Cisco WS-CBS3032-DEL Software Configuration Manual page 497

Chapter 19 Configuring MSTP
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 19-4
Switch A
Root
Root
Root
DP = designated port
RP = root port
F = forwarding
OL-13270-06
Proposal and Agreement Handshaking for Rapid Convergence
Switch B
Proposal
Designated
switch
Agreement
F F
DP RP
Designated
switch
F F
DP RP
Designated
switch
F F
DP RP
Cisco Catalyst Blade Switch 3130 and 3032 for Dell Software Configuration Guide
Switch C
Proposal
Agreement
F F
DP RP
Understanding RSTP
19-11