Route Dampening; Configuring Bgp Route Dampening - Cisco NCS 5500 Series Configuration Manual

Route Dampening

Route Dampening
Route dampening is a BGP feature that minimizes the propagation of flapping routes across an internetwork.
A route is considered to be flapping when it is repeatedly available, then unavailable, then available, then
unavailable, and so on.
For example, consider a network with three BGP autonomous systems: autonomous system 1, autonomous
system 2, and autonomous system 3. Suppose the route to network A in autonomous system 1 flaps (it becomes
unavailable). Under circumstances without route dampening, the eBGP neighbor of autonomous system 1 to
autonomous system 2 sends a withdraw message to autonomous system 2. The border router in autonomous
system 2, in turn, propagates the withdrawal message to autonomous system 3. When the route to network A
reappears, autonomous system 1 sends an advertisement message to autonomous system 2, which sends it to
autonomous system 3. If the route to network A repeatedly becomes unavailable, then available, many
withdrawal and advertisement messages are sent. Route flapping is a problem in an internetwork connected
to the Internet, because a route flap in the Internet backbone usually involves many routes.
The route dampening feature minimizes the flapping problem as follows. Suppose again that the route to
network A flaps. The router in autonomous system 2 (in which route dampening is enabled) assigns network
A a penalty of 1000 and moves it to history state. The router in autonomous system 2 continues to advertise
the status of the route to neighbors. The penalties are cumulative. When the route flaps so often that the penalty
exceeds a configurable suppression limit, the router stops advertising the route to network A, regardless of
how many times it flaps. Thus, the route is dampened.
The penalty placed on network A is decayed until the reuse limit is reached, upon which the route is once
again advertised. At half of the reuse limit, the dampening information for the route to network A is removed.
No penalty is applied to a BGP peer reset when route dampening is enabled, even though the reset withdraws
Note
the route.

Configuring BGP Route Dampening

Perform this task to configure and monitor BGP route dampening.
SUMMARY STEPS
1. configure
2. router bgp as-number
3. address-family { ipv4 | ipv6 } unicast
4. bgp dampening [ half-life [ reuse suppress max-suppress-time ] | route-policy route-policy-name ]
5. commit
DETAILED STEPS
Step 1
configure
Step 2 router bgp as-number
BGP Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.2.x
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Implementing BGP