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The OSPF LSA group pacing feature allows the router to group together OSPF link state advertisements (LSAs) and pace the refreshing, checksumming, and aging functions. The group pacing results in more efficient use of the router.
Prior to the LSA group pacing feature, the Cisco IOS software would perform refreshing on a single timer, and checksumming and aging on another timer. In the case of refreshing, for example, the software would scan the whole database every 30 minutes, refreshing every LSA the router generated, no matter how old it was. Figure 1 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. A large OSPF database (several thousand LSAs) could 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 could 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. So CPU is used only when necessary. However, LSAs being refreshed at frequent, random intervals would require many packets for the few refreshed LSAs the router must send out. That would be inefficient use of bandwidth.
Therefore, 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 packet or more. Thus, the refresh packets are paced, as are the checksumming and aging. The pacing interval is configurable; it defaults to 4 minutes, which is randomized to further avoid synchronization.
Figure 2 illustrates the case of refresh packets. The first timeline illustrates individual LSA timers; the second timeline illustrates individual LSA timers with group pacing.
The router groups together 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.
This feature is supported on these platforms:
OSPF LSA group pacing is enabled by default. For typical customers, the default group pacing interval for refreshing, checksumming, and aging is appropriate and you need not configure this feature.
The group pacing interval is inversely proportional to the number of LSAs the router is refreshing, checksumming, and aging. For example, if you have approximately 10,000 LSAs, decreasing the pacing interval would benefit you. If you have a very small database (40 to 100 LSAs), increasing the pacing interval to 10 to 20 minutes might benefit you slightly.
The default value of pacing between LSA groups is 240 seconds (4 minutes). The range is 10 seconds to 1800 seconds (half an hour). To configure the LSA group pacing interval, perform the following task in router configuration mode:
| Task | Command |
|---|---|
Configure the group pacing of LSAs. | timers lsa-group-pacing seconds |
The following example changes the OSPF pacing between LSA groups to 60 seconds:
router ospf timers lsa-group-pacing 60
This section documents new or modified commands. All other commands used with this feature are documented in the Cisco IOS Release 11.3 command references.
To change the interval at which OSPF LSAs are collected into a group and refreshed, checksummed, or aged, use the timers lsa-group-pacing router configuration command. To restore the default value, use the no form of this command.
timers lsa-group-pacing seconds
seconds | Number of seconds in the interval at which LSAs are grouped and refreshed, checksummed, or aged. The range is 10 seconds to 1800 seconds. The default value is 240 seconds. |
240 seconds
Router configuration
This command first appeared in Cisco IOS Release 11.3 AA.
OSPF LSA group pacing is enabled by default. For typical customers, the default group pacing interval for refreshing, checksumming, and aging is appropriate and you need not configure this feature.
The duration of the LSA group pacing is inversely proportional to the number of LSAs the router is handling. For example, if you have approximately 10,000 LSAs, decreasing the pacing interval would benefit you. If you have a very small database (40 to 100 LSAs), increasing the pacing interval to 10 to 20 minutes might benefit you slightly.
The following example changes the OSPF pacing between LSA groups to 60 seconds:
router ospf
timers lsa-group-pacing 60
Posted: Fri Mar 12 23:10:46 PST 1999
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