Cisco Systems 3750E manual Synchronization of Port Roles, 19-11

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

apoint-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-treelink-typeinterface configuration command.

Figure 19-4 Proposal and Agreement Handshaking for Rapid Convergence

Switch A Proposal Switch B

Root Agreement

F

DP

Root

F

DP

Root

F

DP

DP = designated port

RP = root port

F = forwarding

Designated

switch

F

RP
	Designated					Switch C
F	switch		Proposal
RP
	Designated
	switch		Agreement
F		F			F
RP		DP		RP

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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).

	Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02		19-11