QoS - marking

Within the framework of QoS marking is the process by which we mark or label, or earmark so to speak, a packet of data based on the classification of the particular traffic. Marking can occur at either Layer 2 or Layer 3 or both.

At Layer 2, we can mark the Class of Service (CoS) value that is contained within the 802.1Q VLAN tag. Note that this is done on ports configured with a voice VLAN and on trunks since it is on frames sent from such ports that VLAN tags exist.

At Layer 3, we set the Type of Service (ToS) field in the header of the IPv4 or IPv6 packet to a particular value.

For more information, take a look at QoS CoS vs DSCP.

Once marked, traffic is not yet acted upon by any QoS policies. In order to be acted upon, QoS policies must be applied.