Per-VLAN Spanning Tree Plus (PVST+) | 411
PVST+ in Multi-vendor Networks
Some non-Dell Force10 systems that have hybrid ports participating in PVST+, transmit two kinds of
BPDUs: an 802.1D BPDU and an untagged PVST+ BPDU.
Dell Force10 systems do not expect PVST+ BPDU (tagged or untagged) on an untagged port. If this
happens, FTOS places the port in the Error Disable state. This behavior might result in the network not
converging. To prevent FTOS from executing this action, use the
no spanning-tree pvst err-disable cause
invalid-pvst-bpdu
command. After you configure this command, if the port receives a PVST+ BPDU, the
BPDU is dropped and the port remains operational.
PVST+ Extended System ID
In Figure 23-6, ports P1 and P2 are untagged members of different VLANs. These ports are untagged
because the hub is VLAN unaware. There is no data loop in the above scenario; however, you can employ
PVST+ to avoid potential misconfigurations.
If you enable PVST+ on the Dell Force10 switch in this network, P1 and P2 receive BPDUs from each
other. Ordinarily, the Bridge ID in the frame matches the Root ID, a loop is detected, and the rules of
convergence require that P2 move to the Blocking state because it has the lowest port ID.
To keep both ports in Forwarding state, use Extend System ID. Extend System ID augments the Bridge ID
with a VLAN ID to differentiate BPDUs on each VLAN so that PVST+ does not detect a loop, and both
ports can remain in Forwarding state.
FTOS Behavior: Regarding the bpduguard shutdown-on-violation behavior:
1 If the interface to be shutdown is a port channel, all the member ports are disabled in the hardware.
2 When you add a physical port to a port channel already in Error Disable state, the new member port id
also disabled in the hardware.
3 When you remove a physical port from a port channel in Error Disable state, the Error Disabled state is
cleared on this physical port (the physical port is enabled in the hardware).
4 You can clear the Error Disabled state with any of the following methods:
• Perform a shutdown command on the interface.
• Disable the shutdown-on-violation command on the interface (no spanning-tree pvst edge-port
[bpduguard | [shutdown-on-violation]]).
• Disable spanning tree on the interface (no spanning-tree in INTERFACE mode).
• Disabling global spanning tree (no spanning-tree in CONFIGURATION mode).
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