Cisco Systems 78-11741-02 Blocking Ospf LSA Flooding, Changes the group pacing of LSAs, IPC-237

Configuring OSPF

Blocking OSPF LSA Flooding

Figure 40 OSPF LSAs on Individual Timers with Group Pacing

and at random intervals. Individual LSA timers require many refresh packets that contain few LSAs.

Individual LSA timers

20 LSAs, 1 packet

37 LSAs, 1 packet

15 LSAs, 1 packet

4 min 4 min 4 min É

Individual LSA timers with group pacing

10471

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 from 10 seconds to 1800 seconds (30 minutes). To change the LSA group pacing interval, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# timers lsa-group-pacing	Changes the group pacing of LSAs.
seconds

For an example, see the section “LSA Group Pacing Example” at the end of this chapter.

Blocking OSPF LSA Flooding

By default, OSPF floods new LSAs over all interfaces in the same area, except the interface on which the LSA arrives. Some redundancy is desirable, because it ensures robust flooding. However, too much redundancy can waste bandwidth and might destabilize the network due to excessive link and CPU usage in certain topologies. An example would be a fully meshed topology.

You can block OSPF flooding of LSAs two ways, depending on the type of networks:

•On broadcast, nonbroadcast, and point-to-point networks, you can block flooding over specified OSPF interfaces.

•On point-to-multipoint networks, you can block flooding to a specified neighbor.

On broadcast, nonbroadcast, and point-to-point networks, to prevent flooding of OSPF LSAs, use the following command in interface configuration mode:

Cisco IOS IP Configuration Guide

IPC-237