Specifications

Implementing BGP on Cisco IOS XR Software
Information About Implementing BGP on Cisco IOS XR Software
RC-6
Cisco IOS XR Routing Configuration Guide
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.
For information on route policy filtering for next hops using the next-hop attach point, see the
Implementing Routing Policy Language on Cisco IOS XR Software module of Cisco IOS XR Routing
Configuration Guide.
Scoped IPv4/VPNv4 Table Walk
To determine which address family to process, a next-hop notification is received by first dereferencing
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 processed when an IPv4
unicast next-hop notification is received from the RIB. A mask is maintained in the next hop, indicating
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
• VPNv6 unicast
• IPv4 labeled unicast
• IPv4 unicast
• IPv4 MDT
• IPv4 multicast
• IPv6 unicast
• IPv6 multicast
• IPv6 labeled 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.