1D 16-bit bridge priority ?¬?eld is
split into two ?¬?elds, resulting in these components in the BID:
?– Bridge priority: A 4-bit ?¬?eld still used to carry bridge priority. Because of the limited
bit count, the priority is conveyed in discreet values in increments of 4096 rather than
discreet values in increments of 1, as they would be if the full 16-bit ?¬?eld were
available. The default priority, in accordance with IEEE 802.1D, is 32,768, which is
the midrange value.
?– Extended system ID: A 12-bit ?¬?eld carrying, in this case, the VID for PVST+.
?– MAC address: A 6-byte ?¬?eld with the MAC address of a single switch.
By virtue of the MAC address, a BID is always unique. When the priority and extended
system ID are prepended to the switch MAC address, each VLAN on the switch can be
represented by a unique BID.
If no priority has been con?¬?gured, every switch will have the same default priority, and the
election of the root for each VLAN will be based on the MAC address. This method is a
random means of selecting the ideal root bridge; for this reason, it is advisable to assign a
lower priority to the switch that should serve as the root bridge.
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