300-320 · Question #26
CoS is applied for Layer 2 markings in an Ethernet environment. Which protocol must be configured on the Layer 2 switch port for CoS to work?
The correct answer is C. IEEE 802.1Q. CoS (Class of Service) uses the 3-bit Priority Code Point (PCP) field embedded within the IEEE 802.1Q VLAN tag. For CoS markings to exist in an Ethernet frame, the frame must carry an 802.1Q tag-meaning the switch port must be configured as a trunk (802.1Q) port. Without 802.1Q…
Question
CoS is applied for Layer 2 markings in an Ethernet environment. Which protocol must be configured on the Layer 2 switch port for CoS to work?
Options
- AMPLS
- BIEEE 802.1W
- CIEEE 802.1Q
- DIEEE 802.1S
How the community answered
(35 responses)- A6% (2)
- B3% (1)
- C91% (32)
Explanation
CoS (Class of Service) uses the 3-bit Priority Code Point (PCP) field embedded within the IEEE 802.1Q VLAN tag. For CoS markings to exist in an Ethernet frame, the frame must carry an 802.1Q tag-meaning the switch port must be configured as a trunk (802.1Q) port. Without 802.1Q tagging, there is no 3-bit PCP field and therefore no mechanism to carry CoS. IEEE 802.1W (B) is Rapid Spanning Tree Protocol. IEEE 802.1S (D) is Multiple Spanning Tree Protocol. MPLS (A) uses EXP/TC bits for QoS marking at Layer 2.5, which is separate from Ethernet CoS.
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