Bridge Protocol Data Unit Format and Processing The RSTP BPDU format is the same as the IEEE 802.1D BPDU format except that the protocol version is set to 2. A new 1-byte Version 1 Length field is set to zero, which means that no version 1 protocol information is present. Table 2: RSTP BPDU Flags Configuring Multiple Spanning-Tree Protocol Bridge Protocol Data Unit Format and Processing Bit Function 0 Topology change (TC) 1 Proposal 2–3: 00 01 10 11 Port role: Unknown Alternate port Root port Designated port 4 Learning 5 Forwarding 6 Agreement 7 Topology change acknowledgement (TCA) Configuring Multiple Spanning-Tree Protocol 13