HP Cisco Nexus 5000 Cisco Nexus 5000 Series Switch CLI Software Configuration - Page 142

Information About Rapid PVST+ Chapter 8 Configuring Rapid PVST+ Send feedback to [email protected] Note The TCA flag is used only when the switch is interacting with switches that are running legacy 802.1D STP. See the "Rapid PVST+ Interoperation with Legacy 802.1D STP" section on page 8-16 for information about Rapid PVST+ interaction with 802.1D STP. The proposal and agreement sequence then quickly propagates toward the edge of the network and quickly restores connectivity after a topology change (see the "Synchronization of Port Roles" section on page 8-13). Rapid PVST+ BPDUs Rapid PVST+ and 802.1w use all six bits of the flag byte to add the role and state of the port that originates the BPDU, and the proposal and agreement handshake. Figure 8-3 shows the use of the BPDU flags in Rapid PVST+. Figure 8-3 Rapid PVST+ Flag Byte in BPDU 0 1 2 3 4 5 6 7 Topology change ACK Agreement Proposal Forwarding Learning Port role 00 Unknown 01 Alternate/Backup 02 Root 11 Designated Topology change 182776 Another important change is that the Rapid PVST+ BPDU is type 2, version 2, which makes it possible for the switch to detect connected legacy (802.1D) bridges. The BPDU for 802.1D is version 0. Proposal and Agreement Handshake As shown in Figure 8-4, switch A is connected to switch B through a point-to-point link, and all of the ports are in the blocking state. Assume that the priority of switch A is a smaller numerical value than the priority of switch B. Cisco Nexus 5000 Series Switch CLI Software Configuration Guide 8-8 OL-16597-01