The Solution to the Layer 2 Loop
STP, or 802.1D, was developed to identify parallel Layer 2 paths and block on one of the redundant paths so that a Layer 2 loop would not occur. A single switch with the lowest bridge ID becomes the root bridge, and then all the other nonroot switches determine whether they have parallel paths to the root and block on all but one of those paths. STP communicates using bridge protocol data units (BPDU), and that is how negotiation and loop detection are accomplished.
Example 9-4, which contains annotations, allows you to both review how STP operates and see the commands to verify it at the same time; it uses the topology from the beginning of this chapter.
Example 9-4 STP Verification and Annotations
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