Dell PowerConnect B-FCXs Configuration Manual page 323

These commands change the default VLAN ID, configure port 1/1 as a tagged member of VLANs 1
and 2, and enable the dual-mode feature and PVST+ support on port 1/1. Since VLAN 1 is tagged
in this configuration, the default VLAN ID must be changed from VLAN 1 to another VLAN ID.
Changing the default VLAN ID from 1 allows the port to process tagged frames for VLAN 1. VLAN 2
is specified with the dual-mode command, which makes VLAN 2 the port Port Native VLAN. As a
result, the port processes untagged frames and untagged PVST BPDUs on VLAN 2.
NOTE
Although VLAN 2 becomes the port untagged VLAN, the CLI still requires that you add the port to the
VLAN as a tagged port, since the port is a member of more than one VLAN.
Port 1/1 will process BPDUs as follows:
•
•
•
Note that when VLAN 1 is not the default VLAN, the ports must have the dual-mode feature enabled
in order to process IEEE 802.1Q BPDUs.
For example, the following configuration is incorrect.
PowerConnect(config)#default-vlan-id 1000
PowerConnect(config)#vlan 1
PowerConnect(config-vlan-1)#tagged ethernet 1/1 to 1/2
PowerConnect(config-vlan-1)#exit
PowerConnect(config)#interface ethernet 1/1
PowerConnect(config-if-1/1)#pvst-mode
PowerConnect(config-if-1/1)#exit
PowerConnect(config)#interface ethernet 1/2
PowerConnect(config-if-1/2)#pvst-mode
PowerConnect(config-if-1/2)#exit
In the configuration above, all PVST BPDUs associated with VLAN 1 would be discarded. Since IEEE
BPDUs associated with VLAN 1 are untagged, they are discarded because the ports in VLAN 1 are
tagged. Effectively, the BPDUs are never processed by the Spanning Tree Protocol. STP assumes
that there is no better bridge on the network and sets the ports to FORWARDING. This could cause
a Layer 2 loop.
The following configuration is correct.
PowerConnect(config)#default-vlan-id 1000
PowerConnect(config)#vlan 1
PowerConnect(config-vlan-1)#tagged ethernet 1/1 to 1/2
PowerConnect(config-vlan-1)#exit
PowerConnect(config)#interface ethernet 1/1
PowerConnect(config-if-1/1)#pvst-mode
PowerConnect(config-if-1/1)#dual-mode
PowerConnect(config-if-1/1)#exit
PowerConnect(config)#interface ethernet 1/2
PowerConnect(config-if-1/2)#pvst-mode
PowerConnect(config-if-1/2)#dual-mode
PowerConnect(config-if-1/2)#exit
Setting the ports as dual-mode ensures that the untagged IEEE 802.1Q BPDUs reach the VLAN 1
instance.
PowerConnect B-Series FCX Configuration Guide
53-1002266-01
Process IEEE 802.1Q BPDUs for VLAN 1.
Process untagged PVST BPDUs for VLAN 2.
Drop tagged PVST BPDUs for VLAN 1.
PVST/PVST+ compatibility
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