Understanding Quality of Service (QoS) in Networks – From Ethernet to FortiSwitch

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What Is QoS?

QoS (Quality of Service) refers to the ability of a network to provide different priority levels to different applications, users, or data flows, or to guarantee a certain level of performance.

QoS helps ensure:

• Low latency for real-time traffic (VoIP, video)
• High throughput for bulk data transfers
• Fair bandwidth allocation between users

QoS at Different Layers

1. Layer 2: Ethernet – Class of Service (CoS)

QoS at Layer 2 is applied using the 802.1p standard, which embeds priority information in the Ethernet frame.

 Ethernet Frame (with 802.1Q VLAN Tag)

+-------------+-------------+-------------+-------------+-------------+-------------+

| Dest. MAC   | Src. MAC    | Tag (TPID)  | 802.1p/VID | Ethertype   | Payload     |

| 6 bytes     | 6 bytes     | 2 bytes     | 2 bytes     | 2 bytes     | ...         |

+-------------+-------------+-------------+-------------+-------------+-------------+

802.1p is a 3-bit field inside the VLAN tag (part of 802.1Q).

Values range from 0 to 7, indicating priority levels (0 = lowest, 7 = highest).

2. Layer 3: IP – DSCP (Differentiated Services Code Point)

QoS at the IP layer uses DSCP (within the ToS byte of the IP header) to mark packets.

IPv4 Header (showing DSCP)

+--------+--------+--------+--------+--------+--------+--------+--------+

| Ver | IHL | DSCP (6b) | ECN (2b) | Total Length                        |

+--------+--------+--------+--------+--------+--------+--------+--------+

• DSCP: 6 bits for packet classification (supports 64 values).
• ECN: 2 bits for Explicit Congestion Notification.

Common DSCP Values:

DSCP Name	Binary	Decimal	Purpose
EF	101110	46	Expedited Forwarding (VoIP)
AF31	011010	26	Assured Forwarding
CS0	000000	0	Best Effort (default)

QoS Workflow: End-to-End Flow

1. Classification: Packet is tagged based on source, app, or interface.
2. Marking: Set CoS (802.1p) or DSCP in the packet header.
3. Queuing: Packets are placed into different queues.
4. Scheduling: Algorithms (e.g., Weighted Round Robin or Strict Priority) determine send order.
5. Policing/Shaping: Limit rate and control burst traffic.

Queue Scheduling Algorithms

• Strict Priority (SP): Higher-priority queues always win.
• Weighted Round Robin (WRR): Fairness by rotating queues based on weight.
• WRED/RED: Drops packets before queues overflow (for congestion control).

QoS on FortiSwitch: Implementation in Practice

FortiSwitch provides a robust implementation of QoS using both Layer 2 and Layer 3 mechanisms.

Key Components:

1. 802.1p Map (CoS): Maps priorities 0–7 to specific egress queues.
2. DSCP Map: Maps DSCP values (e.g., EF, AF21) to egress queues.
3. QoS Policies: Configure min/max rates, queue scheduling, drop policies.

FortiSwitch QoS Best Practices:

• Reserve Queue 7 for control traffic (STP, LLDP).
• Avoid enabling trust for both CoS and DSCP on the same port.
• Use strict priority for VoIP, WRR for general traffic.

Diagram: QoS Marking and Queuing Flow

+-----------+        +-------------+        +-------------+        +------------+

| End Device|------->| Access Switch|------->| Aggregation |------->| Internet   |

|  DSCP = EF|        |   CoS = 7    |        | Queuing     |        | Gateway    |

+-----------+        +-------------+        +-------------+        +------------+

        | DSCP Marking        | CoS Mapping         | WRR Queue         |

Summary

QoS ensures that critical traffic gets the bandwidth and latency guarantees it needs. By combining Ethernet Layer 2 CoS and IP Layer 3 DSCP marking, and applying policies on devices like FortiSwitch, network administrators can ensure optimal performance for real-time and business-critical applications.

GUI Configuration Steps:-

1. Configure an 802.1p Map (Layer 2 CoS):
• Navigate to Switch > QoS > 802.1p.
• Click Add Map.
• Enter a name and description for the map.
• Assign each priority level (0–7) to a desired egress queue (0–7).
• Click Add Map to save.​
2. Configure a DSCP Map (Layer 3 QoS):
• Go to Switch > QoS > IP/DSCP.
• Click Add Map.
• Provide a name and description.
• Select the queue number for each DSCP or IP precedence value.
• Click Add Map to save.
3. Set Up a QoS Egress Policy:
• Navigate to Switch > QoS > Egress Policy.
• Click Add Policy.
• Enter a policy name.
• Choose a scheduling mode: Strict, Round Robin, or Weighted Round Robin.
• For each queue, set parameters like min/max rate, drop policy (Tail Drop, RED, or WRED), weight, and ECN marking if applicable.
• Click Add to save the policy.​
4. Apply the QoS Policy to Ports:
• Go to Switch > Ports.
• Select the desired port(s).
• Assign the created QoS policy to the selected port(s).

CLI Configuration Steps

1. Create an 802.1p Map:

config switch qos dot1p-map

    edit "dot1p_map_name"

        set description "Description"

        set priority-0 0

        set priority-1 1

        ...

        set priority-7 7

    next

end

2. Create a DSCP Map:

config switch qos ip-dscp-map

    edit "dscp_map_name"

        set description "Description"

        config map

            edit "entry1"

                set diffserv EF

                set cos-queue 3

            next

            edit "entry2"

                set value 13

                set cos-queue 2

            next

        end

    next

end

3. Define a QoS Egress Policy:

config switch qos qos-policy

    edit "policy_name"

        set rate-by kbps

        set schedule weighted

        config cos-queue

            edit queue-0

                set description "Low Priority"

                set min-rate 1000

                set max-rate 5000

                set drop-policy taildrop

            next

            edit queue-7

                set description "High Priority"

                set min-rate 5000

                set max-rate 10000

                set drop-policy wred

                set ecn enable

            next

        end

    next

end

4. Assign the QoS Policy to Ports:

config switch interface

    edit port1

        set qos-policy "policy_name"

    next

end

Important Notes

Trust Settings: Avoid enabling trust for both Dot1p and DSCP on the same interface. If both are enabled, DSCP takes precedence.

Control Traffic Queue: FortiSwitch reserves queue 7 for control traffic (e.g., STP, LLDP, DHCP). Ensure this queue is not overridden by other configurations. ​

Model-Specific Limitations: Certain models (e.g., FS-108E, FS-124E) have restrictions, such as supporting only one dot1p-map and one ip-dscp-map per switch, and limitations on configuring min-rate or drop-policy settings

Doc Links:

https://docs.fortinet.com/document/fortiswitch/7.6.2/fortiswitchos-administration-guide/609008/qos

https://docs.fortinet.com/document/fortiswitch/7.6.2/fortiswitchos-feature-matrix

https://docs.fortinet.com/document/fortiswitch/7.6.3/fortilink-guide/173281/configuring-qos-with-man...