Cisco Systems 3560 Manual De Usuario

The RSTP provides for rapid recovery of connectivity following the failure of a switch, a switch port, or 
a LAN. It provides rapid convergence for edge ports, new root ports, and ports connected through 
point-to-point links as follows:

Edge ports—If you configure a port as an edge port on an RSTP switch by using the spanning-tree 
portfast interface configuration command, the edge port immediately transitions to the forwarding 
state. An edge port is the same as a Port Fast-enabled port, and you should enable it only on ports 
that connect to a single end station.

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 

, Switch A is connected to Switch B through a point-to-point link, and all 

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.