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OSPF LSA Group Pacing
Description
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.
Original LSA Behavior
Each OSPF LSA has an age, which indicates whether the LSA is still valid. Once the LSA reaches the maximum age (one hour), it is discarded. During the aging process, the originating router sends a refresh packet every 30 minutes to refresh the LSA. Refresh packets are sent to keep the LSA from expiring, whether there has been a change in the network topology or not. Checksumming is performed on all LSAs every 10 minutes. The router keeps track of LSAs it generates and LSAs it receives from other routers. The router refreshes LSAs it generated; it ages the LSAs it received from other routers.
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. 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.
Figure 1 OSPF LSAs on a Single Timer without Group Pacing
Solution
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.
illustrates the case of refresh packets. The first timeline illustrates individual LSA timers; the second timeline illustrates individual LSA timers with group pacing.
Figure 2 OSPF LSAs on Individual Timers with Group Pacing
Benefits
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.
Platforms
This feature is supported on these platforms:
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Cisco 1003, Cisco 1004, Cisco 1005
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Configuration Task
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:
Configuration Example
The following example changes the OSPF pacing between LSA groups to 60 seconds:
router ospftimers lsa-group-pacing 60Command Reference
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.
timers lsa-group-pacing
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
no timers lsa-group-pacingSyntax Description
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.
Default
240 seconds
Command Mode
Router configuration
Usage Guidelines
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.
Example
The following example changes the OSPF pacing between LSA groups to 60 seconds:
router ospf timers lsa-group-pacing 60
