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The TCP/IP Guide

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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  TCP/IP Routing Protocols (Gateway Protocols)
                9  TCP/IP Interior Routing Protocols (RIP, OSPF, GGP, HELLO, IGRP, EIGRP)
                     9  Open Shortest Path First (OSPF)

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OSPF General Operation and Message Types
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 Other Interior Routing Protocols
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OSPF Message Formats
(Page 2 of 7)

Specific 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.

Hello Message Format

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

 

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