Layer 2 Switching Commands 659CoS mapping tables, port default priority, and hardware queue parametersmay be configured on LAG interfaces as well as physical port interfaces.Queue MappingThe priority of a packet arriving at an interface is used to steer the packet tothe appropriate outbound CoS queue through a mapping table. Networkpackets arriving at an ingress port are directed to one of n queues in an egressport(s) based on the translation of packet priority to CoS queue. The CoSmapping tables define the queue used to handle each enumerated type ofuser priority designated in either the 802.1p User Priority, or IP DSCPcontents of a packet. If neither of these fields is trusted to contain ameaningful COS queue designation, the ingress port can be configured to useits default priority to specify the CoS queue.CoS queue mappings use the concept of trusted and untrusted ports.A trusted port is one that takes at face value a certain priority designationwithin arriving packets. Specifically, a port may be configured to trust one ofthe following packet fields:• IEEE 802.1p User Priority• IP Precedence• IP DSCPPackets arriving at the port ingress are inspected and their trusted field valueis used to designate the COS queue that the packet is placed when forwardedto the appropriate egress port. A mapping table associates the trusted fieldvalue with the desired COS queue.Alternatively, a port may be configured as untrusted, whereby it does not trustany incoming packet priority designation and uses the port default priorityvalue instead. All packets arriving at the ingress of an untrusted port aredirected to a specific COS queue on the appropriate egress port(s) inaccordance with the configured default priority of the ingress port. Thisprocess is also used for cases where a trusted port mapping is unable to behonored, such as when a non-IP packet arrives at a port configured to trustthe User Priority or IP DSCP value.
