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Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter15 Understanding and Conf iguring Multiple Spanning Trees
Overview of MST
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MST switches operate as if MAC reduction is enabled.
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For private VLANs (PVLANs), you must map a secondary VLAN to the same instance as the
primary.
IEEE 802.1w RSTP
RSTP, specified in 802.1w, supersedes STP specified in 802.1D, but remains compatible with STP. You
configure RSTP when you configure the MST feature. For more information, see the “ Configuring MST”
section on page 15-9.
RSTP provides the structure on which the MST operates, significantly reduc ing the time to reconfigure
the active topology of a network when its physical topology or configuration parameters chan ge. RSTP
selects one switch as the root of a spanning-tree-connected active topology and assigns port roles to
individual ports of the switch, depending on whether that port is part of the active topology.
RSTP provides rapid connectivity following the failure of a switch, switch port, or a LAN. A new root
port and the designated port on the other side of the bridge transition to the forwarding state thr ough an
explicit handshake between them. RSTP allows switch port configuration so the ports can transition to
forwarding directly when the switch reinitializes.
RSTP provides backward compatibility with 802.1D bridges as follows:
•RSTP selectively sends 802.1D-configured BPDUs and Topology Change Notification (TCN)
BPDUs on a per-port basis.
•When a port initializes, the migration delay timer starts and RSTP BPDUs are transmitted. While
the migration delay timer is active, the bridge processes all BPDUs received on that port.
•If the bridge receives an 802.1D BPDU after a port’s migration delay timer expires, the bridge
assumes it is connected to an 802.1D bridge and starts using only 802.1D BPDUs.
•When RSTP uses 802.1D BPDUs on a port and receives an RSTP BPDU aft er the migration delay
expires, RSTP restarts the migration delay timer and begins using RSTP BPDUs on that port.

RSTP Port Roles

In RSTP, the port roles are defined as follows:
•Root—A forwarding port elected for the spanning tree topology.
•Designated—A forwarding port elected for every switched LAN segment.
•Alternate—An alternate path to the root bridge to that provided by the current root port.
•Backup—A backup for the path provided by a designat ed port toward the leaves of the spanning tre e.
Backup ports can exist only where two ports are connected together in a loopback mode or bridge
with two or more connections to a shared LAN segment.
•Disabled—A port that has no role within the operation of spanning tree.
The system assigns port roles as follows:
•A root port or designated port role includes the port in the active topology.
•An alternate port or backup port role excludes the port from the active topology.