The TCP/IP Guide - OSPF Message Formats

Following this header, the body of the message includes a variable number of fields is included that depends on the message type. Each of the message formats is described in detail in RFC 2328; since some are quite long I will describe their fields only briefly here.

These messages have a Type value of 1 in the header, and the field structure shown in Table 127 and Figure 185 in the body of the message.

**Table 127: OSPF *Hello* Message Format**
Field Name	Size (bytes)	Description
*Network Mask*	4	*Network Mask:* The subnet mask of the network the router is sending to.
*Hello Interval*	2	*Hello Interval:* The number of seconds this router waits between sending Hello messages.
*Options*	1	*Options:* Indicates which optional OSPF capabilities the router supports.
*Rtr Pri*	1	*Router Priority:* Indicates the router's priority, when electing a backup designated router.
*Router Dead Interval*	4	*Router Dead Interval:* The number of seconds a router can be “silent” before it is considered to have failed.
*Designated Router*	4	*Designated Router:* The address of a router designated for certain special functions on some networks. Set to zeroes if there is no designated router.
*Backup Designated Router*	4	*Backup Designated Router:* The address of a backup designated router. Set to all zeroes if there is no backup designated router.
*Neighbors*	Multiple of 4	*Neighbors:* The addresses of each router from which this router has received Hello messages recently.

**Figure 185: OSPF *Hello* Message Format**

OSPF General Operation and Message Types

Other Interior Routing Protocols

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