The designated port is selected on the bridge that has the lowest-cost
path to the root bridge. Designated ports are in the forwarding state, forwarding traf?¬?c
for the segment. In Figure 2-22, the designated port for both segments is on the root
bridge because the root bridge is directly connected to both segments. The 10BASE-T
Ethernet port on switch Y is a nondesignated port because there is only one designated
port per segment. Nondesignated ports are normally in the blocking state to logically
break the loop topology. When a port is in the blocking state, it is not forwarding data
traf?¬?c but can still receive traf?¬?c.
Nondesignated Port (B) Designated Port (F)
Designated Port (F)
Nonroot Bridge
Root Port (F)
Root Bridge
100 BASE-T
10 BASE-T
Switch X Switch Y
Improving Performance with Spanning Tree 51
Switches and bridges running the Spanning Tree Algorithm exchange con?¬?guration
messages with other switches and bridges at regular intervals (every 2 seconds by default).
Switches and bridges exchange these messages using a multicast frame called the BPDU.
One of the pieces of information included in the BPDU is the bridge ID (BID).
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