A.4.4 Summary link advertisements

Summary link advertisements are the Type 3 and 4 link state advertisements. These advertisements are originated by area border routers. A separate summary link advertisement is made for each destination (known to the router) which belongs to the AS, yet is outside the area. For details concerning the construction of summary link advertisements, see Section 12.4.3.

Type 3 link state advertisements are used when the destination is an IP network. In this case the advertisement's Link State ID field is an IP network number (if necessary, the Link State ID can also have one or more of the network's "host" bits set; see Appendix F for details). When the destination is an AS boundary router, a Type 4 advertisement is used, and the Link State ID field is the AS boundary router's OSPF Router ID. (To see why it is necessary to advertise the location of each ASBR, consult Section 16.4.) Other than the difference in the Link State ID field, the format of Type 3 and 4 link state advertisements is identical.

        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   |    3 or 4     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Link State ID                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Advertising Router                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     LS sequence number                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         LS checksum           |             length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         Network Mask                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     TOS       |                  metric                       |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              ...                              |

For stub areas, Type 3 summary link advertisements can also be used to describe a (per-area) default route. Default summary routes are used in stub areas instead of flooding a complete set of external routes. When describing a default summary route, the advertisement's Link State ID is always set to DefaultDestination (0.0.0.0) and the Network Mask is set to 0.0.0.0.

Separate costs may be advertised for each IP Type of Service. The encoding of TOS in OSPF link state advertisements is described in Section 12.3. Note that the cost for TOS 0 must be included, and is always listed first. If the T-bit is reset in the advertisement's Option field, only a route for TOS 0 is described by the advertisement. Otherwise, routes for the other TOS values are also described; if a cost for a certain TOS is not included, its cost defaults to that specified for TOS 0.

Network Mask

For Type 3 link state advertisements, this indicates the destination network's IP address mask. For example, when advertising the location of a class A network the value 0xff000000 would be used. This field is not meaningful and must be zero for Type 4 link state advertisements.

For each specified Type of Service, the following fields are defined. The number of TOS routes included can be calculated from the link state advertisement header's length field. Values for TOS 0 must be specified; they are listed first. Other values must be listed in order of increasing TOS encoding. For example, the cost for TOS 16 must always follow the cost for TOS 8 when both are specified.

TOS

The Type of Service that the following cost concerns. The encoding of TOS in OSPF link state advertisements is described in Section 12.3.

metric

The cost of this route. Expressed in the same units as the interface costs in the router links advertisements.