Foundry Configuration Guide for the FESX, FSX, and FWSX
7 - 66 © Foundry Networks, Inc. December 2005
FastIron SuperX Router(config-vlan-2)# tagged ethernet 1/1
FastIron SuperX Router(config-vlan-2)# exit
FastIron SuperX Router(config)# interface ethernet 1/1
FastIron SuperX Router(config-if-1/1)# dual-mode 2
FastIron SuperX Router(config-if-1/1)# pvst-mode
FastIron SuperX Router(config-if-1/1)# exit
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’s 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’s 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:
• Process IEEE 802.1Q BPDUs for VLAN 1.
• Process untagged PVST BPDUs for VLAN 2.
• Drop tagged PVST BPDUs for VLAN 1.
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:
FastIron SuperX Router(config)# default-vlan-id 1000
FastIron SuperX Router(config)# vlan 1
FastIron SuperX Router(config-vlan-1)# tagged ethernet 1/1 to 1/2
FastIron SuperX Router(config-vlan-1)# exit
FastIron SuperX Router(config)# interface ethernet 1/1
FastIron SuperX Router(config-if-1/1)# pvst-mode
FastIron SuperX Router(config-if-1/1)# exit
FastIron SuperX Router(config)# interface ethernet 1/2
FastIron SuperX Router(config-if-1/2)# pvst-mode
FastIron SuperX Router(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:
FastIron SuperX Router(config)# default-vlan-id 1000
FastIron SuperX Router(config)# vlan 1
FastIron SuperX Router(config-vlan-1)# tagged ethernet 1/1 to 1/2
FastIron SuperX Router(config-vlan-1)# exit
FastIron SuperX Router(config)# interface ethernet 1/1
FastIron SuperX Router(config-if-1/1)# pvst-mode
FastIron SuperX Router(config-if-1/1)# dual-mode
FastIron SuperX Router(config-if-1/1)# exit
FastIron SuperX Router(config)# interface ethernet 1/2
FastIron SuperX Router(config-if-1/2)# pvst-mode
FastIron SuperX Router(config-if-1/2)# dual-mode
FastIron SuperX Router(config-if-1/2)# exit
Setting the ports as dual-mode ensures that the untagged IEEE 802.1Q BPDUs reach the VLAN 1 instance.
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