D-Link DGS-3324SR Product Manual - Page 187
Link-State Acknowledgment Packet, Link-State Advertisement Formats
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xStack DGS/DXS-3300 Series Layer 3 Stackable Gigabit Ethernet Switch User Manual 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 linkstate advertisement headers. The format of the Link-State Acknowledgment packet is shown below: Link-State Acknowledgment Packet Version No. 5 Packet Length Router ID Area ID Checksum Authentication Type Authentication Authentication Link-State Advertisement Header ... Figure 6- 134. Link State Acknowledge Packet 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. Link-State Advertisement Formats There are five distinct types of link-state advertisements. Each link-state advertisement begins with a standard 20-byte linkstate advertisement header. Succeeding sections then diagram the separate link-state advertisement types. Each link-state advertisement describes a piece of the OSPF routing domain. Every router originates a router links advertisement. In addition, whenever the router is elected as the Designated Router, it originates a network links advertisement. Other types of link-state advertisements may also be originated. The flooding algorithm is reliable, ensuring that all routers have the same collection of link-state advertisements. The collection of advertisements is called the linkstate (or topological) database. From the link-state database, each router constructs a shortest path tree with itself as root. This yields a routing table. There are four types of link state advertisements, each using a common link state header. These are: • Router Links Advertisements • Network Links Advertisements • Summary Link Advertisements • Autonomous System Link Advertisements 172