Configuring OSPF and OSPFv3 961LSA PacingOSPF refreshes each self-originated LSA every 30 minutes. Because a routertends to originate many LSAs at the same time, either at startup or whenadjacencies are formed or when routes are first learned, LSA refreshes tend tobe grouped. Further, Area Border Routers (ABRs) attached to the same areatend to originate summary LSAs into the area at the same time. This behaviorleads to periodic bursts of LS Update packets. Update bursts can lead to highCPU utilization, packet loss, and retransmission, if a receiver cannot absorball packets in a burst. These losses occur primarily in two places: 1) at theClass of Service (CoS) queue where the hardware queues packets to the CPU,and 2) when a message buffer is allocated for an incoming packet.This feature makes changes to OSPFv2 to improve the efficiency of LSAflooding, with the expectation that the improvements will greatly reduce oreliminate the packet drops caused by bursts in OSPF control packets. Thechanges are as follows:• Introduce LSA transmit pacing, limiting the rate of LS Update packetsthat OSPF can send• Introduce LSA refresh groups, so that OSPF efficiently bundles LSAs intoLS Update packets when periodically refreshing self-originated LSAsTo configure LSA transmit pacing, use the timers pacing flood command inrouter config mode:ABR-R0(config)#router ospfABR-R0(config-router)#timers pacing flood 50This will cause LSA Update packets to be sent at no less than a 50 millisecondinterval.When OSPF refreshes LSAs, it considers all self-originated LSAs whose age isfrom 1800 to 1800 plus the pacing group size. Grouping LSAs for refreshallows OSPF to combine refreshed LSAs into a minimal number of LSUpdate packets. Minimizing the number of Update packets makes LSAdistribution more efficient. To configure an LSA Refresh window, use thetimers pacing lsa-group command in router-config mode:ABR-R0(config)#router ospfABR-R0(config-router)#timers pacing lsa-group 300This sets the LSA Refresh window to 2100 seconds or about 35 minutes.
