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A bridge ID (BID) is a
single, 8-byte field that is composed of two subfields, as
illustrated in the Figure. The low-order subfield consists of a
6-byte MAC address assigned to the switch. Some switches use one of
the MAC addresses from the pool of 1024 addresses assigned to every
supervisor or backplane. This is a hard-coded number that is
designed not to be changed by the user. The MAC address in the BID
is expressed in the usual hexadecimal (base 16) format.
The high-order BID
subfield is referred to as the bridge priority. The Bridge Priority
field is a 2-byte (16-bit) value. This field can have 216 possible
values that range from 0 to 65,535. The default bridge priority is
the midpoint value, 32,768. Bridge priorities are typically
expressed in a decimal (base 10) format. The key point to remember
concerning STP cost values is that lower costs are better.
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