Hello Packet in OSPFv2

Posted: 6th June 2020 by ccna7guru in ITN

The OSPF Type 1 packet is the Hello packet. Hello packets are used to do the following:

  • Discover OSPF neighbors and establish neighbor adjacencies.
  • Advertise parameters on which two routers must agree to become neighbors.
  • Elect the Designated Router (DR) and Backup Designated Router (BDR) on multiaccess networks like Ethernet. Point-to-point links do not require DR or BDR.

The figure displays the fields contained in the OSPFv2 Type 1 Hello packet.

OSPF Hello Packet Content

OSPF Hello Packet Content
OSPF Hello Packet Content

Important fields shown in the figure include the following:

  • Type – This identifies the type of packet. A value 1 indicates a Hello packet. A value 2 identifies a DBD packet, 3 an LSR packet, 4 an LSU packet, and 5 an LSAck packet.
  • Router ID – A 32-bit value expressed in dotted decimal notation (like an IPv4 address) is used to uniquely identify the originating router.
  • Area ID – This is the number of the area from which the packet originated.
  • Network Mask – This is the subnet mask associated with the sending interface.
  • Hello Interval – This specifies the frequency, in seconds, at which a router sends Hello packets. The default Hello interval on multiaccess networks is 10 seconds. This timer must be the same on neighboring routers; otherwise, an adjacency is not established.
  • Router Priority – This is used in a DR/BDR election. The default priority for all OSPF routers is 1, but can be manually altered from 0 to 255. The higher the value, the more likely the router becomes the DR on the link.
  • Dead Interval – This is the time, in seconds, that a router waits to hear from a neighbor before declaring the neighboring router 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 is not established.
  • Designated Router (DR) – This is the router ID of the DR.
  • Backup Designated Router (BDR) – This is the router ID of the BDR.
  • List of Neighbors – This list identifies the router IDs of all adjacent routers.