STP calls for each switch or bridge to be assigned a unique BID. Typically, the BID is
composed of a priority value (2 bytes) and the bridge MAC address (6 bytes). The default
priority, in accordance with IEEE 802.1D, is 32,768 (1000 0000 0000 0000 in binary, or
0x8000 in hex format), which is the midrange value. The root bridge is the bridge with the
lowest BID.
Example: Selecting the Root Bridge
In Figure 2-23, both switches use the same default priority. The switch with the lowest
MAC address is the root bridge. In the example, switch X is the root bridge, with a BID of
0x8000 (0c00.1111.1111).
Figure 2-23 Root Bridge Selection
There are ?¬?ve STP port states:
?– Blocking
?– Listening
?– Learning
?– Forwarding
?– Disabled
NOTE A Cisco Catalyst switch uses one of its MAC addresses from a pool of MAC
addresses that are assigned either to the backplane or to the supervisor module,
depending on the switch model.
Switch Y
Default Priority 32768
(0x8000)
MAC 0c00.2222.2222
Switch X
Default Priority 32768
(0x8000)
MAC 0c00.1111.1111
BPDU
Switch X Switch Y
52 Chapter 2: Medium-Sized Switched Network Construction
When STP is enabled, every bridge in the network goes through the blocking state and the
transitory states of listening and learning at power-up.
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