© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-1 Optimizing BGP Scalability Using BGP Route Dampening

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-2 Outline Overview BGP Route Dampening Route-Dampening Operation Configuring BGP Route Dampening Releasing Dampened Routes Monitoring Route Dampening Summary

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-3 BGP Route Dampening Designed to reduce router processing load caused by unstable routes Prevents sustained routing oscillations without affecting other well-behaved routes Defined in RFC 2439: BGP Route Flap Dampening A tool designed to help minimize the number of BGP updates Other update reduction tools: –Batching of BGP updates –Per-neighbor update timers

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-4 BGP Route Dampening (Cont.) Minimizes the amount of BGP update processing in the Internet by suppressing unstable (flapping) routes Does not suppress routes that occasionally flap Suppresses routes that are likely to flap in the future based on the history of their behavior –Flap = Remove route –Suppress = Do not use a route after it reappears

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-5 Route-Dampening Operation Each time an EBGP route flaps, it gets 1000 penalty points (IGBP routes are not dampened). The penalty placed on a route decays according to the exponential decay algorithm. When the penalty exceeds the suppress limit, the route is dampened (no longer used or propagated to other neighbors). A dampened route is propagated when the penalty drops below the reuse limit.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-6 Route-Dampening Operation (Cont.) The flap history is forgotten when the penalty drops below half of the reuse limit. A route is never dampened for more time than the maximum suppress limit. An unreachable route with a flap history is put in the history state—it stays in the BGP table but only to maintain the flap history. A penalty is applied on the individual path in the BGP table, not on the IP prefix.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-7 Configuring BGP Route Dampening bgp dampening [half-life reuse suppress max- suppress-time] [route-map map-name] router(config-router)# Configures BGP route dampening BGP dampening parameters: –half-life Decay time in which the penalty is halved –suppress Value when the route starts dampening –reuse Value when the dampened route is reused –max-suppress-time Maximum time to suppress the route –route-map Name of route-map controlling dampening

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-8 Configuring BGP Route Dampening (Cont.) Most Internet service providers use default values: A flapping route is dampened after three successive flaps. A route stays suppressed for approximately 30 minutes. Net result: The route is lost for 30 minutes if a BGP session with a neighbor is cleared three times in succession. Default dampening parameter values are: –half-life15 minutes –suppress2000 –reuse750 –max-suppress-time60 minutes (4x half-life) –per-flap penalty1000 (nonconfigurable)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-9 Configuring BGP Route Dampening (Cont.) set dampening half-life reuse suppress max-suppress- time router(config-route-map)# This command sets the BGP dampening parameters for individual routes matched by a route-map entry. Apply this route-map to the bgp dampening command instead of specifying individual parameters. Applications: –Less aggressive dampening of routes toward root DNS servers (or other servers) –Dampening of smaller prefixes more aggressively –Selective dampening based on BGP neighbor and route-map match criteria

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-10 Releasing Dampened Routes clear ip bgp ip-address flap-statistics [{regexp regexp} | {filter-list list-name} | {ip-address network-mask}] router# Clears the flap statistics but does not release dampened routes clear ip bgp dampening [ip-address network-mask] router# Releases all the dampened routes or just the specified network Flap statistics or dampened routes also cleared when the BGP session with the neighbor is lost

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-11 Monitoring Route Dampening show ip bgp dampened-paths router> Displays the dampened routes show ip bgp flap-statistics [{regexp regexp} | {filter- list access-list} | {ip-address mask [longer-prefix]}] router> Displays flap statistics for all routes with dampening history Can match routes against regular expressions, AS-path access- lists, a specific route, or more specific routes debug ip bgp dampening router# Displays the BGP dampening events

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-12 Monitoring Route Dampening— Example

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-13 Monitoring Route Dampening— Example (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-14 Monitoring Route Dampening— Example (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-15 Monitoring Route Dampening— Example (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-16 Monitoring Route Dampening— Example (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-17 Monitoring Route Dampening— Example (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-18 Monitoring Route Dampening— Example (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-19 Monitoring Route Dampening— Example (Cont.)

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-20 Summary Route dampening is a BGP feature that is designed to reduce BGP processing requirements by minimizing the propagation of unstable routes to BGP peers. A router with route dampening enabled keeps track of all routes and the penalties that are assigned to them. Each time a flap occurs, the flapping route receives 1000 penalty points; if the route penalties reach the suppress limit, the route is no longer forwarded to other neighbors until the assigned penalty has decayed below the reuse limit. You can implement route dampening with the bgp dampening command either globally in the BGP process or selectively using a route-map.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-21 Summary (Cont.) Use the clear ip bgp flap-statistics command to delete all penalty information without releasing the dampened paths. The clear ip bgp dampening command clears dampening information and releases suppressed routes. The show ip bgp dampened-paths command lists all routes that are currently suppressed because of dampening, the show ip bgp flap-statistics command lists all routes that have a penalty that is still above the time-to-forget limit, the show ip bgp flap- statistics prefix command displays detailed dampening information about a specific network, the show ip bgp flap- statistics prefix mask longer-prefix command displays dampening information about a specific network and its subnets, and the debug ip bgp dampening command displays BGP dampening events as they occur in real time.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—7-22