IS-IS - Maximum composite metrics
In the IS-IS protocol, understanding the maximum values for composite metrics can be complex due to varying specifications and implementations:
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Default IS-IS Metrics:
- IS-IS uses a default metric of 10 for links.
- The maximum metric value for a single link is 63.
- Cisco devices can only use the default of 10, and cannot be modified.
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Wide Metrics:
- Introduced to support larger networks and finer path selection control.
- The maximum metric for a single link is extended to 16,777,215 (2^24-1).
- Composite metrics, which are the sum of individual link metrics along a path, can vary.
Cisco's ENSLD 2.0 documentation mentions maximum composite values of 1024 and 64, which may be specific to Cisco's implementation or configuration context. This may simply mean that they can be represented using 10 bits and 5 bits respectively, giving 1024 and 64 values, referring to the ranges of 0 - 1023 and 0 - 63.
Alternatively, values of 1023 and 63 are seen in older or default configurations without wide metric extensions. The values 1024 and 64 might reference specific configurations or hardware/software limits.
RFC 1195 suggests a maximum of 63, but vendor-specific implementations may differ. For Cisco devices, it is advisable to refer to Cisco's documentation.
Links
https://networklessons.com/cisco/ccie-routing-switching-written/is-is-metric-on-cisco-ios https://datatracker.ietf.org/doc/html/rfc1195#page-17