All OSPF link state advertisements (with the exception of network links advertisements) specify metrics. In router links advertisements, the metrics indicate the costs of the described interfaces. In summary link and AS external link advertisements, the metric indicates the cost of the described path. In all of these advertisements, a separate metric can be specified for each IP TOS. The encoding of TOS in OSPF link state advertisements is specified in Table 17. That table relates the OSPF encoding to the IP packet header's TOS field (defined in [RFC 1349]). The OSPF encoding is expressed as a decimal integer, and the IP packet header's TOS field is expressed in the binary TOS values used in [RFC 1349].
OSPF encoding RFC 1349 TOS values ___________________________________________ 0 0000 normal service 2 0001 minimize monetary cost 4 0010 maximize reliability 6 0011 8 0100 maximize throughput 10 0101 12 0110 14 0111 16 1000 minimize delay 18 1001 20 1010 22 1011 24 1100 26 1101 28 1110 30 1111 Table 17: Representing TOS in OSPF.
Each OSPF link state advertisement must specify the TOS 0 metric. Other TOS metrics, if they appear, must appear in order of increasing TOS encoding. For example, the TOS 8 (maximize throughput) metric must always appear before the TOS 16 (minimize delay) metric when both are specified. If a metric for some non-zero TOS is not specified, its cost defaults to the cost for TOS 0, unless the T-bit is reset in the advertisement's Options field (see Section 12.1.2 for more details).