Proposal And Agreement Handshake - Cisco Nexus 3000 series Configuration Manual

Configuring Rapid PVST+
Understanding Rapid PVST+

Proposal and Agreement Handshake

As shown in the following 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.
Figure 10: Proposal and Agreement Handshaking for Rapid Convergence
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 non-edge 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 non-edge
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 network device joins the active topology. As
the network converges, this proposal-agreement handshaking progresses from the root toward the leaves of
the spanning tree.
The switch learns the link type from the port duplex mode: a full-duplex port is considered to have a
point-to-point connection and a half-duplex port is considered to have a shared connection. You can override
the default setting that is controlled by the duplex setting by entering the spanning-tree link-type interface
configuration command.
This proposal/agreement handshake is initiated only when a non-edge port moves from the blocking to the
forwarding state. The handshaking process then proliferates step-by-step throughout the topology.
Cisco Nexus 3000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U1(1)
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