Class of Service and DSCP configuration in voice prioritization

What is 802.1p Class of Service (CoS) ?

The 802.1p method is a Quality of Service (QoS) process defined within the IEEE 802.1q standard. It operates on the MAC layer (Layer 2) and uses a three-bit field within an Ethernet frame header to assign priority levels to packets moving within a Layer 2 network segment. These priority bits are included in VLAN tags and are commonly referred to as "Class of Service" (CoS). When network congestion occurs, higher priority packets are transmitted before lower priority packets.

What is DSCP?

The DSCP method is a Quality of Service (QoS) process defined within RFC 2474. It operates on the IP layer (Layer 3) and uses a six-bit field within an IPv4/IPv6 frame header to assign priority levels to packets moving within a Layer 3 network segment. These priority bits are included in the IPv4/IPv6 headers and are commonly referred to as "DS field" (Differentiated Services) or DSCP. When network congestion occurs, higher priority packets are transmitted before lower priority packets.

Voice packets pass through many different network gateways and routers. In some cases, intermediary network equipment can strip away DSCP headers used for packet prioritization.