Which Type of QoS Marking is Applied to Ethernet Frames?

The CCNA 200-301 test questions and answers refer to a collection of practice questions and correct responses designed to help candidates prepare for the Cisco Certified Network Associate (CCNA) 200-301 exam. These test questions typically cover key topics such as networking fundamentals, IP addressing, security, routing and switching, and troubleshooting.

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08 April 2025

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Which Type of QoS Marking is Applied to Ethernet Frames?

Introduction To CCNA 200-301 Test Questions And Answers

Quality of Service (QoS) is a crucial networking concept that ensures efficient traffic management by prioritizing certain types of data over others. For those preparing for the CCNA 200-301 exam, understanding QoS markings—especially those applied to Ethernet frames—is essential.

In this comprehensive guide by Study4Pass, we will explore:

The different types of QoS markings
How Ethernet frames are marked for QoS
Key differences between Layer 2 and Layer 3 QoS markings
Real-world applications and exam-relevant insights

By the end of this blog, you’ll have a solid grasp of CCNA 200-301 test questions and answers related to QoS markings in Ethernet frames.

What is QoS Marking?

Before diving into Ethernet-specific QoS, let’s define QoS marking:

QoS (Quality of Service) ensures that critical network traffic (like VoIP, video conferencing) gets priority over less important traffic (like email or web browsing).
Marking refers to tagging packets or frames with a priority value so network devices (switches, routers) can apply appropriate traffic handling policies.

Why is QoS Important for the CCNA 200-301 Exam?

QoS is a key topic in the CCNA 200-301 Certification.
You’ll encounter test questions on how different layers handle QoS markings.
Understanding Ethernet frame markings helps troubleshoot real-world network performance issues.

Types of QoS Markings

QoS markings can be applied at different layers of the OSI model:

1. Layer 2 (Data Link Layer) Markings

Applied to Ethernet frames
Uses IEEE 802.1p (Class of Service CoS) in the 802.1Q VLAN tag
Values range from 0 (low priority) to 7 (highest priority)

2. Layer 3 (Network Layer) Markings

Applied to IP packets

Uses:

DSCP (Differentiated Services Code Point) in the IP header (6 bits, 64 possible values)
IP Precedence (older method, 3 bits, 8 possible values)

3. Layer 4 (Transport Layer) Markings

Uses TCP/UDP port numbers to identify traffic types (e.g., VoIP uses UDP ports)
For Ethernet frames, the primary marking method is IEEE 802.1p (CoS).
Which QoS Marking is Applied to Ethernet Frames?
The correct answer is: IEEE 802.1p (Class of Service CoS).

How Does 802.1p Work?

Part of the 802.1Q VLAN tagging standard
Uses 3 bits in the VLAN tag (called the Priority Code Point PCP)

8 priority levels (0-7):

0 (Best Effort) – Default, no priority
1 (Background) – Low priority (e.g., backups)
2 (Spare)
3 (Excellent Effort) – Business-critical data
4 (Controlled Load) – Streaming media
5 (Video) – <100ms latency (video conferencing)
6 (Voice) – <10ms latency (VoIP)
7 (Network Control) – Reserved for network protocols (STP, OSPF)

Where is 802.1p Applied?

Switches use CoS values to prioritize traffic.
Routers typically ignore CoS (since they work at Layer 3).
Comparing Layer 2 vs. Layer 3 QoS Markings

Feature	Layer 2 (802.1p CoS)	Layer 3 (DSCP/IP Precedence)
Header	Ethernet frame (VLAN tag)	IP header (ToS field)
Range	0-7 (3 bits)	0-63 (DSCP), 0-7 (IP Prec)
Persistence	Lost when frame leaves VLAN	End-to-end (remains in IP packet)
Device Handling	Switches prioritize	Routers prioritize

Key Takeaway for CCNA 200-301 Exam

Ethernet frames use 802.1p (CoS)
IP packets use DSCP/IP Precedence
CoS is lost when traffic crosses a router (since VLAN tags are stripped)
Real-World Applications of QoS Marking in Ethernet Frames

1. VoIP Traffic Prioritization

VoIP packets are marked with CoS 6 (Voice)
Switches prioritize these frames to reduce latency

2. Video Conferencing

Video traffic is marked CoS 5
Ensures smooth streaming without buffering

3. Network Management Traffic

STP, OSPF, and other control protocols use CoS 7
Prevents network failures due to congestion
CCNA 200-301 Test Questions and Answers on QoS Markings

To help you prepare, here are some practice questions related to QoS markings:

Conclusion

Understanding QoS markings in Ethernet frames is vital for both the CCNA 200-301 exam and real-world networking. The key points to remember are: Ethernet frames use IEEE 802.1p (CoS) for QoS marking. CoS values range from 0-7, with 7 being the highest priority.