Graceful Restart Procedure Over The Sham-Link; Ecmp And Ospfv3 Sham-Link; Ospf Spf Prefix Prioritization - Cisco ASR 9000 Series Routing Configuration Manual

Implementing OSPF

Graceful Restart Procedure over the Sham-link

OSPFv3 treats the sham link as any other interface during the switch-over or process restart. OSPFv3 assumes
that all the configured sham links are UP and tries to form an adjacency over them.
If the sham link is down prior to the switch-over, OSPFv3 sends the Hello packets to the remote endpoint.
Once the final convergence signal is received from the RIB, OSPFv3 keeps the sham link either up or down
based on the BGP route for each configured sham link in the RIB.
OSPFv3 installs the high AD routes over the sham link only after the BGP convergence is complete.

ECMP and OSPFv3 Sham-link

Equal Cost Multipath (ECMP) mechanism is used to load-balance traffic on the Sham-link if there are multiple
iBGP path for a prefix. If the sham link path and the backdoor path have the same cost, ECMP between the
sham link path and backdoor path is not supported.

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
◦ Permitted prefixes are assigned to a high-priority queue.
◦ Unmatched prefixes, including /32s, are placed in a low-priority queue.
critical priority:
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
OSPF SPF Prefix Prioritization
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