Hello Packet
The OSPF Type 1 packet is the Hello packet. Hello packets are used to:
- 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 and Frame Relay. Point-to-point links do not require DR or BDR.
The figure displays the fields contained in the Type 1 Hello packet. Important fields shown in the figure include:
- Type - Identifies the type of packet. A one (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 (an IPv4 address) used to uniquely identifying the originating router.
- Area ID - Area from which the packet originated.
- Network Mask - Subnet mask associated with the sending interface.
- Hello Interval - 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 - 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 - 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) - Router ID of the DR.
- Backup Designated Router (BDR) - Router ID of the BDR.
- List of Neighbors - List that identifies the router IDs of all adjacent routers.
Click each of the highlighted fields in the figure for more information.