144 CHAPTER 14: MSTP CONFIGURATION
Tabl e 99 Comparison process and result on each device
Device Comparison process
BPDU of port after
comparison
Device A ■ Port AP1 receives the configuration BPDU of Device B
{1, 0, 1, BP1}. Device A finds that the configuration
BPDU of the local port {0, 0, 0, AP1} is superior to the
configuration received message, and discards the
received configuration BPDU.
■ Port AP2 receives the configuration BPDU of Device C
{2, 0, 2, CP1}. Device A finds that the BPDU of the
local port {0, 0, 0, AP2} is superior to the received
configuration BPDU, and discards the received
configuration BPDU.
■ Device A finds that both the root bridge and
designated bridge in the configuration BPDUs of all
its ports are Device A itself, so it assumes itself to be
the root bridge. In this case, it does not make any
change to the configuration BPDU of each port, and
starts sending out configuration BPDUs periodically.
AP1: {0, 0, 0, AP1}
AP2: {0, 0, 0, AP2}
Device B ■ Port BP1 receives the configuration BPDU of Device A
{0, 0, 0, AP1}. Device B finds that the received
configuration BPDU is superior to the configuration
BPDU of the local port {1, 0,1, BP1}, and updates the
configuration BPDU of BP1.
■ Port BP2 receives the configuration BPDU of Device C
{2, 0, 2, CP2}. Device B finds that the configuration
BPDU of the local port {1, 0, 1, BP2} is superior to the
received configuration BPDU, and discards the
received configuration BPDU.
BP1: {0, 0, 0, AP1}
BP2: {1, 0, 1, BP2}
■ Device B compares the configuration BPDUs of all its
ports, and determines that the configuration BPDU of
BP1 is the optimum configuration BPDU. Then, it uses
BP1 as the root port, the configuration BPDUs of
which will not be changed.
■ Based on the configuration BPDU of BP1 and the
path cost of the root port (5), Device B calculates a
designated port configuration BPDU for BP2 {0, 5, 1,
BP2}.
■ Device B compares the calculated configuration BPDU
{0, 5, 1, BP2} with the configuration BPDU of BP2. If
the calculated BPDU is superior, BP2 will act as the
designated port, and the configuration BPDU on this
port will be replaced with the calculated
configuration BPDU, which will be sent out
periodically.
Root port BP1:
{0, 0, 0, AP1}
Designated port BP2:
{0, 5, 1, BP2}
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