Dell Force10 S25-01-GE-24T FTOS 8.4.2.7 Configuration Guide for E-Series TeraS - Page 845

PVST+ in Multi-vendor Networks 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 no spanning-tree pvst err-disable cause invalid-pvst-bpdu. 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 40-5, 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, 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. Figure 40-5. PVST+ with Extend System ID Force10 System P1 untagged in VLAN 10 XP2 untagged in VLAN 20 moves to blocking unless Extended System ID is enabled VLAN unaware Hub Per-VLAN Spanning Tree Plus | 845