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Configuring BGP
Configuring Advanced BGP Features
IPC-328
Cisco IOS IP Configuration Guide
In this case, path 1 would be chosen if the bgp bestpath med confed router configuration command is
enabled. The fourth path has a lower MED, but it is not involved in the MED comparison because there
is an external autonomous system is in this path.
Configuring the Router to Use the MED to Choose a Path in a Confederation
To configure the router to use the MED to select the best path from among paths advertised by a single
subautonomous system within a confederation, use the following command in router configuration
mode:
Note If the bgp always-compare-med router configuration command is enabled, all paths are fully
comparable, including those from other autonomous systems in the confederation, even if the bgp
deterministic med command is also enabled.
Configuring Route Dampening
Route dampening is a BGP feature designed to minimize the propagation of flapping routes across an
internetwork. A route is considered to be flapping when it is repeatedly available, then unavailable, then
available, then unavailable, and so on.
For example, consider a network with three BGP autonomous systems: autonomous system 1,
autonomous system 2, and autonomous system 3. Suppose the route to network A in autonomous system
1 flaps (it becomes unavailable). Under circumstances without route dampening, the eBGP neighbor of
autonomous system 1 to autonomous system 2 sends a withdraw message to autonomous system 2. The
border router in autonomous system 2, in turn, propagates the withdraw message to autonomous system
3. When the route to network A reappears, autonomous system 1 sends an advertisement message to
autonomous system 2, which sends it to autonomous system 3. If the route to network A repeatedly
becomes unavailable, then available, many withdrawal and advertisement messages are sent. This is a
problem in an internetwork connected to the Internet because a route flap in the Internet backbone
usually involves many routes.
Note No penalty is applied to a BGP peer reset when route dampening is enabled. Although the reset
withdraws the route, no penalty is applied in this instance, even if route flap dampening is enabled.
Minimizing Flapping
The route dampening feature minimizes the flapping problem as follows. Suppose again that the route
to network A flaps. The router in autonomous system 2 (where route dampening is enabled) assigns
network A a penalty of 1000 and moves it to history state. The router in autonomous system 2 continues
Command Purpose
Router(config-router)# bgp deterministic med
Configures the router to compare the MED variable when
choosing among routes advertised by different peers in the
same autonomous system.