ECS4660-28F_Management Guide-R03

C HAPTER 51 | IP Routing Commands
Border Gateway Protocol (BGPv4)
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few seconds later, repeatedly. Since routing information is propagated to
other downstream speakers, there is a ripple effect that creates a
cascading storm of updates through the ASes. This causes instability in the
routing tables, as well as the computational overhead required to compute
the best path, and an increase in convergence time.
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and then quickly withdrawn).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 270: 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