17-1 timers lsa-group-pacing seconds

17-1 timers lsa- group -pacing seconds

Syntax Description:

• seconds ” The minimum time between sending groups of timer- expired link-state advertisements (LSAs). The range of values is 10 to 1800 seconds. The default time is 240 seconds.

Purpose: Each OSPF LSA has an age parameter indicating if it is still valid. When the age of an LSA is equal to the maximum age (1 hour ), the LSA is discarded. In order to refresh LSAs, the originating router will send the LSAs every 30 minutes. This periodic refresh process prevents the LSAs from reaching the maximum age and therefore prevents the LSAs from being discarded. LSA refreshing occurs even if the LSA has not changed. In addition, a checksum is performed on all LSAs every 10 minutes. If an LSA is discarded, this means that the information it contained is no longer in the OSPF database. Routes based on the discarded LSA will be removed from the IP routing table.

Before the LSA group-pacing feature was available, refreshing was performed using a single timer. Another timer was used for the checksum and aging function. Every 30 minutes an OSPF router would scan the entire database and refresh every LSA that was originated by the router. This process was CPU intensive because every LSA, after being refreshed, would have identical age timers. After the LSA age timers were refreshed, the router would transmit the LSAs to neighboring OSPF routers. This resulted in periodically high network usage for large OSPF networks. LSA pacing was developed to solve the problem of periodic high CPU usage and network utilization.

Each LSA now has its own timers and LSAs are refreshed independently of other LSAs at random intervals. This paces the refreshing and transmitting of LSAs, which spreads out the CPU and network loads. For most situations the default pacing interval is sufficient. Decreasing the pacing interval may be of benefit for large OSPF networks.

Initial Cisco IOS Software Release: 11.3 (AA)

Configuration Example: Modifying the LSA Group Pacing

LSA group pacing can be modified with the timers lsa-group-pacing router configuration command as shown in the following listing.

 Router A  router ospf 1   log-adjacency-changes    timers lsa-group-pacing 180    network 1.1.1.1 0.0.0.0 area 1   network 10.1.1.0 0.0.0.3 area 0  

Verification

The LSA pacing interval can be verified with the show ip ospf timer lsa-group command.

 rtrA#  show ip ospf timer lsa-group  OSPF Router with ID (1.1.1.1) (Process ID 1) Group size 6, Head 0, Search Index 4,  Interval 180 sec  Next update due in 00:00:22 Current time 134571 Index 0 Timestamp 134593 Index 1 Timestamp 134777 Index 2 Timestamp 134971 Index 3 Timestamp 135153 Index 4 Timestamp 135351 Index 5 Timestamp 135544