Cisco NCS 5500 Series

The OSPFv2OSPF SPF Prefix Prioritization feature provides the administrator with the ability to prioritize
important prefixes to be installed, into the RIB during SPF calculations. Important prefixes converge faster
among prefixes of the same route type per area. Before RIB and FIB installation, routes and prefixes are
assigned to various priority batch queues in the OSPF local RIB, based on specified route policy. The RIB
priority batch queues are classified as "critical," "high," "medium," and "low," in the order of decreasing
priority.
When enabled, prefix alters the sequence of updating the RIB with this prefix priority:
Critical > High > Medium > Low
As soon as prefix priority is configured, /32 prefixes are no longer preferred by default; they are placed in the
low-priority queue, if they are not matched with higher-priority policies. Route policies must be devised to
retain /32s in the higher-priority queues (high-priority or medium-priority queues).
Priority is specified using route policy, which can be matched based on IP addresses or route tags. During
SPF, a prefix is checked against the specified route policy and is assigned to the appropriate RIB batch priority
queue.
These are examples of this scenario:
• If only high-priority route policy is specified, and no route policy is configured for a medium priority:
• Permitted prefixes are assigned to a high-priority queue.
• Unmatched prefixes, including /32s, are placed in a low-priority queue.
• If both high-priority and medium-priority route policies are specified, and no maps are specified for
critical priority:
• Permitted prefixes matching high-priority route policy are assigned to a high-priority queue.
• Permitted prefixes matching medium-priority route policy are placed in a medium-priority queue.
• Unmatched prefixes, including /32s, are moved to a low-priority queue.
• If both critical-priority and high-priority route policies are specified, and no maps are specified for
medium priority:
• Permitted prefixes matching critical-priority route policy are assigned to a critical-priority queue.
• Permitted prefixes matching high-priority route policy are assigned to a high-priority queue.
• Unmatched prefixes, including /32s, are placed in a low-priority queue.
• If only medium-priority route policy is specified and no maps are specified for high priority or critical
priority:
• Permitted prefixes matching medium-priority route policy are assigned to a medium-priority queue.
• Unmatched prefixes, including /32s, are placed in a low-priority queue.
Use the [no] spf prefix-priority route-policy rpl command to prioritize OSPFv2OSPF prefix installation
into the global RIB during SPF.
SPF prefix prioritization is disabled by default. In disabled mode, /32 prefixes are installed into the global
RIB, before other prefixes. If SPF prioritization is enabled, routes are matched against the route-policy
criteria and are assigned to the appropriate priority queue based on the SPF priority set. Unmatched
prefixes, including /32s, are placed in the low-priority queue.
Routing Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.3.x
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Implementing OSPF
OSPFv2OSPF SPF Prefix Prioritization