Modify the LSA group step

The ospf lsa step-by-step feature allows the router to organize OSPF connection status notifications and set the pace for refreshing, verifying, and survival. The result of the group step is more effective with the vro.
The router organizes the ospf lsa together and sets the pace for the refresh, checksum, and survival functions to prevent impact on CPU and network resources. This feature is useful for large OSPF networks. The ospf lsa group does not require time-saving. For common users, the default set-step interval is suitable for refreshing, checksum, and survival. You do not need to configure this feature.
I. Original LSA features

Each ospf lsa has a lifetime, indicating whether the LSA is still valid. Once the LSA reaches the maximum lifetime of 1 hour), it will be discarded. During the lifetime, the source router sends a refresh packet every 30 minutes to refresh the LSA. Send a refresh package to prevent LSA from expiration, regardless of whether the network topology is changed. The Checksum is completed on all LSA every 10 minutes. The router keeps track of the LSA it generates and the LSA it receives from other routers. Refresh the LSA generated by the vro, and calculate the LSA lifetime received from other vrouters.
Before the LSA group's step-by-step feature is available, Cisco IOS completes refresh on one timer and checksum and Lifetime Calculation on the other timer. For example, when refreshing, the software scans the entire database every 30 minutes and refreshes every LSA generated by the router, no matter how old it is. Figure 11-1 indicates that all LSAs are refreshed immediately. This process wastes CPU resources because only a small number of databases need to be refreshed. A large OSPF database contains thousands of LSAs, which have different lifetime. Refresh on a timer causes synchronization of all LSA lifetime, resulting in multiple CPU processes immediately. Moreover, a large number of LSAs can also cause a sharp increase in network transmission volumes, consuming a large amount of network resources in a short time.
All LSAs are refreshed. Three packets are required for 120 external LSAs on the Ethernet.

All LSAs are refreshed immediately before the final step.
Figure 1 ospf lsa with no set steps on a single Timer
Ii. Solution

This problem is solved by enabling each LSA to have its own timer. We also use this refresh example to refresh each LSA after 30 minutes, regardless of other LSA. Therefore, the CPU is used only when necessary. However, frequent, random, and refresh LSA requires many packages provided by those routers that must be sent out and rarely refreshed. This will reduce the bandwidth utilization.
During frequent and random intervals, another LSA needs to be refreshed. The refreshed package only contains several LSA single LSA timers.

Single LSA timer with set steps
Figure 2 ospf lsa with fixed steps on a single Timer
Therefore, a router delay interval is used to replace a single timer until the LSA refresh function is completed. The accumulated LSA is a group, refreshed, and sent out of one or more packages. In this way, the refresh package step, checksum, and computing lifetime are also the same. The set step interval is configurable and the default value is 4 minutes. This can be avoided at will.
The set interval is inversely proportional to the number of LSA instances whose vrouters are refreshing, performing checksum, and calculating the lifetime. For example, if there are nearly 10,000 LSAs, it is helpful to reduce the fixed interval. If there is a small database 40 ~ 100 LSA), then the set interval is increased to 10 ~ 20 minutes will be helpful.
The default value of the step between LSA groups is 240 seconds and 4 minutes ). Value Range: 10 seconds to 1800 seconds ). To modify the interval of the LSA group, run the following command in vro configuration mode:

Command	Function
Lsa-group-pacing seconds	Modify the LSA group step.