Technical Tip: QOS queue for control traffic on FortiSwitch

[sachitdas_FTNT]

Description	This article describes what is control traffic, which QoS queue is used in FortiSwitch for control traffic and a few protocol examples that are included in the control plane.
Scope	FortiSwitch and FortiGate version 6.4.x and above Relaled QOS documents: https://docs.fortinet.com/document/fortiswitch/7.2.1/administration-guide/609008/qos https://docs.fortinet.com/document/fortiswitch/7.2.1/fortilink-guide/173281/configuring-qos-with-man...
Solution	Control plane traffic includes traffic generated, processed and forwarded by the FortiSwitch CPU. Some protocol examples are MCLAG, loop guard, OSPF, BFD, RIP, ISIS, VRRP, ARP, EAPOL, MRP, DHCP, IGMP, capwap, fortilink, lldp. There are some cases where the FortiSwitch CPU will handle non-control traffic. Example: If hardware routing is not supported or the hardware table is full, normal L3 routed traffic would flow through the CPU) For STP from a 3rd party (or any unit), BPDU would traverse both control and dataplane.  In the hardware (dataplane), BPDUs higher relative priority is given than other protocols which allows the switch to react quickly to changes in the network topology. The CPU then handles the BPDU traffic in the spanning tree daemon.  This process also has a higher relative priority for similar reasons. There are 8 QoS queues in FortiSwitch 0 to 7, queue 7 is used for control traffic. To further explain, the traffic on the FortiSwitch ports should be sent into and out Q7.   However, when traffic is delivered to the local CPU (via protocol trap) those priorities are shuffled internally on FortiSwitch by design. FortiSwitch split out different traffic types into different Queues with different rate limits.  These are not the same Q as we would see on switch ports.