16.9. Building the non-zero-TOS portion of the routing table

The OSPF protocol can calculate a different set of routes for each IP TOS (see Section 2.4). Support for TOS-based routing is optional. TOS-capable and non-TOS-capable routers can be mixed in an OSPF routing domain. Routers not supporting TOS calculate only the TOS 0 route to each destination. These routes are then used to forward all data traffic, regardless of the TOS indications in the data packet's IP header. A router that does not support TOS indicates this fact to the other OSPF routers by clearing the T-bit in the Options field of its router links advertisement.

The above sections detailing the routing table calculations handle the TOS 0 case only. In general, for routers supporting TOS-based routing, each piece of the routing table calculation must be rerun separately for the non-zero TOS values. When calculating routes for TOS X, only TOS X metrics can be used. Any link state advertisement may specify a separate cost for each TOS (a cost for TOS 0 must always be specified). The encoding of TOS in OSPF link state advertisements is described in Section 12.3.

An advertisement can specify that it is restricted to TOS 0 (i.e., non-zero TOS is not handled) by clearing the T-bit in the link state advertisement's Option field. Such advertisements are not used when calculating routes for non-zero TOS. For this reason, it is possible that a destination is unreachable for some non-zero TOS. In this case, the TOS 0 path is used when forwarding packets (see Section 11.1).

The following lists the modifications needed when running the routing table calculation for a non-zero TOS value (called TOS X). In general, routers and advertisements that do not support TOS are omitted from the calculation.

Calculating the shortest-path tree (Section 16.1).

Routers that do not support TOS-based routing should be omitted from the shortest-path tree calculation. These routers are identified as those having the T-bit reset in the Options field of their router links advertisements. Such routers should never be added to the Dijktra algorithm's candidate list, nor should their router links advertisements be examined when adding the stub networks to the tree. In particular, if the T-bit is reset in the calculating router's own router links advertisement, it does not run the shortest-path tree calculation for non-zero TOS values.

Calculating the inter-area routes (Section 16.2).

Inter-area paths are the concatenation of a path to an area border router with a summary link. When calculating TOS X routes, both path components must also specify TOS X. In other words, only TOS X paths to the area border router are examined, and the area border router must be advertising a TOS X route to the destination. Note that this means that summary link advertisements having the T-bit reset in their Options field are not considered.

Examining transit areas' summary links (Section 16.3).

This calculation again considers the concatenation of a path to an area border router with a summary link. As with inter-area routes, only TOS X paths to the area border router are examined, and the area border router must be advertising a TOS X route to the destination.

Calculating AS external routes (Section 16.4).

This calculation considers the concatenation of a path to a forwarding address with an AS external link. Only TOS X paths to the forwarding address are examined, and the AS boundary router must be advertising a TOS X route to the destination. Note that this means that AS external link advertisements having the T-bit reset in their Options field are not considered.

In addition, the advertising AS boundary router must also be reachable for its advertisements to be considered (see Section 16.4). However, if the advertising router and the forwarding address are not one in the same, the advertising router need only be reachable via TOS 0.