Route Dampening; Load Sharing And Multipath - Cisco Nexus 3000 Series Configuration Manual

Information About Advanced BGP
S e n d d o c u m e n t c o m m e n t s t o n e x u s 3 k - d o c f e e d b a c k @ c i s c o . c o m

Route Dampening

Route dampening is a BGP feature that minimizes the propagation of flapping routes across an
internetwork. A route flaps when it alternates between the available and unavailable states in rapid
succession.
For example, consider a network with three BGP autonomous systems: AS1, AS2, and AS3. Suppose
that a route in AS1 flaps (it becomes unavailable). Without route dampening, AS1 sends a withdraw
message to AS2. AS2 propagates the withdrawal message to AS3. When the flapping route reappears,
AS1 sends an advertisement message to AS2, which sends the advertisement to AS3. If the route
repeatedly becomes unavailable, and then available, AS1 sends many withdrawal and advertisement
messages that propagate through the other autonomous systems.
Route dampening can minimize flapping. Suppose that the route flaps. AS2 (in which route dampening
is enabled) assigns the route a penalty of 1000. AS2 continues to advertise the status of the route to
neighbors. Each time that the route flaps, AS2 adds to the penalty value. When the route flaps so often
that the penalty exceeds a configurable suppression limit, AS2 stops advertising the route, regardless of
how many times that it flaps. The route is now dampened.
The penalty placed on the route decays until the reuse limit is reached. At that time, AS2 advertises the
route again. When the reuse limit is at 50 percent, AS2 removes the dampening information for the route.
The router does not apply a penalty to a resetting BGP peer when route dampening is enabled, even
Note
though the peer reset withdraws the route.

Load Sharing and Multipath

BGP can install multiple equal-cost eBGP or iBGP paths into the routing table to reach the same
destination prefix. Traffic to the destination prefix is then shared across all the installed paths.
The BGP best-path algorithm considers the paths as equal-cost paths if the following attributes are
identical:
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BGP selects only one of these multiple paths as the best path and advertises the path to the BGP peers.
Paths received from different AS confederations are considered as equal-cost paths if the external
Note
AS_path values and the other attributes are identical.
When you configure a route reflector for iBGP multipath, and the route reflector advertises the selected
Note
best path to its peers, the next hop for the path is not modified.
Cisco Nexus 3000 Series NX-OS Unicast Routing Configuration Guide, NX-OS Release 5.0(3)U1(1)
6-6
Weight
Local preference
AS_path
Origin code
Multi-exit discriminator (MED)
IGP cost to the BGP next hop
Chapter 6 Configuring Advanced BGP