Chapter 29
| IP Routing Commands
Border Gateway Protocol (BGPv4)
– 918 –
Route damping provides a relief mechanism to minimize the effects of route
flapping. It can reduce the propagation of updates for flapping routes without
impacting the route convergence time for stable routes. When enabled, a route is
assigned a penalty each time it flaps (i.e., announced and then quickly withdrawn).
If the penalty exceeds 2000 (the suppress limit) the route is suppressed. After the
route remains stable for a specified interval (half-life), the penalty is reduced by half.
Subsequently, the penalty is reduced every 15 minutes. When the penalty falls
below a specified value (reuse limit), the route is unsuppressed.
The penalty never exceeds the maximum penalty, which is computed from
specified attributes as shown below:
Maximum penalty = reuse-limit * 2^(max-suppress-time/half-life)
When a route is being “damped,” any updates or withdrawals for this route received
from a peer are ignored. This limits the effects of route flapping to a single peering
connection. Since most ASes are connected by high-speed links, it is not always
necessary to use route dampening. However, when invoked, it may be necessary to
fine tune the penalty attributes to ensure fair treatment to unstable routes.
Configuration Guidelines
1. Use the bgp dampening command to enable route dampening.
2. Use the bgp dampening command to adjust the penalty attributes of half-life,
reuse-limit, suppress-limit, and max-suppress-time.
3. Use the clear ip bgp dampening command to clears route dampening
information and unsuppresses any suppressed routes.
BGP Command List
Table 185: Border Gateway Protocol Commands – Version 4
Command Function Mode
General Configuration
router bgp Enables BGPv4 routing process and enters router
configuration mode
GC
ip as-path access-list Configures an autonomous system path access list GC
ip community-list Configures a community list GC
ip extcommunity-list Configures an extended community list GC
ip prefix-list Configures an address prefix list GC
aggregate-address Configures an aggregate address in the routing table RC
bgp client-to-client
reflection
Configures route reflection between clients via route
reflector
RC
bgp cluster-id Configures cluster identifier for multiple route reflectors in
the same cluster
RC
bgp confederation
identifier
Configures the identifier for a confederation containing
smaller multiple internal autonomous systems
RC
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