Normally, IS-IS metrics can have values up to 63. These metrics are narrow metrics. IS-IS generates two type, length, and value (TLV) codings, one for an IS-IS adjacency (code, length, and value (TLV) 2) and the second for an IP prefix (TLV 128 and TLV 130). During SPF, if the total cost of the path to a destination exceeds 1023, then according to ISO/IEC 10587, the path is ignored.
To overcome these restrictions, a second pair of TLVs is available, one for IP prefixes (TLV 135) and the second for IS-IS adjacency (TLV 22). With these TLVs, IS-IS metrics can have values up 16,777,215 and the maximum path metric allowed is 4,261,412,864. This allows more flexibility while designing a domain. These metrics are wide metrics.
NoteConfigured narrow and wide metrics for a particular interface must be identical in value while migrating from metric style narrow to metric style wide. Only if the metric style is “narrow” or “wide” (that is, no “transition”) is it okay to have different values (because one of the values is not used).