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4.8.3 Spanning Tree Algorithm
In Ethernet networks, only one active path may exist between any two bridges or switches. Multiple
active paths between switches can cause loops in the network. When loops occur, some switches
recognize stations on both sides of the switch. This situation causes the forwarding algorithm to
malfunction allowing duplicate frames to be forwarded. Spanning tree algorithms provide path
redundancy by defining a tree that spans all of the switches in an extended network and then forces
certain redundant data paths into a standby (blocked) state. At regular intervals, the switches in the
network send and receive spanning tree packets that they use to identify the path. If one network
segment becomes unreachable, or if spanning tree costs change, the spanning tree algorithm
reconfigures the spanning tree topology and re-establishes the link by activating the standby path.
Spanning tree operation is transparent to end stations, which do not detect whether they are connected
to a single LAN segment or a switched LAN of multiple segments.
Spanning Tree Protocol (STP) is a Layer 2 protocol designed to run on bridges and switches. The
specification for STP is defined in IEEE 802.1d. The main purpose of STP is to ensure that you do
not run into a loop situation when you have redundant paths in your network. STP detects/disables
network loops and provides backup links between switches or bridges. It allows the device to interact
with other STP compliant devices in your network to ensure that only one path exists between any
two stations on the network.
After a stable network topology has been established, all bridges listen for hello BPDUs (Bridge
Protocol Data Units) transmitted from the root bridge. If a bridge does not get a hello BPDU after a
predefined interval (Max Age), the bridge assumes that the link to the root bridge is down. This
bridge then initiates negotiations with other bridges to reconfigure the network to re-establish a valid
network topology. The process to create a new topology can take up to 50 seconds. During this time,
end-to-end communications are interrupted.
The use of Spanning Tree is not recommended for ports that are connected to end stations, because by
definition, an end station does not create a loop within an Ethernet segment. Additionally, when a
teamed adapter is connected to a port with Spanning Tree enabled, users may experience unexpected
connectivity problems. For example, consider a teamed adapter that has a lost link on one of its
physical adapters. If the physical adapter were to be reconnected (also known as fallback), the
intermediate driver would detect that the link has been reestablished and would begin to pass traffic
through the port. Traffic would be lost if the port was temporarily blocked by the Spanning Tree
Protocol.
