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2.2 | |
Single Area OSPF Concepts | |
| 2.2.6 | |
OSPF Hello protocol |
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When a router starts an
OSPF routing process on an interface, it sends a hello packet and
continues to send hellos at regular intervals. The rules that govern
the exchange of OSPF hello packets are called the Hello protocol. At Layer 3 of the OSI model, the hello packets are addressed to the multicast address 224.0.0.5. This address is “all OSPF routers”. OSPF routers use hello packets to initiate new adjacencies and to ensure that neighbor routers are still functioning. Hellos are sent every 10 seconds by default on broadcast multi-access and point-to-point networks. On interfaces that connect to NBMA networks, such as Frame Relay, the default time is 30 seconds. On multi-access networks the Hello protocol elects a designated router (DR) and a backup designated router (BDR). Although the hello packet is small, it
consists of the OSPF packet header.
The hello packet carries information
that all neighbors must agree upon before an adjacency is formed, and
link-state information is exchanged.
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For the
hello packet the type field is set to 1. 
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