Ospf Spf Prefix Prioritization - Cisco NCS 6000 Series Configuration Manual

Implementing OSPF

OSPF SPF Prefix Prioritization

The OSPF SPF Prefix Prioritization feature enables an administrator to converge, in a faster mode, important
prefixes during route installation.
When a large number of prefixes must be installed in the Routing Information Base (RIB) and the Forwarding
Information Base (FIB), the update duration between the first and last prefix, during SPF, can be significant.
In networks where time-sensitive traffic (for example, VoIP) may transit to the same router along with other
traffic flows, it is important to prioritize RIB and FIB updates during SPF for these time-sensitive prefixes.
The OSPF 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:
• If both high-priority and medium-priority route policies are specified, and no maps are specified for
• If both critical-priority and high-priority route policies are specified, and no maps are specified for
• If only medium-priority route policy is specified and no maps are specified for high priority or critical
• Permitted prefixes are assigned to a high-priority queue.
• Unmatched prefixes, including /32s, are placed in a low-priority queue.
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.
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.
priority:
• Permitted prefixes matching medium-priority route policy are assigned to a medium-priority queue.
Routing Configuration Guide for Cisco NCS 6000 Series Routers, IOS XR Release 6.4.x
OSPF SPF Prefix Prioritization
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