A.4.2 Router links advertisements

Router links advertisements are the Type 1 link state advertisements. Each router in an area originates a router links advertisement. The advertisement describes the state and cost of the router's links (i.e., interfaces) to the area. All of the router's links to the area must be described in a single router links advertisement. For details concerning the construction of router links advertisements, see Section 12.4.1.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            LS age             |     Options   |       1       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Link State ID                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Advertising Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     LS sequence number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |             length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    0    |V|E|B|        0      |            # links            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Link ID                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Link Data                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Type      |     # TOS     |        TOS 0 metric           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      TOS      |        0      |            metric             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      TOS      |        0      |            metric             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Link ID                              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Link Data                             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |

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 TOS. Router links advertisements are flooded throughout a single area only.

bit V

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).

bit E

When set, the router is an AS boundary router (E is for external)

bit B

When set, the router is an area border router (B is for border)

# links

The number of router links described by this advertisement. This must be the total collection of router links (i.e., interfaces) to the area.

The following fields are used to describe each router link (i.e., interface). Each router link is typed (see the below Type field). 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.

Type

A quick description of the router link. One of the following. Note that host routes are classified as links to stub networks whose network mask is 0xffffffff.

                 Type   Description
                 __________________________________________________
                 1      Point-to-point connection to another router
                 2      Connection to a transit network
                 3      Connection to a stub network
                 4      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 said advertisement in the link state database. See Section 12.2 for more details.

                       Type   Link ID
                       ______________________________________
                       1      Neighboring router's Router ID
                       2      IP address of Designated Router
                       3      IP network/subnet number
                       4      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 connections, it specifies the interface's MIB-II [RFC 1213] ifIndex value. For the 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. See Section 16.1.1 for more details.

# TOS

The number of different TOS metrics given for this link, not counting the required metric for TOS 0. For example, 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.

For each link, separate metrics may be specified for each Type of Service (TOS). The metric for TOS 0 must always be included, and was discussed above. Metrics for non-zero TOS are described below. The encoding of TOS in OSPF link state advertisements is described in Section 12.3. Note that the cost for non-zero TOS values that are not specified defaults to the TOS 0 cost. Metrics must be listed in order of increasing TOS encoding. For example, the metric for TOS 16 must always follow the metric for TOS 8 when both are specified.

TOS

IP Type of Service that this metric refers to. The encoding of TOS in OSPF link state advertisements is described in Section 12.3.

metric

The cost of using this outbound router link, for traffic of the specified TOS.