Cisco Catalyst 2928 Software Configuration Manual page 348

Understanding RSTP
Root ports—If the RSTP selects a new root port, it blocks the old root port and immediately
•
transitions the new root port to the forwarding state.
• Point-to-point links—If you connect a port to another port through a point-to-point link and the local
port becomes a designated port, it negotiates a rapid transition with the other port by using the
proposal-agreement handshake to ensure a loop-free topology.
As shown in
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. 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 nonedge ports to the blocking state, and sends an
agreement message (a BPDU with the agreement flag set) through its new root port.
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.
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 17-4
Switch A
Root
Root
Root
DP = designated port
RP = root port
F = forwarding
Catalyst 2928 Switch Software Configuration Guide
17-10
Figure 17-4, Switch A is connected to Switch B through a point-to-point link, and all
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
Switch C
Proposal
Agreement
F F
DP RP
Chapter 17 Configuring MSTP
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