Allied Telesis AT-9724TS Field Description Bit, Area, Field Description Type, Link ID, Link Data

Router Links Advertisements
Octets
0	1				2		3		4
	Link-State Age					Options		Link-State Type
					Link-State ID
				Advertising Router
				Link-State Sequence Number
Link-State Checksum							Length
Reserved	V	E	B	Reserved		Number of Links
					Link-ID
					Link Data
Type				No. of TOS			TOS 0 Metric
TOS				0			Metric
					...

TOS

0

Metric

...

Link-ID

Link Data

Figure 6- 107. Routers Links Advertisements

In router links advertisements, the Link State ID field is set to the router’s OSPF Router ID.The T-bit is set in the advertisement’s Option field if and only if the router is able to calculate a separate set of routes for each IP Type of Service (TOS). Router links advertisements are flooded throughout a single area only.

Field	Description
V – bit	When set, the router is an endpoint of an active virtual link that is using the described area as a Transit area (V
E – bit	is for Virtual link endpoint).
	When set, the router is an Autonomous System (AS) boundary router (E is for External).
B – bit	When set, the router is an area border router (B is for Border).
Number of Links	The number of router links described by this advertisement.This must be the total collection of router links to
	the area.

The following fields are used to describe each router link. Each router link is typed.The Type field indicates the kind of link being described. It may be a link to a transit network, to another router or to a stub network.The values of all the other fields describing a router link depend on the link’s Type. For example, each link has an associated 32-bit data field. For links to stub networks this field specifies the network’s IP address mask. For other link types the Link Data specifies the router’s associated IP interface address.

Field	Description
Type	A quick classification of the router link. One of the following:Type Description Point-to-point connection to
Link ID	another router. Connection to a transit network. Connection to a stub network.Virtual link.
	Identifies the object that this router link connects to.Value depends on the link’s Type.When connecting to an
	object that also originates a link state advertisement (i.e. another router or a transit network) the Link ID is
	equal to the neighboring advertisement’s Link State ID.This provides the key for looking up an advertisement in
	the link state database.Type Link ID Neighboring router’s Router ID. IP address of Designated Router. IP
Link Data	network/subnet number. Neighboring router’s Router ID.
	Contents again depend on the link’s Type field. For connections to stub networks, it specifies the network’s IP
	address mask. For unnumbered point-to-point connection, it specifies the interface’s MIB-II ifIndex value. For
	other link types it specifies the router’s associated IP interface address.This latter piece of information is
No. of TOS	needed during the routing table build process, when calculating the IP address of the next hop.
	The number of different Type of Service (TOS) metrics given for this link, not counting the required metric for
TOS 0 Metric	TOS 0. If no additional TOS metrics are given, this field should be set to 0.
	The cost of using this router link for TOS 0.

Allied Telesyn AT-9724TS High-Density Layer 3 Stackable Gigabit Ethernet Switch 114