Cisco ASR 9000 Series Routing Configuration Manual page 42

BGP Next Hop Tracking
• 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.
For information on route policy filtering for next hops using the next-hop attach point, see the Implementing
Routing Policy Language on Cisco ASR 9000 Series Router module of Cisco ASR 9000 Series
Aggregation Services Router Routing Configuration Guide (this publication).
Scoped IPv4/VPNv4 Table Walk
To determine which address family to process, a next-hop notification is received by first de-referencing the
gateway context associated with the next hop, then looking into the gateway context to determine which
address families are using the gateway context. The IPv4 unicast and VPNv4 unicast address families share
the same gateway context, because they are registered with the IPv4 unicast table in the RIB. As a result, both
the global IPv4 unicast table and the VPNv4 table are is processed when an IPv4 unicast next-hop notification
is received from the RIB. A mask is maintained in the next hop, indicating if whether the next hop belongs
to IPv4 unicast or VPNv4 unicast, or both. This scoped table walk localizes the processing in the appropriate
address family table.
Reordered Address Family Processing
The Cisco IOS XR software walks address family tables based on the numeric value of the address family.
When a next-hop notification batch is received, the order of address family processing is reordered to the
following order:
• IPv4 tunnel
• VPNv4 unicast
• IPv4 labeled unicast
• IPv4 unicast
• IPv4 multicast
• IPv6 unicast
New Thread for Next-Hop Processing
The critical-event thread in the spkr process handles only next-hop, Bidirectional Forwarding Detection (BFD),
and fast-external-failover (FEF) notifications. This critical-event thread ensures that BGP convergence is not
adversely impacted by other events that may take a significant amount of time.
show, clear, and debug Commands
The show bgp nexthops command provides statistical information about next-hop notifications, the amount
of time spent in processing those notifications, and details about each next hop registered with the RIB. The
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
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Implementing BGP