The Edge Control Protocol (ECP) provides reliable transport for the Port Extender Control and Status Protocol (PE-CSP) protocol. It includes a basic acknowledgement and retransmission facility. The Edge Control Protocol Data Unit (ECPDU) structure (ECPDU Structure) is slightly modified from the IEEE 802.1Q-2014. Preceding the ECPDU structure is the Destination and Source Address.
DA (Destination MAC address)—The destination MAC address of the Ethernet frame that contains an ECPDU is specified by the Upper Layer Protocol (ULP). When ECP is carrying PE CSP, the destination address used is the Bridge Group Address, nearest non-TPMR Bridge group address (01-80-c2-00-00-03) as listed in Table-8-1 of the 802.1Q-2014. The DA is 6 octets long.
The BPE device accepts either the nearest non-TPMR bridge group address or the individual MAC address sent in its LLDP PE TLV.
SA (Source MAC address)—The source MAC address of the bridge. The SA is six octets long.
EtherType—ECP Ethertype (0x8940). The ECP EtherType is two octets long.
Version—The ECP protocol version. The version is 4 bits long, and the value is 1.
SubType—Defines the ULP type included in the PDU. The value of Subtype is 0x2 when BPE CSP is the ULP. See table 43-1 of IEEE-802.1Q-2014. The SubType field is a 10 bits long.
Sequence Number—The Sequence Number field identifies the sequential order of the PDU with respect to other ECPDUs. The first sequence number can start anywhere, but each subsequent sequence number is incremented by one. The Sequence Number is 2 octets long.
ULPDU—The ULPDU contains the Upper Layer Protocol Data Unit if the operation type value is “ECP Request”. The ULPDU field is absent if the operation type value is “ECP Acknowledgment”. The length of this field is upper layer dependent. The ULPDU in this case is PE-CSP.