IEEE 802.1d (referred as Dot1d) and Cisco’s per VLAN Spanning Tree (PVST) BPDU encapsulations are supported on a per-SAP basis for the 7450 ESS and 7750 SR. STP is associated with a VPLS service like PVST is associated per VLAN. The main difference resides in the Ethernet and LLC framing and a type-length-value (TLV) field trailing the BPDU.
Table 1 shows differences between Dot1d and PVST Ethernet BPDU encapsulations based on the interface encap-type field.
Each SAP has a Read-Only operational state that shows which BPDU encapsulation is currently active on the SAP. The states are:
Dot1d — This state specifies that the switch is currently sending IEEE 802.1d standard BPDUs. The BPDUs are tagged or non-tagged based on the encapsulation type of the egress interface and the encapsulation value defined in the SAP. A SAP defined on an interface with encapsulation type dot1q continues in the dot1d BPDU encapsulation state until a PVST encapsulated BPDU is received, in which case, the SAP converts to the PVST encapsulation state. Each received BPDU must be properly IEEE 802.1q tagged if the interface encapsulation type is defined as dot1q. PVST BPDUs is silently discarded if received when the SAP is on an interface defined with a null encapsulation type.
PVST — This state specifies that the switch is currently sending proprietary encapsulated BPDUs. PVST BPDUs are only supported on Ethernet interfaces with the encapsulation type set to dot1q. The SAP continues in the PVST BPDU encapsulation state until a dot1d encapsulated BPDU is received, in which case, the SAP reverts to the dot1d encapsulation state. Each received BPDU must be properly IEEE 802.1q tagged with the encapsulation value defined for the SAP. PVST BPDUs are silently discarded if received when the SAP is on an interface defined with a null encapsulation type.
Dot1d is the initial and only SAP BPDU encapsulation state for SAPs defined on Ethernet interface with encapsulation type set to null.
Each transition between encapsulation types optionally generates an alarm that can be logged and optionally transmitted as an SNMP trap on the 7450 ESS or 7750 SR.