42 Chapter 2: Spanning Tree Protocol

The only interface on the three switches that does not forward is SW3’s 0/27 port, which is the same spanning-tree topology shown in Figure 2-1. The process is now complete, with all ports in forwarding state except SW3’s E0/27 interface.

Table 2-3 outlines the state of each port and shows why it is in that state.

Port costs can be configured, or you can use the default values. Table 2-4 lists the default port costs defined by IEEE; Cisco uses these same defaults. The IEEE revised the cost values because the original values, set in the early 1980s, did not anticipate the growth of Ethernet to support 10-Gigabit Ethernet.

Table 2-4 Default Port Costs According to IEEE

Reacting to Changes in the Network

After the STP topology has been set, it does not change unless the network topology changes. This section covers STP, but if you want more information, the Cisco Press book Cisco LAN Switching, by Kennedy Clark and Kevin Hamilton, explains STP in great depth. It is worth the time to think about a single example of how STP changes its topology when reacting to network changes, because you can learn a couple of important terms that you will see in real life when working with STP.

Spanning Tree Protocol 43

The root bridge sends a new hello BPDU every 2 seconds by default. Each bridge forwards the hello, changing the cost to reflect that bridge’s added cost to reach the root. Each bridge uses this repetitive hearing of hellos from the root as a way to know that its path to the root is still working, because the hello follows the same path as all the data frames. When a bridge ceases to receive the hellos, something has failed, so it reacts and starts the process of changing the spanning tree.

The hello BPDU defines the timers used by all the bridges when choosing when to react:

■Hello Time—How long the root waits before sending the periodic hello BPDUs, which then are forwarded by successive switches/bridges. The default is 2 seconds.

■MaxAge—How long any bridge should wait, after beginning to not hear hellos, before trying to change the STP topology. Usually this is a multiple of the hello time; the default is 20 seconds.

■Forward Delay—Delay that affects the time involved when an interface changes from blocking state to forwarding state. A port stays in listening state and then learning state for the number of seconds defined by the forward delay. This timer is covered in more depth shortly.

As a quick review, when the network is stable, the STP process works like this:

1.The root sends a hello BPDU, with a cost of 0, out all its interfaces.

2.The neighboring bridges forward hello BPDUs out their nonroot designated ports, identifying the root, but with their cost added.

3.Each bridge in the network repeats Step 2 as it receives these hello BPDUs.

4.The root repeats Step 1 every hello time.

5.If a bridge does not get a hello BPDU in hello time, it continues as normal. If a bridge fails to receive a hello BPDU for an entire MaxAge time, the bridge reacts.

For example, imagine that the link between SW1 and SW3 fails, as shown in Figure 2-5.