OSPF Hello Protocol and Packets Explained

OSPF is an advanced routing protocol. Unlike traditional distance vector routing protocols, which share routing information with all neighbors, OSPF first establishes neighborship and then shares routing information only with its neighbors.

An OSPF router uses the OSPF hello protocol to discover neighbors, establish adjacencies, and maintain relationships with other OSPF routers. The OSPF hello protocol periodically sends OSPF hello packets out of each enabled OSPF interface, and in environments that support multicast. Hello packets are also known as hello messages.

It sends hello messages to the address 224.0.0.5. It is a multicast address. It belongs to OSPF. Only OSPF-running routers listen to this address. Based on network type and topology, it uses different time intervals to send hello packets. In broadcast and point-to-point networks, it sends hello packets every 10 seconds. In non-broadcast and point-to-multipoint networks, it sends them every 30 seconds.

A Hello packet contains all the information that another OSPF router needs to decide whether it forms a neighborship with the sending router. The receiving router compares the hello packet's parameters with locally configured parameters. If both parameters match, it replies with its hello packet. Upon receiving this hello packet, the sending router sends another hello packet as acknowledgment. The receiving router also sends an acknowledgment hello packet to confirm the acknowledgment hello packet. At this stage, both routers are neighbors.

Let us take an example. The following image shows a network with two routers. OSPF is running on both. When this network starts, both routers send hello packets from all OSPF-running interfaces and hope to receive OSPF hello packets from other routers connected to those interfaces.

R1's hello packet reaches R2. It contains all the necessary parameters R2 needs to build neighborship. R2 matches the hello packet's parameters with its parameters. These parameters are known as the OSPF neighborship requirement. If the parameters match, R2 replies with a hello packet. The packet reaches R1. R1 sends another hello packet to acknowledge R2. At this stage, both routers become neighbors.

After becoming neighbors, routers continue to send hello packets at a specific interval, known as the hello interval. Routers use these hello packets to verify each other. If a router does not receive a hello packet from its neighbor after a specific time known as the dead interval, it removes the neighbor from the neighbor list. This way, OSPF routers use hello packets to build and maintain the neighborship.

Key points:-

• OSPF uses the hello protocol to send hello packets.
• OSPF uses hello packets to build and maintain neighborship.
• Hello packets use IP protocol type 89.
• Hello packets are sent to the multicast IP address 224.0.0.5.
• Only OSPF-speaking routers listen to IP address 224.0.0.5.
• A hello packet includes everything an OSPF router needs to build and maintain neighborship.

Conclusion

OSPF running routers share routing information only with neighbors. To become neighbors, they use hello packets. A hello packet includes all essential parameters an OSPF router needs to become a neighbor. After becoming OSPF neighbors, routers use it to maintain the neighborship.