What Does CoS Mean in 200-301?
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.
Full Definition
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.
Exam Trap — Don't Get Fooled
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.
Related 200-301 Terms
Frequently Asked Questions
What does CoS mean on the 200-301 exam?
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.
How does CoS appear as a trap on the 200-301?
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.
How important is CoS on the 200-301 exam?
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.