Cisco Nexus 7000 Series - Capabilities Negotiation; Route Dampening

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Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
OL-20002-02
Chapter 11 Configuring Advanced BGP
Information About Advanced BGP
Figure 11-3 route reflector
The route reflector and its client peers form a cluster. You do not have to configure all iBGP peers to act
as client peers of the route reflector. You must configure any nonclient peer as fully meshed to guarantee
that complete BGP updates reach all peers.
Capabilities Negotiation
A BGP speaker can learn about BGP extensions supported by a peer by using the capabilities negotiation
feature. Capabilities negotiation allows BGP to use only the set of features supported by both BGP peers
on a link.
If a BGP peer does not support capabilities negotiation, Cisco NX-OS will attempt a new session to the
peer without capabilities negotiation if you have configured the address family as IPv4. Any other
multiprotocol configuration (such as IPv6) requires capabilities negotiation.
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.
ZA
B
C
D
AS10AS20
eBGP
iBGP
iBGP
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