3.7.1 Rewrite user priority

User priority rewriting is functionality that updates the user priority in the VLAN tag of a frame detected by flow detection. The user priority is the three highest-order bits of the Tag Control field shown in the following figure:

Figure 3-12: VLAN Tag Header Formats

When the user priority is updated for frames that have multiple VLAN tags, the user priority in the first VLAN tag encountered when counting from the MAC address side is updated. When the user priority is updated for frames that have multiple VLAN tags, the user priority in the first VLAN tag encountered when counting from the MAC address side is updated.

Figure 3-13: Schematic diagram of a multi-frame format with VLAN Tag

You cannot update the user priority for the following frames:

• Frames sent from a port for which VLAN tunneling is set

• 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

• IPv4 or IPv6 packets with an unknown receiver address

You cannot specify user priority rewriting and user priority inheritance at the same time.

If neither user priority rewriting nor user priority inheritance is used, the user priority is set as described in the following table.

Table 3-9: User priority when sending frames

User priority for frames to be sent User's priority	Applicable frames
3	Frames received without a VLAN tag and sent with a VLAN tag Frames forwarded from the access line to the backbone line by VLAN tunneling
User priority of received frames	Frames with a VLAN tag that are forwarded from the access line to the backbone line by VLAN tunneling Frames received with a VLAN tag on a port for which neither tag translation nor VLAN tunneling is configured, and sent with a VLAN tag