2.4. TOS-based routing

OSPF can calculate a separate set of routes for each IP Type of Service. This means that, for any destination, there can potentially be multiple routing table entries, one for each IP TOS. The IP TOS values are represented in OSPF exactly as they appear in the IP packet header.

Up to this point, all examples shown have assumed that routes do not vary on TOS. In order to differentiate routes based on TOS, separate interface costs can be configured for each TOS. For example, in Figure 2 there could be multiple costs (one for each TOS) listed for each interface. A cost for TOS 0 must always be specified.

When interface costs vary based on TOS, a separate shortest path tree is calculated for each TOS (see Section 2.1). In addition, external costs can vary based on TOS. For example, in Figure 2 Router RT7 could advertise a separate type 1 external metric for each TOS. Then, when calculating the TOS X distance to Network N15 the cost of the shortest TOS X path to RT7 would be added to the TOS X cost advertised by RT7 for Network N15 (see Section 2.2).

All OSPF implementations must be capable of calculating routes based on TOS. However, OSPF routers can be configured to route all packets on the TOS 0 path (see Appendix C), eliminating the need to calculate non-zero TOS paths. This can be used to conserve routing table space and processing resources in the router. These TOS-0-only routers can be mixed with routers that do route based on TOS. TOS-0-only routers will be avoided as much as possible when forwarding traffic requesting a non-zero TOS.

It may be the case that no path exists for some non-zero TOS, even if the router is calculating non-zero TOS paths. In that case, packets requesting that non-zero TOS are routed along the TOS 0 path (see Section 11.1).