172.28.1.2 0 0 100 I
Note the h beside the route in both displays—the route is being marked as a route that is flapping. Once dampened, how does a route come out of this state? The penalty against the route is halved until the route's penalty has fallen below the reuse limit once every 15 minutes (by default). Once the penalty against the route has fallen below the reuse limit, the route will be advertised to BGP neighbors again. There are five attributes of a route when dampening is configured that you need to be concerned with:
Penalty— The penalty that is applied to the route each time it flaps; the default is 1000.
Suppress limit— Once the penalty reaches this mark, the route will be dampened; the default is 2000.
Half-life— Each time the half-life passes, the penalty that is currently assessed against the route is halved; the default is 15 minutes.
Reuse limit— The penalty must drop below this number for the route to be advertised again; the default is 750.
Maximum suppress limit— The maximum number of half-lives a route can be suppressed; the default is 4 half-lives.
These can be configured as part of the bgp dampen command. To give an example of how this works, look at the penalty that occurs over time for a given route as shown in Figure 8-13.
Figure 8-13. Route Dampening Affects
Here, a given route is withdrawn and re-advertised by an eBGP router twice in 5 minutes; each time the route flaps, a penalty of 1000 is applied for a total of 2000. When the second flap occurs, the route is dampened.
After 15 minutes, the penalty in force against the route will have decayed exponentially to 1000. Immediately after this, the route flaps twice more, raising the total penalty to 3000. 15 minutes later, at the 30-minute mark, the penalty has now decayed to 1500.
At the 45-minute mark, the penalty will have decayed half of its value to 750, and the route can be reused again.