Full form: Class of Service
Also known as: Class of Service, 802.1p, PCP
Quick Definition
A 3-bit priority field in the 802.1Q VLAN tag used to mark Ethernet frames for QoS.
CoS (Class of Service) uses a 3-bit Priority Code Point (PCP) field within the 802.1Q VLAN tag to mark Ethernet frames with a priority value (0–7). Higher values indicate higher priority: 0 = best effort, 5 = voice, 6–7 = network control. CoS is a Layer 2 marking that is only present on tagged trunk frames and is not preserved when the 802.1Q tag is stripped at the edge.
CoS only exists in the 802.1Q tag and is therefore only meaningful on trunk links. It is stripped when the frame leaves a VLAN-tagged segment. DSCP (Layer 3) is more reliable for end-to-end QoS marking.
CoS (Class of Service) uses a 3-bit Priority Code Point (PCP) field within the 802.1Q VLAN tag to mark Ethernet frames with a priority value (0–7). Higher values indicate higher priority: 0 = best effort, 5 = voice, 6–7 = network control. CoS is a Layer 2 marking that is only present on tagged trunk frames and is not preserved when the 802.1Q tag is stripped at the edge.
CoS only exists in the 802.1Q tag and is therefore only meaningful on trunk links. It is stripped when the frame leaves a VLAN-tagged segment. DSCP (Layer 3) is more reliable for end-to-end QoS marking.
CoS falls under the IP Services domain of the 200-301 exam. Understanding it in context with related terms like dscp and qos is essential for answering scenario-based questions correctly.