Introduction to Hello Packets

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Hello, Packets Type 1 are OSPF packets sent periodically on all interfaces, including virtual links, to establish and maintain neighbor adjacency. It is a multicast packet with a multicast and broadcast capability, enabling dynamic discovery of neighboring routers. Hello packets advertise parameters on which routers agree to become neighbors. It also elects the Designated Router (DR) and Backup Designated Router (BDR) on multi-access networks. Point-to-point links do not require DR or BDR. The figure below illustrates the hello packet:

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  • Type—This is an eight-bit field that identifies the type of packet. It has three types: Type 1 indicates a Hello packet, Type 2 indicates a DBD packet, Type 3 indicates an LSR packet, Type 4 indicates an LSU packet and Type 5 is an LSAck packet.
  • Router ID—This is a 32-bit field expressed in dotted decimal notation that uniquely identifies the originating router.
  • Area ID– Area ID is also a 32 bit file that identifies the originated area of the packet.
  • Network Mask– Network mask associated with the sending interface

Note:– the above four fields are the part of OSPF header and the below six fields are the parts of OSPF hello Field.

  • Hello Interval– This field defines how often we send the hello packet. Hello interval also defines the frequency, in seconds, at which a router sends Hello packets. The default Hello interval on multi-access networks is 10 seconds. This hello-interval timer must be the same on neighbouring routers if not, an adjacency is not possible.
  • Router Priority– This field is used in a DR/BDR election. The default priority for all OSPF routers is 1 but we can also manually configure router priority from 0 to 255. The higher the value, the router becomes the DR on the link.
  • Dead Interval– This field defines how long we should wait for hello packets before declaring the neighbor dead and out of service. By default, the router Dead Interval is four times the Hello interval. This timer must be the same on neighboring routers; otherwise, an adjacency will not be established.
  • Designated Router (DR)– This is the Router ID of the DR.
  • Backup Designated Router (BDR)– This field defines the Router ID of the BDR.
  • List of Neighbors– This field identifies the router IDs of all adjacent routers.