Proposal And Agreement Handshake - Cisco Nexus 7000 Series Configuration Manual

Configuring Rapid PVST+ Using Cisco NX-OS
Rapid PVST+
Figure 6: 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 device to detect connected legacy (802.1D) bridges. The BPDU for 802.1D is type 0, version 0.

Proposal and Agreement Handshake

In this figure, 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. Switch A sends a proposal message (a configuration BPDU with the proposal flag set) to switch B, proposing
itself as the designated switch.
After receiving the proposal message, switch B selects as its new root port the port from which the proposal
message was received, forces all nonedge ports to the blocking state, and sends an agreement message (a
BPDU with the agreement flag set) through its new root port.
After receiving the agreement message from switch B, switch A also immediately transitions its designated
port to the forwarding state. No loops in the network can form because switch B blocked all of its nonedge
ports and because there is a point-to-point link between switches A and B.
When switch C connects to switch B, a similar set of handshaking messages are exchanged. Switch C selects
the port connected to switch B as its root port, and both ends of the link immediately transition to the forwarding
state. With each iteration of this handshaking process, one more switch joins the active topology. As the
network converges, this proposal-agreement handshaking progresses from the root toward the leaves of the
spanning tree as shown in this figure.
Cisco Nexus 7000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.x
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