Introduction

IS IS uses two types of metric. These are used to calculate the path's cost as part of the SPF algorithm.

Metric

• There are two metric styles: narrow and wide and one state of transitional.
• ISIS does not automatically calculate the metric based on bandwidth link or delay like other protocols (OSPF or EIGRP).
• Additionally there is a transitional mode that can be used when migrating form narrow to wide.
• Total cost is the sum of all metric values for each outgoing interface to the destination.
• Interfaces can be exclude from SPF calculation using the command isis metric maximum.

Narrow

• Default metric type.
• Metric default is 10 for each interface.
• Range for narrow metric range for an interface is 1 to 63 (6 bits).
• The maximum total value of all hops can be 1023 (10 bits)

Wide

• Wide metric type expands the metric to use 24 bits for TLV-22.
• The new metric style, link metrics can have a maximum value of 16,777,215 with a total path metric of 4,261,412,864.
• Cisco recommends running the wide metric style.
• If there is a mismatch between metric styles, the adj will be maintained, but routes will not be accepted with different metric style.

Transitional

• One way to transition to a different metric-style is by configuring all routers to metric-style transitional.
• Once all routers are sent you can change to either narrow or wide style.
• During the transitional phase, both types will be recognized.

Relevant IOS Commands

metric

If set under the isis router configuration mode, specifies the default metric value for each level.
If set under the interface mode, set the metric of each interface.

R1(config-router)#        metric (cost) [ level-1 | level-2 ]
R1(config-if)#               isis metric (value |maximum) [ level-1 | level-2 ]

metric-style

Defines the type of metric. Use transitional when migrating to another style.

R1(config-router)#        metric-style (narrow | wide | transition) [level-1 | level-1-2 | level-2]

Show clns protocol

show clns protocol

R5#sh clns protocol 

IS-IS Router: <Null Tag>
  System Id: 0050.0500.5005.00  IS-Type: level-1
  Manual area address(es): 
    49.0045
  Routing for area address(es): 
    49.0045
  Interfaces supported by IS-IS:
    GigabitEthernet1/0 - IP
  Passive interface: 
    Loopback0
  Redistribute:
    static (on by default)
  Distance for L2 CLNS routes: 110
  RRR level: none
  Generate narrow metrics: level-1-2      <---metric info
  Accept narrow metrics:   level-1-2
  Generate wide metrics:   level-1
  Accept wide metrics:     level-1

Additional Resources

IP Routing Intermediate System-to-Intermediate System Protocol - Cisco
Learn ISIS Metric and Metric Style Part 7 - Rekrowten blog

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