Introduction to Hello Packets

Hello Packets Type 1 are OSPF packet which sent periodically on all interfaces including virtual links to establish and maintain neighbour adjacency. It is a multicast packet having a multicast and broadcast capability also enabling dynamic discovery of neighbouring routers. Hello packets advertises parameters on which routers agree to become neighbours. 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:

Hello Packets

  • Type– This is eight-bit field Identifies the type of packet. It has three types: Type 1 indicates a Hello packet. Type 2 indicates a DBD packet and Type 3 packet indicate an LSR packet, Type 4 indicates an LSU packet, and Type 5 an LSAck packet.
  • Router ID– This is 32- bit field expressed; in dotted decimal notation used to uniquely identifying the originating router.
  • Area ID– Area ID is also a 32 bit filed 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 neighbour dead and out of service. By default, the router Dead Interval is four times the Hello interval. This timer must be the same on neighbouring routers otherwise, an adjacency does not establish.
  • 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.