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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter15 Configuring MSTP Understanding RSTP
To be consistent with Cisco STP implementations, this guide documents the port state as blocking
instead of discarding. Designated ports start in the listening state.
Rapid Convergence
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 conn ec te d th rou gh
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, a nd you sh ould e nab le it on ly on port s
that connect to a single end station.
•Root ports—If the RSTP selects a new root port, it blocks the ol d root po rt a nd imm edi ate ly
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 w ith t he o th er p ort by usi ng the
proposal-agreement handshake to ensure a loop-free topology.
As shown in Figure 15-2, 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 bloc king stat e, an d s en ds 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 immediate ly 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 b etwee n Switc hes A an d B.
When Switch C is connected to Switch B, a similar set of handshak ing m essag es ar e exchan ged.
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.
Table15-1 Port State Comparison
Operational Status STP Port State
(802.1D) RSTP Port State Is Port Included in the
Active Topology?
Enabled Blocking Discarding No
Enabled Listening Discarding No
Enabled Learning Learning Yes
Enabled Forwarding Forwarding Yes
Disabled Disabled Discarding No