Rapid Spanning Tree Protocol - Cisco WS-C6506 Software Manual

Understanding How Multiple Spanning Tree Works
These sections describe MST:
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Rapid Spanning Tree Protocol

RSTP significantly reduces the time to reconfigure the active topology of the network when changes to
the physical topology or its configuration parameters occur. 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 forwarding through an explicit
handshake between them. RSTP allows switch port configuration so that the ports can transition to
forwarding directly when the switch reinitializes.
RSTP, specified in 802.1w, supersedes STP, which is specified in 802.1D, while remaining compatible
with STP. RSTP provides the structure on which the MST operates. You configure RSTP when you
configure the MST feature. For more information, see the
Switch" section on page
RSTP provides backward compatibility with 802.1D bridges as follows:
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RSTP Port Roles
RSTP uses the following definitions for port roles:
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Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
7-18
Rapid Spanning Tree Protocol, page 7-18
MST-to-SST Interoperability, page 7-19
Common Spanning Tree, page 7-21
MST Instances, page 7-21
MST Configuration, page 7-21
MST Region, page 7-22
Message Age and Hop Count, page 7-23
MST-to-PVST+ Interoperability, page 7-24
7-51.
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 that are received on that port.
RSTP BPDUs are not visible on the port; only version 3 BPDUs are visible.
If the bridge receives an 802.1D BPDU after a port's Migration Delay timer expires, the bridge
assumes that 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 after the migration delay
expires, RSTP restarts the Migration Delay timer and begins using RSTP BPDUs on that port.
Root—A forwarding port that is elected for the spanning-tree topology.
Designated—A forwarding port that is elected for every switched LAN segment.
Alternate—An alternate path to the root bridge to that provided by the current root port.
Chapter 7 Configuring Spanning Tree
"Configuring Multiple Spanning Tree on the
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