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the transmitting bridge ID. If the CBPDUs all have the same root ID, cost,
and transmitting bridge ID, then the port identifier is used as a tiebreaker.
Example 1. Message 1 has a lower root ID, so the bridge saves the
message.
Message 1
root ID
12
Example 2. Root ID is the same for Message 1 and Message 2, but cost is
lower in Message 1. The bridge saves Message 1.
Message 1
root ID
29
Example 3. Root ID and cost are the same for Message 1 and Message 2,
but the transmitting bridge ID is lower in Message 1. The bridge saves
Message 1.
Message 1
root ID
35
The following case describes how one bridge interprets CBPDUs, thus
How a bridge
handles CBPDUs
contributing to the Spanning Tree configuration. For purposes of this case,
the following convention is used to depict a CBPDU:
root ID.cost.transmitter ID.
1 When Spanning Tree is first started on a network, the bridge thinks that it is
the root bridge and transmits a CBPDU from each of its ports with the
following information:
Its own bridge ID as the root ID (for example, 85)
Zero (0) as the cost (because it thinks it is the root bridge)
Its own bridge ID as the transmitting ID (for example, 85)
Spanning Tree and the Bridged Network
cost
transmitter root ID
15
35
cost
transmitter root ID
15
80
cost
transmitter root ID
80
39
Message 2
cost
31
12
Message 2
cost
29
18
Message 2
cost
35
80
5-11
transmitter
32
transmitter
38
transmitter
40
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