3Com CoreBuilder 3500, Implementation Manual, Page: 123 / 592

How the Spanning Tree Protocol Works 123
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.
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.
How a Single Bridge
Interprets CBPDUs
The following case describes how
a single bridge
interprets CBPDUs and
contributes to the Spanning Tree configuration.
1
When Spanning Tree is first started on a network, the bridge acts as if 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, for the moment, it is the root bridge)
■
Its own bridge ID as the transmitting ID (for example,
85
)
Thus, its CBPDU looks like this:
85.0.85
.
2 The bridge receives CBPDUs on each of its ports from all other bridges
and saves the
best CBPDU from each port.
The bridge determines the best CBPDU by comparing the information in
each message that arrives at a particular port to the message that is
currently stored at that port. In general, the lower the value of the
CBPDU, the
better
it is. When the bridge comes across a better CBPDU
than it has stored, it replaces the old message with the new one.
Message 1 Message 2
root ID cost transmitter root ID cost transmitter
29 15 80 29 18 38
Message 1 Message 2
root ID cost transmitter root ID cost transmitter
35 80 39 35 80 40