Problem: CPUHOG Messages During LSA Refresh Period-Cause: Router Is Not Running LSA Group-Pacing Code

Problem: CPUHOG Messages During LSA Refresh Period ‚ Cause: Router Is Not Running LSA Group -Pacing Code

This problem occurs when the Cisco IOS Software code is not Release 12.0 or later. In Cisco IOS Software Release 12.0, the LSA group pacing feature was introduced to eliminate this CPU problem that can occur every 30 minutes.

In previous versions of Cisco IOS Software, all LSAs refresh every 30 minutes to synchronize the age of all LSAs. Therefore, there is a significant flood every 30 minutes to refresh all LSAs at the same time. This flooding causes the CPUHOG messages every 30 minutes. Imagine a situation in which a couple thousand LSAs are refreshing at the same time.

Figure 9-88 shows the flowchart to follow to solve this problem.

Figure 9-88. Problem-Resolution Flowchart

Debugs and Verification

Example 9-243 shows the CPUHOG messages that appear in the router's log every 30 minutes.

Example 9-243 Router Is Seeing CPUHOG Messages Every 30 Minutes

 R1#  show log  %SYS-3-CPUHOG: Task ran for 2424 msec (15/15), process = OSPF Router %SYS-3-CPUHOG: Task ran for 2340 msec (10/9), process = OSPF Router %SYS-3-CPUHOG: Task ran for 2264 msec (0/0), process = OSPF Router 

Solution

LSA group pacing looks at the LSA every periodic interval (every 4 minutes, by default) and refreshes only those LSAs that are past their refresh time. This is an efficient way of reducing a large flood by chopping it down to smaller LSA floods. No extra configuration is required for this feature, but for large numbers of LSAs ( generally 10,000 or more), it is recommended to use small intervals (for example, every 2 minutes); for few 100s of LSAs, use a large interval, such as 20 minutes.

If 10,000 LSAs need to be refreshed, keeping the refresh interval smaller will check the LSA every 2 or 4 minutes to see how many LSAs have reached the refresh interval, which is 30 minutes. The advantage of checking this frequently is that fewer LSAs would need to be refreshed every 2 or 4 minutes, and this will not cause a huge storm of LSA updates. If the number of LSAs is small, it really doesn't matter whether the refresh occurs at 2 minutes or 20 minutes. That is why it's better to increase the timer so that all the LSAs that are few in number can be refreshed at once.

Example 9-244 shows how to configure the LSA refresh interval.

Example 9-244 Configuring the LSA Refresh Interval

 R1(config)#  router ospf 1   R1(config-router)#timer lsa-group-pacing ?  <10-1800>  Interval between group of LSA being refreshed or maxaged