Spanning Tree Protocol 45
The first, listening, allows each device to wait to make sure that there are no new, better hellos with a new, better root. The second state, learning, allows the bridge to learn the new location of MAC addresses without allowing forwarding and possibly causing loops. These states help prevent the switches from flooding frames until all the switches have converged and learned the new MAC addresses.
Using default timers, SW3 requires 50 seconds before it can place port fastethernet 0/27 in forwarding state. First, SW3 waits MaxAge seconds before deciding that it is no longer receiving the same root BPDU in its root port (20 seconds is the default). At that point, SW3 places port fastethernet 0/27 in listening state for Forward Delay seconds (15 seconds is the default). After that, SW3 places fastethernet 0/27 in learning state for another Forward Delay seconds before transitioning the port to forwarding state. So, a total of MaxAge plus twice
Forward Delay, or 50 seconds, is required.
Table 2-5 summarizes spanning tree’s intermediate states.
Table 2-5 |
Spanning-Tree Intermediate States |
|
||
|
|
|
|
|
|
|
Forwards |
Learns MACs Based on |
Transitory or Stable |
|
State |
Data Frames? |
Received Frames? |
State? |
|
|
|
|
|
|
Blocking |
No |
No |
Stable |
|
|
|
|
|
|
Listening |
No |
No |
Transitory |
|
|
|
|
|
|
Learning |
No |
Yes |
Transitory |
|
|
|
|
|
|
Forwarding |
Yes |
Yes |
Stable |
|
|
|
|
|
SW3 also must tell the other switches to timeout the entries in their bridging tables. For instance, SW2's bridging table lists Bob’s MAC address as being out port 0/26, but now it will be out port 0/27. So, SW3 sends a special Topology Change Notification (TCN) BPDU out port 0/27. When SW2 receives the TCN, it decides to timeout all MAC table entries based on the MaxAge timer (default 20 seconds). Because SW3 sends the TCN BPDU as soon as it puts its interface into a listening state, SW2 has removed the entry for Bob's MAC address before SW3 begins forwarding. (SW2 forwards the TCN to the root switch as well, who makes sure all other bridges/switches know to timeout MAC table entries quickly as well.)
Spanning Tree Protocol Summary
Spanning trees accomplish the goal of allowing physical redundancy, but with only one currently active path through a bridged network. Spanning tree uses the following features to accomplish the goal:
■All bridge interfaces eventually stabilize at either forwarding or blocking state. The forwarding interfaces are considered a part of the spanning tree.