Types of OSPF Packets

OSPF uses five different packets in its protocols. Table 3-2 describes the different OSPF packet types. This section discusses each protocol and the role that the packets play.

122 Chapter 3: OSPF Communication

Table 3-2 Overview of OSPF Packet Types

Packet Name

Type/Number

Function

Hello

1

Discovers and maintains neighbors.

Database description

2

Summarizes database contents.

Link state request

3

Requests LSAs that need to be downloaded to requesting router. Only sent during Exchange, Loading, or Full state.

Link state update

4

Contains a list of the LSAs that are to be updated. Often used in flooding, as discussed later in this chapter.

Link state acknowledgment

5

Acknowledges the packets sent out during flooding to ensure efficient use of floods.

All OSPF packets share a common header. Figure 3-21 illustrates a breakdown (by field) of the common header found at the beginning of each packet issued by an OSPF subprotocol (Hello, exchange, flooding). In OSPF, these packets are sent out to the multicast address (224.0.0.5) that is listened to only by OSPF routers.

Figure 3-21 Common OSPF Packet Header

0 12 3 01234567890123456789012345678901

Version #

Type

Packet Length

Router ID

Area ID

Checksum

AuType

Authentication

Authentication

The OSPF version number is set in the header and it is currently at version 2. The type of packet allows OSPF to correctly interpret the packet when it is received (type shown in Table 3-2). The packet length is expressed in bytes and is the total size of the packet including the header.

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