OSPF Interfaces 471Delay The transmit delay is the estimated time (in seconds) that it takes for thesystem to transmit a link state update packet on the interface. The systemincreases the age of the link state advertisements (LSAs) that arecontained in the update packets by the value that you specify for thedelay.This delay setting has more significance for interfaces that are connectedto very low speed links because, on slower speed links, it is moreprobable that the router may send out back-to-back data packets morequickly than other routers and hosts can receive them. To avoid thissituation, set the transmit delay to configure the router to wait a specifiednumber or seconds between transmissions.The delay value that you specify for an interface also increases the age ofall LSAs that are transmitted over the interface by the same value. Thissetting may also affect how soon the LSA is flushed from an area router’sdatabase. Reasons that an LSA is flushed from a router’s link statedatabase include:n LSA is overwritten by a newer instance of the LSA. For example,when a router receives similar LSAs (LSAs that have identical sequenceand checksums), it then compares the ages of each LSA, and storesthe LSA that has the least age value in the LSA database. This LSA isthen used for routing table calculations.n LSA ages out. When an LSA reaches the maximum age allowed bythe system, the router first refloods the LSA onto the network. Whenit is no longer needed to ensure database synchronization (forexample, when the LSA is no longer contained in neighbor LSAs), it isthen flushed from the database.Hello Interval The Hello interval (in seconds) determines how often the interfacetransmits Hello packets to other routers. The hello interval value must beidentical among all routers that are attached to a common network. Hellopackets notify other routers that the sending router is still active on thenetwork. If a router does not send Hello packets for the period of timethat is specified by the dead interval, that router is considered inactive byits neighbors, and all participating OSPF routers within the affected areasconverge on the new topology. Therefore, the smaller the Hello interval,the faster that topological changes are discovered; as a result, however,more routing traffic occurs. The default value for the Hello interval is10 seconds.
