OSPF Hello Protocol/ Packets

It is the protocol used by OSPF for several reasons 

This protocol discovers neighbors by sending Hello packets on OSPF speaking interfaces of a Router

Contents of a hello packet are as following

• Router ID for originating router (Router on which Packet was generated)

• Hello Timer for originating Router’s forwarding interface

• Dead Timer for originating Router’s forwarding interface

• Area ID of originating router’s interface where hello is being sent

• Network Mask of routers forwarding interface

• Neighbors of originating router

• Router’s priority of originating router

• DR/BDR Location (IP Address)

• Authentication information and type

There are several parameters in Hello packets which both routers should agree to become neighbors, agreeing routers will become neighbors and those who do not agree will simply discard the packets

If these parameters matches the hello packet is declared as valid and if originating router is not in neighbor table for receiving interface it is added into neighbor table otherwise dead timer is reset.

Router includes router ID for all neighbors for the link on which packet will be sent, in hello packet so if a router receives a Hello packet and finds his router ID listed, router knows that two-way communication is established.

Adjacency or neighbor relation can be formed only after two way communication is established not necessarily all neighbors will become adjacent, an adjacency will form or not also depends upon the type of network these two routers are attached with

Hello packets also works as keep alive packets

Hello Interval

• Hello packets are sent on all OSPF enabled interfaces on regular intervals called Hello Interval

• In Cisco default Hello interval is 10 seconds for broadcast networks and 30 seconds for non-broadcast networks

• Hello interval is configured on per interface basis

• Hello interval can be changed with ip ospf hello-interval command

Dead Interval

• If a router haven’t received a Hello packet from a neighbor within a period of time known as dead interval, it will declare the neighbor down

• In Cisco by default dead interval is four times to Hello interval

• Dead interval can be changed with ip ospf dead-interval command

• It ensures bidirectional communication between neighbors, as both the routers haven’t sent and received the Hello packets they won’t become neighbors

• It elects DR, BDR on broadcast and non-broadcast multi-access networks

For any correction, improvement or suggestion please do not hesitate to reach out at usman@cyberswats.com