Dell EMC SmartFabric OS10 User Guide Release 10.5.1

When EBGP routes become unavailable, they “flap” and the router issues both WITHDRAWN and UPDATE notices. A flap occurs when a route is withdrawn, readvertised after being withdrawn, or has an attribute change.

The constant router reaction to the WITHDRAWN and UPDATE notices causes instability in the BGP process.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, configure penalties (a numeric value) for routes that flap. When that penalty value reaches a configured 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 flap, the penalty value decrements or decays. If the route flaps 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 about 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 flapping route’s penalty value. If the penalty value is less than the reuse value, the flapping route again advertises or is no longer suppressed (1 to 20000, default 750). Withdrawn routes are deleted from the history state.
   • suppress — Number compares to the flapping route’s penalty value. If the penalty value is greater than the suppress value, the flapping 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 flap statistics or for specific 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.

Configure 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

View dampened paths

OS10# show ip bgp dampened-paths
 
BGP local router ID is 80.1.1.1
Status codes: s suppressed, S stale, d dampened, h history, * valid, > best
Origin codes: i - IGP, e - EGP, ? - incomplete
      Network        From       Reuse       Path           
d*    3.1.2.0/24     80.1.1.2   00:00:12    800 9 8 i
d*    3.1.3.0/24     80.1.1.2   00:00:12    800 9 8 i
d*    3.1.4.0/24     80.1.1.2   00:00:12    800 9 8 i
d*    3.1.5.0/24     80.1.1.2   00:00:12    800 9 8 i
d*    3.1.6.0/24     80.1.1.2   00:00:12    800 9 8 i
Total number of prefixes: 5