Understanding LSA Group Pacing

The OSPF LSA group pacing feature allows the router to group OSPF LSAs and pace the refreshing, checksumming, and aging functions. The group pacing results in more efficient use of the router.

NOTE Cisco has made LSA group pacing the default behavior for OSPF in Cisco IOS Software

Release 12.0 and later.

Each OSPF LSA is recorded and tracked with an age that indicates whether the LSA is still valid. When the LSA reaches the maximum age (60 minutes), the LSA is discarded from the link-state database. A router keeps track of LSAs that it generates and LSAs that it receives from other routers. The router refreshes LSAs that it generated; it ages the LSAs it received from other routers.

During normal OSPF operation, LSA refresh packets are sent to keep the LSA from expiring, regardless of whether a change has occurred in the network topology. The originating router sends a refresh packet every 30 minutes to refresh the LSA; this prevents the LSA age from ever reaching 60 minutes.

Sending an update every 30 minutes is not an ideal solution if that is the only method used. OSPF also performs checksumming on all LSAs, in the link-state database, every 10 minutes.

Prior to the LSA group pacing feature, Cisco IOS Software would perform LSA refreshing on a single timer, and checksumming and aging on another timer. For example, in the case of refreshing, the software would scan the entire database every 30 minutes, refreshing every LSA the router generated, no matter how old it was. Figure 3-26 illustrates all the LSAs being refreshed at once. This process wasted CPU resources because only a small portion of the database needed to be refreshed.

Figure 3-26 OSPF LSAs on a Single Timer

All LSAs refreshed, for example, 120 external LSAs on Ethernet need 3 packets.

30 Minutes

30 Minutes

30 Minutes

Prior to OSPF packet pacing, all LSAs refreshed at once.

Prior to OSPF packet pacing, all LSAs refreshed at once.

A large OSPF database (several thousand LSAs) might have thousands of LSAs with different ages. Refreshing on a single timer resulted in the age of all LSAs becoming synchronized, which resulted in much CPU processing at once. Furthermore, a huge number of LSAs might cause a sudden increase of network traffic, consuming a large amount of network resources in a short period of time.

This problem is solved by each LSA having its own timer. Again using the example of refreshing, each LSA gets refreshed when it is 30 minutes old, independent of other LSAs. Therefore, the CPU is used only when necessary. However, LSAs being refreshed at frequent, random intervals would require many packets for the few refreshed LSAs that the router must send out. That would be inefficient use of bandwidth.

The router delays the LSA refresh function for an interval of time instead of performing it when the individual timers are reached. The accumulated LSAs constitute a group, which is then refreshed and sent out in one or more packets. Therefore, the refresh packets are paced, as are the checksumming and aging. Figure 3-27 illustrates the case of refresh

130 Chapter 3: OSPF Communication packets. The first time line illustrates individual LSA timers; the second time line illustrates individual LSA timers with group pacing.

Figure 3-27 OSPF LSAs on Individual Timers

If You Used Individual LSA Timers

At frequent, random intervals, another LSA needs to be refreshed. This would require many refresh packets, containing only a few LSAs.

Transmission with Group Pacing

20 LSAs, 1 Packet

37 LSAs, 1 Packet

20 LSAs, 1 Packet

37 LSAs, 1 Packet

< '

5 LSAs

1 Pack

et

4 Min

4 Min

4 Min

4 Min

4 Min

4 Min

4 Min

The router groups OSPF LSAs and paces the refreshing, checksumming, and aging functions so that sudden hits on CPU usage and network resources are avoided. This feature is most beneficial to large OSPF networks.

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