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19-11
Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02
Chapter 19 Configuring MSTP
Understanding RSTP
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 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).
Proposal
Switch A Switch B
FF
RPDP
FF
RPDP
FF
RPDP
FF
RPDP
Switch C
88760
Agreement
Root
Designated
switch
Root
Designated
switch
Proposal
Root
Designated
switch
Agreement
DP = designated port
RP = root port
F = forwarding