Cisco Nexus 5000 Series Switch CLI Software Configuration Guide, NX-OS 4.0(1a)N1 (OL-16597-01, January 2009)

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Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
OL-16597-01
Chapter 8 Configuring Rapid PVST+
Information About Rapid PVST+
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
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.
0 1 2 3 4 5 6 7
Topology change
Proposal
Port role
Learning
Forwarding
Agreement
Topology change ACK
00 Unknown
01 Alternate/Backup
02 Root
11 Designated
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