Rockwell Automation 1783-EMS08T Stratix 6000 Ethernet Managed Switch User Manual - Manual (Page 54)

54 Rockwell Automation Publication 1783-UM001D-EN-P - January 2013
Chapter 4 Switch Management
STP uses a spanning-tree algorithm to select one switch of a redundantly
connected network as the root of the spanning tree. The algorithm calculates the
best loop-free path through a switched Layer 2 network by assigning a role to
each port based on the role of the port in the active topology:
• Root—A forwarding port elected for the spanning-tree topology
• Designated—A forwarding port elected for every switched LAN segment
• Alternate—A blocked port providing an alternate path to the root bridge
in the spanning tree
• Backup—A blocked port in a loopback configuration
The switch that has all of its ports as the designated role or as the backup role is
the root switch. The switch that has at least one of its ports in the designated role
is called the designated switch.
Spanning tree forces redundant data paths into a standby (blocked) state. If a
network segment in the spanning tree fails and a redundant path exists, the
spanning-tree algorithm recalculates the spanning-tree topology and activates the
standby path. Switches send and receive spanning-tree frames, called bridge
protocol data units (BPDUs), at regular intervals. The switches do not forward
these frames but use them to construct a loop-free path. BPDUs contain
information about the sending switch and its ports, including switch and MAC
addresses, switch priority, port priority, and path cost. Spanning tree uses this
information to select the root switch and root port for the switched network and
the root port and designated port for each switched segment.
Rapid Spanning Tree Protocol
Rapid Spanning Tree Protocol (RSTP) is an enhanced version of STP that uses
point-to-point wiring and provides rapid convergence of the spanning tree.
When a point-to-point connection fails, the alternate connection transitions to
the forwarding state.
RSTP is implemented on the switch in these ways:
• You can enable RSTP or STP mode on the RSTP Configuration tab. By
default, both RSTP and STP are disabled.
• A single instance of RSTP exists for the entire network, regardless of the
number of VLANs. This implementation is known as Common Spanning
Tree (CST).
• RSTP parameters are port-dependent, or used for determining a specific
port’s behavior.
• If port mirroring is configured on a port, or a port becomes disabled via
the switch’s web interface, RSTP becomes disabled on the port.