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Classes of Service
RS400 185 ROS™ v3.5
7 Classes of Service
ROS™ CoS provides the following features:
• Support for 4 Classes of Service
• Ability to prioritize traffic by ingress port.
• Ability to prioritize traffic by the priority field in 802.1Q tags.
• Ability to prioritize traffic based on its source or destination MAC address.
• Ability to prioritize traffic by the TOS field in the IP header.
7.1 CoS Operation
CoS provides the ability to expedite the transmission of certain frames and port traffic over
others.
The CoS of a frame can take on one of four values: Normal, Medium, High or Critical.
The default policies of the switch enforce a Normal CoS for all traffic.
Note: Use the highest supported CoS with caution, as it is always used by the switch for handling
network management traffic such as RSTP BPDUs.
If this CoS is used for regular network traffic, upon traffic bursts, it may result in loss of some
network management frames which in its turn may result in loss of connectivity over the network.
The CoS feature has two main phases, inspection and forwarding:
7.1.1 Inspection Phase
In the inspection phase the CoS priority of a received frame is determined from:
• The priority field in 802.1Q tags
• The Differentiated Services Code Point (DSCP) component of the Type Of Service (TOS)
field, if the frame is IP.
• The default CoS for the port.
• A specific CoS based upon the source and destination MAC address (as set in the Static
MAC Address Table).
Note that a frame’s CoS will be determined once the first examined parameter is found in the
frame.
Received frames are first examined to determine if their destination or source MAC address is
found in the Static MAC Address Table. If yes, the CoS configured for the static MAC address
is used. If neither destination or source MAC address is in the Static MAC Address Table, the
frame is then examined for 802.1Q tags and the priority field is mapped to a CoS. If a tag is not
present, the frame is examined to determine if it is an IP frame. If the frame is IP and
inspecting TOS is enabled, the CoS is determined from the DSCP field. If the frame is not IP or
inspecting TOS is disabled, the default CoS for the port is used.