D-Link xStack DES-3800 Series User Manual page 141

xStack DES-3800 Series Layer 3 Stackable Fast Ethernet Managed Switch
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:
Link-State Update Packet
4
Packet Length
Version No.
Router ID
Area ID
Checksum
Authentication Type
Authentication
Authentication
Number of Advertisements
Link-State Advertisements ...
Figure 8- 20. Link-State Update Packet
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:
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