HP ProCurve 9304M Installation And Configuration Manual page 283

Configuring Spanning Tree Protocol (STP) and Advanced STP Features
Figure 8.34 Interaction of IEEE 802.1Q, PVST, and PVST+ regions
PVST BPDUs tunneled through
the IEEE 802.1Q region
802.1D BPDUs 802.1D BPDUs
PVST+ Region IEEE 802.1Q Region PVST+ Region
dual mode dual mode
port port
Do not
connect
PVST BPDUs PVST BPDUs
(over ISL trunks) (over ISL trunks)
PVST Region
VLAN Tags and Dual Mode
To support the IEEE 802.1Q (Common Spanning Tree) portion of PVST+, a port must be a member of VLAN 1.
Cisco devices always use VLAN 1 to support the IEEE 802.1Q portion of PVST+.
For the port to also support the other VLANs (the PVST+ VLANs) in tagged mode, the dual-mode feature must be
enabled on the port. The dual-mode feature enables the port to send and receive both tagged and untagged
frames. When the dual-mode feature is enabled, 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's Port Native VLAN . By default, the Port Native VLAN is the
1
same as the device's Default VLAN , which by default is VLAN 1. Thus, to support IEEE 802.1Q in a typical
configuration, the port must be able to send and receive untagged frames for VLAN 1 and tagged frames for the
other VLANs.
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.
NOTE: Support for the IEEE 802.1Q spanning tree always uses VLAN 1, regardless of whether the devices are
configured to use tagged or untagged frames on the VLAN.
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.
Starting with release 07.6.04, 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:
1.Cisco PVST/PVST+ documentation refers to the Default VLAN as the Default Native VLAN .
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