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Spanning Tree Operation

159

Root Priority/ID: 0x8000 / 00:b0:64:75:6b:c0

Cost Of Path To Root: 0x00000000 (0)

Bridge Priority/ID: 0x8000 / 00:b0:64:75:6b:c0

Port Priority/ID: 0x80 / 0x03

Message Age: 0/256 seconds
(

exactly 0seconds)

Maximum Age: 5120/256
seconds (

exactly 20seconds)

Hello Time: 512/256 seconds
(

exactly 2seconds)

Forward Delay: 3840/256
seconds (

exactly 15seconds)

Extra bytes (Padding):

........ 00 00 00 00 00 00 00 00

Frame Check Sequence: 0x2e006400

Notice the cost of path to root. It is zero because this switch is actually the

root bridge. We'll discuss path costs in more detail in the next section,
"Selecting the Designated Port."

The network analyzer output above also shows the BPDU timers, which

are used to prevent bridging loops because the timers determine how long it
will take the spanning tree to converge after a failure.

BPDUs are susceptible to propagation delays, which happen because of

packet length, switch processing, bandwidth, and utilization problems. This
can create an unstable network because temporary loops might occur in the
network when BPDUs are not received on time to the remote switches in the
network. The STP uses timers to force ports to wait for the correct topology
information.

As you can see in the output, the hello time is exactly 2 seconds, the max-

imum age is exactly 20 seconds, and the forward delay is exactly 15 seconds.

Selecting the Designated Port

After you have selected the root bridge, all switches must become buddies
with the root bridge. Each switch listens to BPDU on all active ports, and if
more than one BPDU is received, the switch knows it has a redundant link
to the root bridge. The switch has to determine which port will become the
root port and which port will be put into blocking state.

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