Rockwell ArmorStratix 5700 series, Instructions Manual

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Rockwell Automation Publication 1783-UM007F-EN-P - September 2016 361
Configure Switch Features Chapter 7
Spanning Tree Protocol (STP) STP is a Layer 2 link management protocol that provides path redundancy
while preventing loops in the network. For a Layer 2 Ethernet network to
function properly, only one active path can exist between any two stations.
Multiple active paths among end stations cause loops in the network. If a loop
exists in the network, end stations can receive duplicate messages. Switches can
also learn end-station MAC addresses on multiple Layer 2 interfaces. These
conditions result in an unstable network. Spanning-tree operation is
transparent to end stations, which cannot detect whether they are connected to
a single LAN segment or a switched LAN of multiple segments.
The 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 that is elected for the spanning-tree topology.
• Designated—A forwarding port that is 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 assigned to the Designated role or the
Backup role is the root switch. The switch that has at least one of its ports
assigned to 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 elect the root switch and root port for the switched
network and the root port and designated port for each switched segment.