Figure 2-22 shows the ?¬?nal state of a Layer 2 network after spanning tree has performed
the operations to eliminate loops.
Figure 2-22 Loop Avoidance
Spanning-Tree Operation
STP performs three steps to provide a loop-free logical network topology:
1. Elects one root bridge: STP has a process to elect a root bridge. Only one bridge can
act as the root bridge in a given network. On the root bridge, all ports are designated
ports. Designated ports are in the forwarding state and are designated to forward traf?¬?c
for a given segment. When in the forwarding state, a port can send and receive traf?¬?c.
In Figure 2-22, switch X is elected as the root bridge.
2. Selects the root port on the nonroot bridge: STP establishes one root port on each
nonroot bridge. The root port is the lowest-cost path from the nonroot bridge to the root
bridge. Root ports are in the forwarding state. Spanning-tree path cost is an
accumulated cost calculated on the bandwidth. In Figure 2-22, the lowest-cost path to
the root bridge from switch Y is through the 100BASE-T FastEthernet link.
3. Selects the designated port on each segment: On each segment, STP establishes one
designated port.
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