IP Event Dampening

Routing protocols provide network convergence functionality in IP networks, including TelePresence campus and branch networks. However, these protocols are impeded by links that “flap” or change state repeatedly. Although not a protocol in itself, IP Event Dampening complements the functioning of routing protocols to improve availability by minimizing the impact of flapping on routing protocol convergence.

Whenever the line protocol of an interface changes state, or flaps, routing protocols are notified of the status of the routes affected by the change in state. Every interface state change requires all affected devices in the network to recalculate best paths, install or remove routes from the routing tables, and then advertise valid routes to peer routers. An unstable interface that flaps excessively can cause other devices in the network to consume substantial amounts of system processing resources and cause routing protocols to lose synchronization with the state of the flapping interface.

The IP Event Dampening feature introduces a configurable exponential decay mechanism to suppress the effects of excessive interface flapping events on routing protocols and routing tables in the network. This feature allows the network administrator to configure a router to automatically identify and selectively dampen a local interface that is flapping. Dampening an interface removes the interface from the network until the interface stops flapping and becomes stable.

Configuring the IP Event Dampening feature improves convergence times and stability throughout the network by isolating failures so that disturbances are not propagated, which reduces the utilization of system processing resources by other devices in the network and improves overall network stability.

IP Event Dampening uses a series of administratively defined thresholds to identify flapping interfaces, to assign penalties, to suppress state changes (if necessary), and to make stabilized interfaces available to the network. These thresholds are as follows:

• Suppress threshold: The value of the accumulated penalty that triggers the router to dampen a flapping interface. The flapping interface is identified by the router and assigned a penalty for each up and down state change, but the interface is not automatically dampened. The router tracks the penalties that a flapping interface accumulates. When the accumulated penalty reaches the default or preconfigured suppress threshold, the interface is placed in a dampened state. The default suppress threshold value is 2000.
• Half-life period: Determines how fast the accumulated penalty can decay exponentially. When an interface is placed in a dampened state, the router monitors the interface for additional up and down state changes. If the interface continues to accumulate penalties and the interface remains in the suppress threshold range, the interface remains dampened. If the interface stabilizes and stops flapping, the penalty is reduced by half after each half-life period expires. The accumulated penalty reduces until the penalty drops to the reuse threshold. The default half-life period timer is five seconds.
• Reuse threshold: When the accumulated penalty decreases until the penalty drops to the reuse threshold, the route is unsuppressed and made available to the other devices on the network. The default value is 1000 penalties.
• Maximum suppress time: Represents the maximum amount of time an interface can remain dampened when a penalty is assigned to an interface. The default maximum penalty timer is 20 seconds.

IP Event Dampening is configured on a per-interface basis (where default values are used for each threshold) as follows:

Router(config-)# interface FastEthernet0/0
Router(config-if)# dampening

IP Event Dampening can be complemented with the use of route summarization, on a per-routing protocol basis, to further compartmentalize the effects of flapping interfaces and associated routes.