Technical Tip: How to change the IS-IS metric style

This article describes how to change the metric style used for Intermediate System-Intermediate System (IS-IS) routing on the FortiGate.

IS-IS originally utilized a 10-bit path metric (which allowed for a total path cost of 1023), and each interface/link could be assigned a 6-bit metric value (from 1 to 63). See also: RFC 1195.

This small metric value became a limiting factor for growing networks, and so RFC 3784 (specifically with the addition of TLVs 22 and 135) extended IS-IS to allow support for a 24-bit default interface metric (from 1 to 16777214) with a maximum path metric of 32 bits (up to 4,261,412,864).

The former, smaller path metric is defined as narrow in IS-IS parlance, whereas the expanded path metric is referred to as wide. By default, the IS-IS metric style on the FortiGate is set to narrow, and this can be changed using set metric-style under config router isis:

FortiGate # config router isis

FortiGate (isis) # set metric-style ?

narrow                  Use old style of TLVs with narrow metric.
wide                    Use new style of TLVs to carry wider metric.
transition              Send and accept both styles of TLVs during transition.
narrow-transition       Narrow and accept both styles of TLVs during transition.
narrow-transition-l1    Narrow-transition level-1 only.
narrow-transition-l2    Narrow-transition level-2 only.
wide-l1                 Wide level-1 only.
wide-l2                 Wide level-2 only.
wide-transition         Wide and accept both styles of TLVs during transition.
wide-transition-l1      Wide-transition level-1 only.
wide-transition-l2      Wide-transition level-2 only.
transition-l1           Transition level-1 only.
transition-l2           Transition level-2 only.

In general, the recommendation is to use wide metric-style for IS-IS going forward, with the transition-related options being useful for cases where an older IS-IS network is still using narrow metrics and is in the process of being updated.

Note that it is still possible to set both narrow and wide metrics on IS-IS-enabled interfaces, even if the metric-style is set to only send/receive certain types/levels.  To set the metric for each interface, use the following options:

config router isis

    config isis-interface

        edit <name>

            set metric-l1 <0-63, default = 10>
            set metric-l2 <0-63, default = 10>
            set wide-metric-l1 <1-16777214, default = 10>
            set wide-metric-l2 <1-16777214, default = 10>

        next

    end

end

Related article:

Technical Tip: FortiGate Intermediate System-Intermediate System (IS-IS) configuration example