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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 advertise-
ment (i.e. another router or a transit network) the Link ID is equal to the neigh-
boring advertisement Link’ s 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 net-
works, 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.
Field
Description
TOS
IP Type of Service that this metric refers to.
Metric The cost of using this outbound router link, for traffic of the specified
TOS.