IEEE 802.1d (referred as dot1d) and Cisco per-VLAN Spanning Tree (PVST) BPDU encapsulations are supported on a per SAP basis. The STP is associated with a VPLS service like PVST is per VLAN. The difference between the two encapsulations is in the Ethernet and LLC framing and a type-length-value (TLV) field trailing the BPDU.The encapsulation format cannot be configured by the user; the system automatically determines the encapsulation format based on the BPDUs received on the port.
The following table shows differences between Dot1d and PVST Ethernet BPDU encapsulations based on the interface encap-type field:
| Field | dot1d encap-type null | dot1d encap-type dot1q | PVST encap-type null | PVST encap-type dot1q |
|---|---|---|---|---|
Destination MAC |
01:80:c2:00:00:00 |
01:80:c2:00:00:00 |
N/A |
01:00:0c:cc:cc:cd |
Source MAC |
Sending Port MAC |
Sending Port MAC |
N/A |
Sending Port MAC |
EtherType |
N/A |
0x81 00 |
N/A |
0x81 00 |
Dot1p and CFI |
N/A |
0xe |
N/A |
0xe |
Dot1q |
N/A |
VPLS SAP ID |
N/A |
VPLS SAP encap value |
Length |
LLC Length |
LLC Length |
N/A |
LLC Length |
LLC DSAP SSAP |
0x4242 |
0x4242 |
N/A |
0xaaaa (SNAP) |
LLC CNTL |
0x03 |
0x03 |
N/A |
0x03 |
SNAP OUI |
N/A |
N/A |
N/A |
00 00 0c (Cisco OUI) |
SNAP PID |
N/A |
N/A |
N/A |
01 0b |
CONFIG |
Standard 802.1d |
Standard 802.1d |
N/A |
Standard 802.1d |
TLV: Type & Len |
N/A |
N/A |
N/A |
58 00 00 00 02 |
TLV: VLAN |
N/A |
N/A |
N/A |
VPLS SAP encap value |
Padding |
As Required |
As Required |
N/A |
As Required |
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 are silently discarded if received when the SAP is on an interface defined with encapsulation type null.
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.