Cisco Catalyst 2960 Series, Configuration Manual

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A port with the root or a designated port role is included in the active topology. A port with the alternate or
backup port role is excluded from the active topology.
In a stable topology with consistent port roles throughout the network, the RSTP ensures that every root port
and designated port immediately transition to the forwarding state while all alternate and backup ports are
always in the discarding state (equivalent to blocking in IEEE 802.1D). The port state controls the operation
of the forwarding and learning processes.
Table 31: Port State Comparison
Is Port Included in the Active
Topology? RSTP Port State STP Port State
(IEEE 802.1D) Operational Status
No Discarding Blocking Enabled
No Discarding Listening Enabled
Yes Learning Learning Enabled
Yes Forwarding Forwarding Enabled
No Discarding Disabled Disabled
To be consistent with Cisco STP implementations, this guide defines 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 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.
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.
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
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Information About MSTP