DELL S50V User Manual

Per-VLAN Spanning Tree Plus | 845

Some non-Dell Force10 systems which 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 error-disable state. This behavior might result in the network not 
converging. To prevent FTOS from executing this action, use the command 

. After you configure this command, if the port receives a PVST+ 

BPDU, the BPDU is dropped, and the port remains operational.

because the hub is VLAN unaware. There is no data loop in the above scenario, however, PVST+ can be 
employed to avoid potential mis-configurations. 

If PVST+ is enabled 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 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.

VLAN unaware
        Hub

Force10 System

P1
untagged in VLAN 10

P2
untagged in VLAN 20 

X

moves to blocking unless
Extended System ID is 
enabled