D-Link DES-6500 - Switch User Manual page 160

Link-State Update Packet
Link-State Update packets are OSPF packet type 4. These packets implement the flooding of link-state advertisements. Each
Link-State Update packet carries a collection of link-state advertisements one hop further from its origin. Several link-state
advertisements may be included in a single packet.
Link-State Update packets are multicast on those physical networks that support multicast/broadcast. In order to make the
flooding procedure reliable, flooded advertisements are acknowledged in Link-State Acknowledgment packets. If retransmission
of certain advertisements is necessary, the retransmitted advertisements are always carried by unicast Link-State Update packets.
The format of the Link-State Update packet is shown below:
The body of the Link-State Update packet consists of a list of link-state advertisements. Each advertisement begins with a
common 20-byte header, the link-state advertisement header. Otherwise, the format of each of the five types of link-state
advertisements is different.
Link-State Acknowledgment Packet
Link-State Acknowledgment packets are OSPF packet type 5. To make the folding of link-state advertisements reliable, flooded
advertisements are explicitly acknowledged. This acknowledgment is accomplished through the sending and receiving of Link-
State Acknowledgment packets. Multiple link-state advertisements can be acknowledged in a single Link-State Acknowledgment
packet.
Depending on the state of the sending interface and the source of the advertisements being acknowledged, a Link-State
Acknowledgment packet is sent either to the multicast address AllSPFRouters, to the multicast address AllDRouters, or as a
unicast packet.
The format of this packet is similar to that of the Data Description packet. The body of both packets is simply a list of link-state
advertisement headers.
The format of the Link-State Acknowledgment packet is shown below:
Each acknowledged link-state advertisement is described by its link-state advertisement header. It contains all the information
required to uniquely identify both the advertisement and the advertisement's current instance.
xStack DES-6500 Modular Layer 3 Chassis Ethernet Switch User Manual
Link-State Update Packet
Version No.
Checksum
Figure 6- 125. Link-State Update Packet
Link-State Acknowledgment Packet
Version No.
Checksum
Link-State Advertisement Header ...
Figure 6- 126. Link State Acknowledge Packet
4
Packet Length
Router ID
Area ID
Authentication Type
Authentication
Authentication
Number of Advertisements
Link-State Advertisements ...
Packet Length
5
Router ID
Area ID
Authentication Type
Authentication
Authentication
145