Ruijie RG-S2900G-E Series - Page 785

Configuration Guide QoS Configuration
of the ToS field for IPv4 packet header or Traffic Class field for IPv6 packet header,
called Differentiated Services Code Point (DSCP) value.
In a DiffServ-compliant network, every device has the same transmission service policy
for the messages with the same classification information, and vice versa. The class
information in the packet can be assigned by all the systems along the way, such as hosts,
devices, or other network devices. It’s based on a policy set by a manager, or contents in
the packet, or both. The assignment of class information in order to identify packets
usually consumes enormous resources of the network device. To reduce the processing
overhead on the backbone network, such assignment is often used on the network edge.
Based on the class information, the devices can provide different priorities for different
traffic, or limit the amount of resources allocated per traffic class, or appropriately discard
the packets of less important, or perform other operations as appropriate. This behavior of
these independent devices is call per-hop behavior in the DiffServ architecture.
If all devices in the network are providing consistent per-hop behavior, this network
forms the end-to-end QoS solution for the DiffServ architecture.
QoS processing flow
Classifying
The process of classifying involves putting the messages to the dataflow indicated with
CoS value according to the trust policy or the analysis of the message contents. As a result,
the core task of classifying is to determine the CoS value of a message. It happens when
the port is receiving the inbound messages. When a port is associated with a policy-map
that represents a QoS policy, the classification will take effect and be applied on all the
messages input through that port.
For general non-IP messages, the switch classifies the messages according to the
following criteria:
 If the message itself does not contain any QoS information, which means the layer-2
packet header has no User Priority bits, it gets the QoS information of the message
by using the default CoS value of the message input port. Like the User Priority bits
of the message, the default CoS value of the port ranges 0~7.
 If the message itself contains QoS information, which means the layer-2 packet
header has User Priority bits, it gets the CoS information directly from the message.
Note
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 message or the input port
of the message without analyzing the message 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
message on that port, to determine the DSCP value of the message. Note that, if a