After receiving Switch B’s agreement message, Switch A also immediately transitions its designated
port to the forwarding state. No loops in the network are formed 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 is connected 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 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.
In a switch stack, the cross-stack rapid transition (CSRT) feature ensures that a stack member receives
acknowledgments from all stack members during the proposal-agreement handshaking before moving
the port to the forwarding state. CSRT is automatically enabled when the switch is in MST mode.
The switch learns the link type from the port duplex mode: a full-duplex port is considered to have a
point-to-point connection; 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 using the spanning-tree link-type interface
configuration command.
Figure 15: Proposal and Agreement Handshaking for Rapid Convergence
Synchronization of Port Roles
When the switch receives a proposal message on one of its ports and that port is selected as the new root port,
the RSTP forces all other ports to synchronize with the new root information.
The switch is synchronized with superior root information received on the root port if all other ports are
synchronized. An individual port on the switch is synchronized if
•
That port is in the blocking state.
•
It is an edge port (a port configured to be at the edge of the network).
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
254
Information About MSTP
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