Cisco 7609-S Software Configuration Guide - Page 280

Understanding How STP Works Chapter 20 Configuring STP and Prestandard IEEE 802.1s MST STP Port States These sections describe the STP port states: • STP Port State Overview, page 20-6 • Blocking State, page 20-8 • Listening State, page 20-9 • Learning State, page 20-10 • Forwarding State, page 20-11 • Disabled State, page 20-12 STP Port State Overview Propagation delays can occur when protocol information passes through a switched LAN. As a result, topology changes can take place at different times and at different places in a switched network. When a Layer 2 LAN port transitions directly from nonparticipation in the spanning tree topology to the forwarding state, it can create temporary data loops. Ports must wait for new topology information to propagate through the switched LAN before starting to forward frames. They must allow the frame lifetime to expire for frames that have been forwarded using the old topology. Each Layer 2 LAN port on a Cisco 7600 series router using STP exists in one of the following five states: • Blocking-The Layer 2 LAN port does not participate in frame forwarding. • Listening-First transitional state after the blocking state when STP determines that the Layer 2 LAN port should participate in frame forwarding. • Learning-The Layer 2 LAN port prepares to participate in frame forwarding. • Forwarding-The Layer 2 LAN port forwards frames. • Disabled-The Layer 2 LAN port does not participate in STP and is not forwarding frames. A Layer 2 LAN port moves through these five states as follows: • From initialization to blocking • From blocking to listening or to disabled • From listening to learning or to disabled • From learning to forwarding or to disabled • From forwarding to disabled Figure 20-2 illustrates how a Layer 2 LAN port moves through the five states. 20-6 Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX OL-4266-08