Chapter 8 - Spanning Tree
The Spanning Tree Protocol (STP) can be used to detect and disable network loops, and to provide
backup links between switches, bridges or routers. This allows the switch to interact with other bridging
devices (that is, an STP-compliant switch, bridge or router) in your network to ensure that only one
route exists between any two stations on the network, and provide backup links which automatically
take over when a primary link goes down.
STP uses a distributed algorithm to select a bridging device (STP- compliant switch, bridge or router)
that serves as the root of the spanning tree network. It selects a root port on each bridging device
(except for the root device) which incurs the lowest path cost when forwarding a packet from that
device to the root device. Then it selects a designated bridging device from each LAN which incurs the
lowest path cost when forwarding a packet from that LAN to the root device. All ports connected to
designated bridging devices are assigned as designated ports. After determining the lowest cost
spanning tree, it enables all root ports and designated ports, and disables all other ports. Network
packets are therefore only forwarded between root ports and designated ports, eliminating any
possible network loops.
Figure 8: The Spanning Tree Protocol
Once 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 (Maximum Age), the bridge assumes that the link to the Root Bridge is down. This
bridge will then initiate negotiations with other bridges to reconfigure the network to reestablish a
valid network topology.
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