Configuration > Quality of Service

Differential Services

IP packets coming from an upstream device or application include a Type of Service (ToS) byte containing a DSCP (codepoint) that can be used for QoS prioritization. Differential services uses the DSCP in the IP packet to assign an 802.1p priority to the packet. You can also assign a new DSCP to the packet, which can be carried (along with the 802.1p priority) in the packet to downstream devices.

You can use this option to read the DSCP of an incoming IPv4 packet and, without changing this codepoint, assign the 802.1p priority to the packet. This means that a priority value of 0-7 must be configured for a DSCP before the switch will attempt to perform a QoS match on the packet’s DSCP bits.

To enable Differential Services:

  1. Identify the DSCP in packets received from an upstream or edge switch.

  2. Assign the DSCPs identified in step 1 to a different DSCP policy or No-override, as explained below.

  3. If QoS is using a DSCP policy, you must delete or change this assignment before you can change the priority setting on the codepoint. Otherwise, the switch blocks the change.

  1. Assign the priority you want to use for the DSCP policy.

  1. Select Quality of Service from the Configuration tab, and then select DSCP Policy Table from the left pane.

  2. Select the codepoint for the DSCP policy. This codepoint will be written to the header of every packet containing the codepoint to which the DSCP policy is assigned.

  3. Use the Priority drop-down menu to select the priority you want to assign to the DSCP policy. Possible values are 0-7 and No Override.

  4. By default, the priority is No-override for all codepoints except 101110, which means that a priority has not been assigned. When the assignment is No-override, QoS does not affect the packet queuing priority or VLAN tagging, and the packets are handled as follows:

802.1Q Status

Outbound 802.1p Priority

Received and forwarded on a tagged VLAN


Received on untagged VLAN and forwarded on tagged VLAN

0 (normal)

Forwarded on untagged VLAN


  1. Assign the DSCP policy to the codepoint in inbound packets.

  2. Assigning DSCP policies requires knowledge of the codepoints set in IP packets by the upstream devices and applications.

  1. Select Quality of Service from the Configuration tab, and then select DiffServe TOS from the left pane.

  2. Select the codepoint (in inbound packets) that will use the DSCP policy.

  3. Use the DSCP Policy drop-down menu to select the DSCP policy you want to use to prioritize inbound packets with the selected codepoint.

  1. Enable differential services.

  1. Select Quality of Service from the Configuration tab, and then select Type of Service from the left pane.

  2. Click the Type of Service drop-down arrow and select Differential Services.

  1. Click Apply Changes.

Differential Services Operating Notes

Different applications may use the same DSCP in their IP packets. Also, the same application may use multiple DSCPs if the application originates on different clients, servers, or other devices. Using an edge switch enables you to select the packets you want and mark them with predictable DSCPs that can be used by downstream switches to honor policies set in the edge switch.

Differential Services associates a DSCP policy with a specific DSCP (codepoint) in an incoming IPv4 packet.  (A DSCP policy consists of the codepoint to be added to the outbound packet and an associated 802.1p priority.) This allows the new priority and, optionally, a new DSCP to be sent to downstream devices.

For additional information, refer to the Advanced Traffic Management Guide for your switch.