3.10.2 CoS and queuing priorities

CoS values are used as an index for showing the priority of frames on the Switch. The queuing priority indicates how easily a frame can be discarded for each queue.

The following table describes the specifiable range of CoS values and queuing priority values.

Table 3-12 CoS and queuing priorities specified
Item	Range
CoS value	0~7
Queuing priority	1~3

You cannot specify a CoS value and user priority inheritance at the same time.

In addition, frames for which flow-control priority determination and user priority inheritance are not performed use the default CoS and queuing priority. The following tables show CoS defaults and queuing priorities for each frame type.

Table 3-13: Default CoS and queuing priorities for each frame type
Frame type	Default
Frame type	CoS value	Queuing priority
Unicast frames	Conforms to the result of user priority mapping	3
Broadcast frames	0
Multicast frames	0

Mirrored frames also depend on CoS and queuing preference of the source frame.

Note that the correspondence between the CoS values and the determined queuing priority is fixed for the frames indicated in the table below regardless of whether priority determination and user priority inheritance for flow control are set.

The following table indicates the frames whose values cannot be changed by either priority determination or user priority inheritance.

Table 3-14: Frame list that cannot be changed by priority determination
Frame type	CoS value	Queuing priority
Frames spontaneously sent by the Switch	7	3
The following frames received by the Switch: ARP frame #1#2 Frames used for line test	5	-
The following frames received by the Switch: Frames that are regarded as movement detection of MAC addressing to the local device	2	-
Of the frames received by the Switch by Layer 3 forwarding, the following packets and frames: IPv4 and IPv6 packets exceeding the MTU Frames whose TTL is set to 1 Frames whose hop limit is set to 1 Frames with an IP option Frames with an IPv6 extension header	2	-
Of the frames received on the Switch by Layer 3 forwarding, the following packets: IPv4 or IPv6 packets with an unknown receiver address	2	-
The following frames for which the Switch perform Layer 3 forwarding: Fragmented frames on the Switch Frames with an IP option Frames with an IPv6 extension header Forwarding frames that are temporarily retained on the Switch due to unresolved ARP or NDP	3	-

Legend:-can be changed by flow control priority determination

Note #1: You can change the queuing preference of ARP broadcast frames from 3 to 2 using the configuration command flow action-change arp-discard-class.
Note #2: You can use the configuration command flow action-change arp-reply-cos to change CoS of ARP reply frames received by the Switch whose destination MAC address is a broadcast address from 5 to 0.