VLAN tags and dual mode
The dual-mode feature enables a port to send and receive both tagged and untagged frames. When the dual-mode feature is enabled on a port, the port is an untagged member of one of its VLANs and is at the same time a tagged member of all its other VLANs. The untagged frames are supported on the port Port Native VLAN .
The dual-mode feature must be enabled on a Brocade port in order to interoperate with another vendor device. Some vendors use VLAN 1 by default to support the IEEE 802.1Q-based standard spanning tree protocols, such as 802.1d and 802.1w for sending untagged frames on VLAN 1. On Brocade switches, by default, the Port Native VLAN is the same as the Default VLAN , which is VLAN 1. Thus, to support IEEE 802.1Q in a typical configuration, a port must be able to send and receive untagged frames for VLAN 1 and tagged frames for the other VLANs, and interoperate with other vendor devices using VLAN 1.
If you want to use tagged frames on VLAN 1, you can change the default VLAN ID to an ID other than 1. You also can specify the VLAN on which you want the port to send and receive untagged frames (the Port Native VLAN). The Port Native VLAN ID does not need to be the same as the default VLAN. Make sure that the untagged (native) VLAN is also changed on the interoperating vendor side to match that on the Brocade side.
To support the IEEE 802.1Q with non-standard proprietary protocols such as PVST and PVST+, a port must always send and receive untagged frames on VLAN 1 on both sides. In this case, enable the dual-mode 1 feature to allow untagged BPDUs on VLAN 1and use Native VLAN 1 on the interoperating vendor side. You should not use VLAN 1 for tagged frames in this case.