The above criteria take effect only when the QoS trust mode of the port is
enabled. Enabling the QoS trust mode of a port does not mean getting the
QoS information directly from the packet or the input port of the packet
without analyzing the packet contents.
If the policy-map associated with the port is using the ACL classifying based on the mac
access-list extended, the associated ACLs will be matched by getting the source MAC
address, destination MAC address and Ethertype domain of the packet on that port, to
determine the DSCP value of the packet. Note that, if a port is associated with a
policy-map but has no DSCP value set for it, the device will assign the priority for the
packets of this classification through the default behavior: following the priority
information contained in the layer-2 packet header of the packet or the default priority of
the port.
The above three criteria may apply simultaneously on the same port. In
this case, they will take effect according to the sequence 3, then 2 and
then 1. In other words, the ACLs work first for the classifying operation.
When it fails, the criteria 2 will be used, and so on. Here, if the QoS trust
mode of the port is enabled, criteria 2 and 1 will be used to get the QoS
information directly from the packet or the port; otherwise, default DSCP
value 0 will be assigned for the packets failing the classifying operation.
For IP packets, the device classifies the packets according to the following criteria:
If the port trust mode is Trust ip-precedence, it extracts from the ip precedence field (3
bits) of the IP packet and fills the CoS field (3 bits) of the output packet.
If the port trust mode is Trust cos, it extracts directly the CoS field (3 bits) of the packet
and overwrite the ip precedence field (3 bits) of the packet. There are the following two
cases. Case 1 is that the layer-2 packet header does not contain User Priority bits, and
now the CoS value is got from the default CoS value of the packet input port. Case 2 is
that the layer-2 packet header contains User Priority bits, and now the CoS is got
directly from the packet header.
If the Policy-map associated with the port is using the ACLs classifying based on the ip
access-list (extended), the associated ACLs will be matched by getting the source IP
address, destination IP address, Protocol field and layer-4 TCP/UDP port field of the
packet, to determine the DSCP value of the packet, and the CoS value is determined
according to the mapping from DSCP to CoS. Note that, if a port is associated with a
policy-map but has no DSCP value set for it, the switch will use the above criteria 1 and
2 to determine the priority.
Just like the criteria for non-IP packet classifying, the above classifying criteria can apply on
the same port at the same time. In this case, they will take effect according to the sequence
3, then 2 and then 1.
For details of the CoS-to-DSCP map and IP-precedence-to-DSCP map, see the descriptions
below.
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