Cisco NCS 5500 Series Configuration Manual page 145

Implementing BGP
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.
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
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.
This route policy filtering is possible because RIB identifies the source protocol of route that resolved a next
hop as well as the mask length associated with the route. The nexthop route-policy command is used to
specify the route-policy.
Next Hop as the IPv6 Address of Peering Interface
BGP can carry IPv6 prefixes over an IPv4 session. The next hop for the IPv6 prefixes can be set through a
nexthop policy. In the event that the policy is not configured, the nexthops are set as the IPv6 address of the
peering interface (IPv6 neighbor interface or IPv6 update source interface, if any one of the interfaces is
configured).
If the nexthop policy is not configured and neither the IPv6 neighbor interface nor the IPv6 update source
interface is configured, the next hop is the IPv4 mapped IPv6 address.
Scoped IPv4/VPNv4 Table Walk
BGP Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.2.x
Information about Implementing BGP
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