Brocade FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide - Manual (Page 319)

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.
Configuring PVST+ support
PVST+ support is automatically enabled when the port receives a PVST BPDU. You can manually
enable the support at any time or disable the support if desired.
If you want a tagged port to also support IEEE 802.1Q BPDUs, you need to enable the dual-mode
feature on the port. The dual-mode feature is disabled by default and must be enabled manually.
A port that is in PVST+ compatibility mode due to auto-detection reverts to the default MSTP mode
when one of the following events occurs:
• The link is disconnected or broken
• The link is administratively disabled
• The link is disabled by interaction with the link-keepalive protocol
This allows a port that was originally interoperating with PVST+ to revert to MSTP when connected to a
Brocade device.
Enabling PVST+ support manually
To immediately enable PVST+ support on a port, enter commands such as the following.
device(config)#interface ethernet 1/1
device(config-if-1/1)#pvst-mode
Syntax: [no] pvst-mode
NOTE
If you disable PVST+ support, the software still automatically enables PVST+ support if the port
receives a BPDU with PVST+ format.
VLAN tags and dual mode
FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide 319
53-1003086-04