Hello Packets

Hello Type 1 packets are OSPF packet which sent periodically on all interfaces including virtual links to  establish and maintain neighbor adjacency. It is a multicast packet having a multicast and broadcast capability also enabling dynamic discovery of neighboring routers. Hello packet advertise parameters on which routers agree to become neighbors. It also elect 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 illustrate the hello packet:

  • Type– This is eight bit field Identifies the type of packet. It has three type: Type 1 indicates a Hello packet. Type 2 indicate a DBD packet and Type 3 packet indicate an LSR packet, Type 4 indicate 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 field 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 neighboring 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 prior to declare 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 do not established.
  • Designated Router (DR)– This is the Router ID of the DR.
  • Backup Designated Router (BDR)– This field define the Router ID of the BDR.
  • List of Neighbors– This field identifies the router IDs of all adjacent routers.