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Implementing BGP on Cisco IOS XR Software
Information About Implementing BGP on Cisco IOS XR Software
RC-5
Cisco IOS XR Routing Configuration Guide
OL-14356-01
the neighbor and peering with the neighbor is terminated immediately after the configuration if 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
Note Reachability and recursed metric events trigger a best-path recalculation.
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:
• To avoid the aggregate routes, the prefix length must be greater than a specified value.
• The source protocol must be from a selected list, ensuring that BGP routes are not used to resolve
next hops that could lead to oscillation.