PVST+ in Multi-Vendor NetworksSome non-Dell Networking systems which have hybrid ports participating in PVST+ transmit two kinds ofBPDUs: an 802.1D BPDU and an untagged PVST+ BPDU.Dell Networking systems do not expect PVST+ BPDU (tagged or untagged) on an untagged port. If thissituation occurs, Dell Networking OS places the port in an Error-Disable state. This behavior might resultin the network not converging. To prevent Dell Networking OS from executing this action, use the nospanning-tree pvst err-disable cause invalid-pvst-bpdu command. After you configurethis command, if the port receives a PVST+ BPDU, the BPDU is dropped and the port remains operational.Enabling PVST+ Extend System IDIn the following example, ports P1 and P2 are untagged members of different VLANs. These ports areuntagged because the hub is VLAN unaware. There is no data loop in this scenario; however, you canemploy PVST+ to avoid potential misconfigurations.If you enable PVST+ on the Dell Networking switch in this network, P1 and P2 receive BPDUs from eachother. Ordinarily, the Bridge ID in the frame matches the Root ID, a loop is detected, and the rules ofconvergence require that P2 move to blocking state because it has the lowest port ID.To keep both ports in a Forwarding state, use extend system ID. Extend system ID augments the bridge IDwith a VLAN ID to differentiate BPDUs on each VLAN so that PVST+ does not detect a loop and bothports can remain in a Forwarding state.Figure 93. PVST+ with Extend System ID• Augment the bridge ID with the VLAN ID.PROTOCOL PVST modeextend system-idPer-VLAN Spanning Tree Plus (PVST+) 649
