Alternate Priority

The congestion notification alternate priority is a non-CNPV used to protect the congestion notification domain from a non-congestion controlled flow packet with the same priority as a configured CNPV on the port from triggering congestion notification. At least one 802.1p priority on a port must be a non-CNPV. Any non-CNPV can be used as a congestion notification alternate priority.

When a packet ingresses a port at the edge of a congestion notification domain and has the same priority as a CNPV configured on the ingress port, the packet‘s priority must be remapped to an alternate priority. Should a non-congestion notification packet trigger congestion in a CNPV queue, the source for this packet will not know what to do with the CNM PDU it receives back from the congestion point. The remapping of the priority to a non-CNPV value at the congestion notification domain edge guards against this possibility.

An alternate priority can be set both globally or on a port basis. The global alternate priority is only used if the port-priority choice set using the set dcb cn port-priority choice command is set to default and the priority choice set using the set dcb cn priority choice command is set to admin. The port-priority alternate priority is only used if the port-priority choice is set to admin. (See Priority Choice for a priority choice discussion.) This administratively set global or port-based alternate priority defaults to 0. Otherwise, the auto alternate priority is used. The auto alternate priority for a CNPV defaults to either the next lowest non-CNPV priority if a lower one exists on the port or the next highest non-CNPV priority on the port.

Use the set dcb cn priority alt-pri command to change the global alternate priority default.

Use the set dcb cn port-priority alt-pri command to change the alternate priority default on a port basis. This command overrides any global configuration for the specified port.