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PowerConnect B-Series FCX Configuration Guide 277
53-1002266-01
PVST/PVST+ compatibility
8
FIGURE 53 Interaction of IEEE 802.1Q, PVST, and PVST+ regions
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 Dell PowerConnect 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 Dell PowerConnect 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 Dell PowerConnect 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.
PVST+Region IEEE 802.1Q Region
PVST+Region
PVST Region
PVST BPDUs
(over ISL trunks)
PVST BPDUs
(over ISL trunks)
Do not
connect
dual mode
port
dual mode
port
802.1D BPDUs
802.1D BPDUs
PVST BPDUs tunneled through
the IEEE 802.1Q region