Westermo U200 Operator manal - Manual (Page 22)

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8 Rapid Spanning Tree Protocol (RSTP)
The R/T200 switch series supports the Rapid Spanning Tree Protocol (RSTP) according to
IEEE802.1w with fall-back to the Spanning Tree Protocol (STP - IEEE802.1D). The STP
fallback feature means that the R/T200 switches can be used together with switches that only
have support for STP.
RSTP/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 might receive
duplicate messages with broadcast storm and an unstable network as result.
The STP uses a spanning-tree algorithm to select one switch of a redundantly connected
network as the root (focal point) 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 port in the spanning
tree
Switches that have ports with these assigned roles are called root or designated switches.
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. When two ports on a switch are part of a loop, the spanning-
tree port priority and path cost settings determine which port is put in the forwarding state and
which is put in the blocking state. The spanning-tree port priority value represents the location
of a port in the network topology and how well it is located to pass traffic. The path cost value
represents the media speed.
The stable, active spanning-tree topology of a switched network is determined by these
elements:
• The unique bridge ID (switch priority and MAC address) associated with each VLAN
on each switch
• The spanning-tree path cost to the root switch
• The port identifier (port priority and MAC address) associated with each port
When the switches in a network are powered up, each switch functions as if is the root switch.
Each switch sends a configuration BPDU through all of its ports. The BPDUs communicate
and compute the spanning-tree topology. Each configuration BPDU contains this information:
• The unique bridge ID of the switch that the sending switch identifies as the root switch
• The spanning-tree path cost to the root
• The bridge ID of the sending switch
• Message age
• The identifier of the sending port
• Values for the hello, forward-delay, and max-age protocol timers
When a switch receives a configuration BPDU that contains superior information (lower bridge
ID, lower path cost, and so forth), it stores the information for that port. If this BPDU is