Connectivity Guide

Route dampening
When EBGP routes become unavailable, they “ap” and the router issues both WITHDRAWN and UPDATE notices. A ap occurs when a
route is withdrawn, readvertised after being withdrawn, or has an attribute change.
The constant router reaction to the WITHDRAWN and UPDATE notices causes instability in the BGP process. To minimize this instability,
congure penalties (a numeric value) for routes that ap. When that penalty value reaches a congured limit, the route is not advertised,
even if the route is up, the penalty value is 1024.
As time passes and the route does not ap, the penalty value decrements or decays. If the route aps again, it is assigned another penalty.
The penalty value is cumulative and adds underwithdraw, readvertise, or attribute change.
When dampening applies to a route, its path is described by:
History entry Entry that stores information on a downed route.
Dampened path Path that is no longer advertised.
Penalized path Path that is assigned a penalty.
1 Enable route dampening in ROUTER-BGP mode.
dampening [half-life | reuse | max-suppress-time]
• half-life — Number of minutes after which the penalty decreases (1 to 45, default 15). After the router assigns a penalty of
1024 to a route, the penalty decreases by half after the half-life period expires.
• reuse — Number compares to the apping route’s penalty value. If the penalty value is less than the reuse value, the apping
route again advertises or is no longer suppressed (1 to 20000, default 750). Withdrawn routes are removed from the history state.
• suppress — Number compares to the apping route’s penalty value. If the penalty value is greater than the suppress value, the
apping route no longer advertises and is suppressed (1 to 20000, default 2000).
• max-suppress-time — Maximum number of minutes a route is suppressed (1 to 255, default is four times the half-life value
or 60 minutes).
2 View all ap statistics or for specic routes meeting the criteria in EXEC mode.
show ip bgp flap-statistics [ip-address [mask]]
• ip-address [mask]— Enter the IP address and mask.
• filter-list as-path-name — Enter the name of an AS-PATH ACL.
• regexp regular-expression — Enter a regular express to match on.
When you change the best path selection method, path selections for the existing paths remain unchanged until you reset it by using the
clear ip bgp command in EXEC mode.
Congure values to reuse or restart route
OS10(config)# router bgp 102
OS10(conf-router-bgp-102)# address-family ipv4 unicast
OS10(conf-router-bgpv4-af)# dampening 2 2000 3000 10
View dampened (nonactive) routes
OS10# show ip bgp flap-statistics
BGP local router ID is 13.176.123.28
Status codes: s suppressed, S stale, d dampened, h history, * valid, > best
Origin codes: i - IGP, e - EGP, ? - incomplete
Network
From Reuse Path
Total number of prefixes: 0
350
Layer 3