Cisco CCNA Routing and Switching ICND2 200-101 Official Cert Guide

What IEEE 802.1D Spanning Tree Does

Introduction to STP

STP prevents loops by placing each switch port in either a forwarding state or a blocking state. Interfaces in the forwarding state act as normal, forwarding and receiving frames. However, interfaces in a blocking state do not process any frames except STP messages (and some other overhead messages). Interfaces that block do not forward user frames, do not learn MAC addresses of received frames, and do not process received user frames.

Figure 1-4 shows a simple STP tree that solves the problem shown in Figure 1-3 by placing one port on SW3 in the blocking state.

Figure 1-4 What STP Does: ...