Hello and dead intervals

The OSPF Hello protocol is used to discover and maintain communications with neighboring routers. Hello packets are sent out at a regular interval for this purpose. The DR sends out the Hello packets. In a broadcast network, the multicast address of 224.0.0.5 is used to send out Hello packets. New routers on the network listen for and reply to these packets to join the OSPF area. If a new router never receives a Hello packet, other routers will not know it is there and will not communicate with it. However, once a new router is discovered the DR adds it to the list of routers in that area and it is integrated into the routing calculations.

Dead interval is the time other routers will wait before declaring a neighbor dead (offline). Setting a reasonable dead interval is very important. If this interval is too short, routers will be declared offline when they are just slow or momentarily inaccessible, and link-state updates will happen more than they need to, using more bandwidth. If the dead interval is too long, it will slow down network traffic overall if online routers attempt to contact offline ones instead of re-routing traffic.

FortiOS also supports OSPF fast-hello, which provides a way of sending multiple Hello packets per second. This is achieved by setting a dead-interval to one second. The hello-multiplier, which can be any number between 4 and 10, determines the number of Hello packets that will be sent every second. The CLI syntax for OSPF fast- hello follows:

config ospf-interface edit ospf1

set interface port1

set network-type broadcast set dead-interval 1

set hello-multiplier 4

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