3.5.1 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 3-1 VLAN Tag Header Format

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 3-2 Schematic diagram of a frame format with multiple VLAN Tags

If a user priority rewrite is specified in a relay frame that is received without a VLAN Tag and is sent as a VLAN Tag, the user priority of VLAN Tag at the time of transmission becomes the priority after the rewrite.

If user priority rewriting is not performed, the user priority shown in the following table will be used.

Table 3-9: User priority when sending frames
User priority for frames to be sent User's priority	Applicable frames
7	Frames spontaneously sent by the Switch DHCP when DHCP snooping is enabled ARP packets when DHCP snooping dynamic ARP checking is enabled GSRP Flush request frame
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 IGMP packet with IGMP snooping enabled, Hello message of PIM protocol, and IPv4 packet with 224.0.0.0/24 as the destination MLD packet with MLD snooping enabled, IPv6 packet with ff02::/16 as the destination ARP packet when the relay function of Layer 2 authentication ARP packet is enabled Linktrace messages when CFM is enabled