To minimize the number of adjacencies, LSDBs, and related SPF and PRC computations that are performed, it is recommended that you have configured all Level 1 routers as Level 1 by using the is-type command . We recommend that you use the metric-style wide command because some features, such as setting prefix tags and MPLS traffic engineering, require that routers that are running IS-IS generate the new-style TLVs that have wider metric fields. If you use the default narrow metric style for IS-IS, the router generates and accepts old-style type, length, and value objects (TLVs).
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4305-4309). . Kindle Edition.
3.8.e [i] Metrics, wide metric
Cisco IOS Software allows for wide metrics with the support of a 24-bit metric field. Using the new metric style, link metrics now have a maximum value of 16777215 (2 24-1) with a total path metric of 4261412864 (254 x 2 24). Deploying IS-IS in the IP network with wide metrics is recommended to enable finer granularity and to support future applications such as Traffic Engineering.
Running different metric styles within one network poses a serious problem: Link-state protocols calculate loop-free routes because all routers (within one area) calculate their routing table based on the same link- state database. This principle is violated if some routers look at old-style (narrow), and some at new-style (wider) TLVs. However, if the same interface cost is used for both the old- and new-style metrics, then the SPF will compute a loop-free topology.
Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4317-4318). . Kindle Edition.