the configured maximum number of prefixes is fewer than the number of prefixes that have already been
received from the neighbor for the address family.
BGP Next Hop Tracking
BGP receives notifications from the Routing Information Base (RIB) when next-hop information changes
(event-driven notifications). BGP obtains next-hop information from the RIB to:
•
Determine whether a next hop is reachable.
•
Find the fully recursed IGP metric to the next hop (used in the best-path calculation).
•
Validate the received next hops.
•
Calculate the outgoing next hops.
•
Verify the reachability and connectedness of neighbors.
BGP is notified when any of the following events occurs:
•
Next hop becomes unreachable
•
Next hop becomes reachable
•
Fully recursed IGP metric to the next hop changes
•
First hop IP address or first hop interface change
•
Next hop becomes connected
•
Next hop becomes unconnected
•
Next hop becomes a local address
•
Next hop becomes a nonlocal address
Reachability and recursed metric events trigger a best-path recalculation.Note
Event notifications from the RIB are classified as critical and noncritical. Notifications for critical and noncritical
events are sent in separate batches. However, a noncritical event is sent along with the critical events if the
noncritical event is pending and there is a request to read the critical events.
•
Critical events are related to the reachability (reachable and unreachable), connectivity (connected and
unconnected), and locality (local and nonlocal) of the next hops. Notifications for these events are not
delayed.
•
Noncritical events include only the IGP metric changes. These events are sent at an interval of 3 seconds.
A metric change event is batched and sent 3 seconds after the last one was sent.
The next-hop trigger delay for critical and noncritical events can be configured to specify a minimum batching
interval for critical and noncritical events using the nexthop trigger-delay command. The trigger delay is
address family dependent.
The BGP next-hop tracking feature allows you to specify that BGP routes are resolved using only next hops
whose routes have the following characteristics:
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.1.x
12 OL-30423-03
Implementing BGP
BGP Next Hop Tracking
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