Allied Telesis AT-9724TS Link-State Request Packet, Field Description Options, Bit, MS bit

	OSPF Database Description Packet
	Octets
0		1					2	3	4
	Version No.						2	Packet Length
							Router ID
							Area ID
			Checksum					Authentication Type
							Authentication
							Authentication
	Reserved		I	M	MS		Reserved	Options
							DD Sequence No.
							Link-State Advertisement Header
						Figure 6- 102. Database Description Packet
Field		Description
Options		The optional capabilities supported by the router.
I – bit		The Initial bit.When set to 1, this packet is the first in the sequence of Database Description packets.
M – bit		The More bit.When set to 1, this indicates that more Database Description packets will follow.
MS – bit		The Master Slave bit.When set to 1, this indicates that the router is the master during the Database Exchange
DD Sequence Number		process.A zero indicates the opposite.
		User to sequence the collection of Database Description Packets.The initial value (indicated by the Initial bit
		being set) should be unique.The DD sequence number then increments until the complete database
		description has been sent.

The rest of the packet consists of a list of the topological database’s pieces. Each link state advertisement in the database is described by its link state advertisement header.

Link-State Request Packet

Link-State Request packets are OSPF packet type 3.After exchanging Database Description packets with a neighboring router, a router may find that parts of its topological database are out of date.The Link-State Request packet is used to request the pieces of the neighbor’s database that are more up to date. Multiple Link-State Request packets may need to be used.The sending of Link-State Request packets is the last step in bringing up an adjacency.

A router that sends a Link-State Request packet has in mind the precise instance of the database pieces it is requesting, defined by LS sequence number, LS checksum, and LS age, although these fields are not specified in the Link-State Request packet itself.The router may receive even more recent instances in response.

The format of the Link-State Request packet is shown below:

Allied Telesyn AT-9724TS High-Density Layer 3 Stackable Gigabit Ethernet Switch 110