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:

OSPF Link-State Acknowledgement Packet

 

 

 

Octets

 

 

 

 

 

 

0

1

2

3

4

 

 

 

 

 

 

 

Version No.

 

5

 

Packet Length

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Router ID

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Area ID

 

 

 

 

 

 

 

 

 

 

Checksum

Authentication Type

Authentication

Authentication

Link-State Advertisement Header

Figure 6- 105. 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 link-state 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 link- state (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

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