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Route Tag Structure

On E-series routers, an IS-IS route tag is a 32-bit (4-octet) nonzero number that is stored as sub-TLV 1 inside the extended IP reachability TLV (type 135). TLV type 135, in turn, is part of an IS-IS LSP. The route tag is therefore advertised when LSPs are transmitted in an IS-IS network.

Because TLV type 135 is a new-style TLV tuple, it has a data structure that includes a wide metric field of four octets. As a result, to use IS-IS route tags you must issue the metric-style wide command (in Router Configuration mode) to specify that the router generate and accept only new-style TLV tuples.

For a discussion of IS-IS support for TLV tuples, see Extensions for Traffic Engineering.

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