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Page 63
Chapter 3: Configuration
LGB1108A
3.4 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 (an 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 3-17. 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.