Understanding the Spanning Tree
Protocol
Switches either forward or filter Layer 2 frames. The way they make the
forwarding/filtering decision can lead to loops in a network with redundant
links. Spanning Tree is a protocol that detects potential loops and breaks
them.
A Layer 2 switch is functionally the same thing as a transparent bridge.
Transparent bridges:
■ Learn MAC (Media Access Control) addresses by looking at the
source address of incoming frames. They build a table mapping MAC
address to port number.
■ Forward broadcasts and multicasts out all ports except the on which
they came. (This is called flooding.)
■ Forward unknown unicasts out all ports except the one on which they
came. An unknown unicast is a message bound for a unicast MAC
address that is not in the switch’s table of addresses and ports.
■ Do not make any changes to the frames as they forward them.
Spanning Tree Protocol (STP) works by selecting a root bridge, then selecting
one loop-free path from the root bridge to every other switch. (STP uses the
term bridge because it was written before there were switches.) Consider the
following switched network (see Figure 3-1).