Allied Telesis AT-9724TS User Manual
Page 115
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
is for Virtual link endpoint).
E – bit
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
another router. Connection to a transit network. Connection to a stub network.Virtual link.
Link ID
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
network/subnet number. Neighboring router’s Router ID.
Link Data
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
needed during the routing table build process, when calculating the IP address of the next hop.
No. of TOS
The number of different Type of Service (TOS) metrics given for this link, not counting the required metric for
TOS 0. If no additional TOS metrics are given, this field should be set to 0.
TOS 0 Metric
The cost of using this router link for TOS 0.
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Allied Telesyn AT-9724TS High-Density Layer 3 Stackable Gigabit Ethernet Switch
Octets
Router Links Advertisements
Link-State ID
Advertising Router
Link-State Checksum
Length
Link-State Sequence Number
0
1
2
3
4
Link-State Age
Options
Link-State Type
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