Layer 3 DSCP marking

By changing or honoring the settings of the DSCP codepoint in IP packet headers, QoS allows you to control the DSCP and associated 802.1p priority values in outbound IP packets that are sent to downstream devices.

You can later configure downstream devices to read and use the DSCP policy that QoS sets. When marking the DSCP bits in IP packets, a QoS policy is not dependent on VLAN-tagged ports to carry 802.1p packet priorities to downstream devices.

When configuring a Layer 3 DSCP policy, specify:
  • Bit values for the DSCP codepoint (the upper six bits in the ToS/Traffic Class byte in IP packet headers), entered in either binary format, the decimal equivalent, or an ASCII standard (hexadecimal) name

  • An 802.1p priority value that is associated with the new DSCP bit valuesCertain DSCP codepoints (such as Assured Forwarding and Expedited Forwarding) have default 802.1p priorities.

A DSCP policy assigns a DSCP codepoint and 802.1p priority value to IPv4 and IPv6 packets. You can classify traffic on an edge switch and use Layer 3 DSCP-marking (instead of only 802.1p priority) to assign and preserve QoS policies on downstream devices. In this case, if you reconfigure the 802.1p priority associated with the DSCP codepoint, the new 802.1p assignment takes effect starting on the switch on which it is configured and is used in packets sent to downstream devices.

If you configure a different 802.1p priority for a DSCP codepoint, the new DSCP policy overrides the 802.1p priority value in packets which enter the switch with the specified codepoint. The Layer 2 802.1p priority setting (0 through 7) determines the outbound port queue to which a packet is sent.