When using a LAG, it is possible to take an operational link degradation into consideration by setting a configurable degradation threshold. The following alternative settings are available through configuration:
port-threshold value
weight-threshold value
hash-weight-threshold value
When the LAG operates under normal circumstances and is included in an IS-IS or OSPF routing instance, the LAG must be associated with an IGP link cost. This LAG cost can either be statically configured in the IGP context or set dynamically by the LAG based upon the combination of the interface speed and reference bandwidth.
Under operational LAG degradation however, it is possible for the LAG to set a new updated dynamic or static threshold cost taking the gravity of the degradation into consideration.
As a consequence, there are some IGP link cost alternatives available, for which the most appropriate must be selected. The IGP uses the following priority rules to select the most appropriate IGP link cost:
Static LAG cost (from the LAG threshold action during degradation)
Explicit configured IGP cost (from the configuration under the IGP routing protocol context)
Dynamic link cost (from the LAG threshold action during degradation)
Default metric (no cost is set anywhere)
For example:
Static LAG cost overrules the configured metric.
Dynamic cost does not overrule configured metric or static LAG cost.