Intermediate System-to-Intermediate System (IS-IS) is a dynamic routing protocol that operates within an autonomous system (or domain). IS-IS provides support for hierarchical routing, which enables you to partition large routing domains into smaller areas. When used separately from Shortest Path Bridging MAC (SPBM), IS-IS uses a two-level hierarchy, dividing the domain into multiple Level 1 areas and one Level 2 area. When used separately from SPBM, the Level 2 area serves as backbone of the domain, connecting to all the Level 1 areas. SPBM currently uses only Level 1 areas.
Important
The IEEE 802.1aq standard currently only defines the use of one hierarchy, Level 1. Level 2 function is disabled.
You can configure the Level 1 metric as 1 to 16,777,215.
You can also configure the Level 1 metric as auto. When you use this setting, the network route is determined by summing the lowest value metrics, which are inversely proportional to port speed. This ensures that the fastest port speed determines the network route when you configure IS-IS and SPBM.
The Auto parameter is supported on Auto-sense enabled ports also.
Any speed change within the interface causes the port to recalculate the L1 metric. In the case of MultiLink Trunking (MLT), this update is triggered by every individual port that forms the trunk.
Note
You cannot configure the value to auto on IS-IS logical interfaces because these interfaces only support a static metric configuration.
When updating from an earlier release, all L1 metrics previously configured on static interfaces are maintained. However, the auto-sense interfaces use the auto metric by default.
The L1 auto metric cannot be used with the statically configured L1 metric. When you enable the auto metric, the statically configured metric is deleted. When you enable a statically configured L1 metric, the auto metric is disabled.
To avoid conflicts, L1 auto metric should be set on both ends of the link. Otherwise, the metric is selected based on the IS-IS protocol - the highest value between the two peers.
The IS-IS L1 metric is calculated in megabits per second (Mbps), based on the operational link speed. For example: a L1 metric = 2 000 000 / link speed on a port with a speed of 1000 Mbps, then the L1 metric = 2000000 / 1000 = 2000.
The auto-sense IS-IS L1 metric settings are configured globally and are applied to all auto-sense ports where IS-IS is in use. All auto-sense ports will have the same L1 metric settings applied to them.
By default, the L1 auto metric is disabled on static IS-IS interfaces and enabled on auto-sense ports.