Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 125
The Hello Protocol

Routers periodically transmit Hello packets to not only find other OSPF routers on their subnet but also to transmit and make sure that certain parameters are set to the same values within all the routers on that subnet. The Hello packet format is shown here. The Hello packet stays on the local subnet; it is not forwarded by the router. The Hello packet contains:

•  The router’s selection of the DR (designated router) and BDR (backup designated router)

•  The router’s priority used to determine the DR and BDR

•  Configurable timers that include the Hello Interval (time a router expects to hear hellos) and the RouterDeadInterval (the time period before a router is declared down)

•  A list of neighboring routers that this router has received hellos from

The most basic exchange between routers is called the Hello protocol. This protocol allows OSPF routers to discover one another (in a single area) and allows for the building of relationships between routers. This is the protocol that allows for the DR and BDR to be selected. Once the DR is selected, adjacencies are formed (discussed next).

The Hello Protocol

For multiaccess networks, when a router transmits a Hello packet it is sent using the ALL-SPF-Routers (which means all OSPF routers) multicast address of 224.0.0.5

OSPF routers build and maintain their relationships by periodic exchanges of Hello packets. Included in the transmitted Hello packets is a list of all the routers a router has heard from (i.e., received Hello packets from). When a router sees its address in a received Hello packet, it knows that the router that transmitted that packet has seen it. Once this is accomplished, the DR and the BDR are selected. Any DR with a priority of 0 counts itself out of the selection. There is one DR and DBR per subnet or LAN segment.

These packets are continually sent every Hello period specified in the packet. This is how a router can detect that another router is down (DeadInterval), which it uses to wait and build a new database with the Dykstra algorithm.