STP Overview 145
After the comparison processes described in the table above, a spanning tree with
Device A as the root bridge is stabilized, as shown in
Figure 48.
Device C ■ Port CP1 receives the configuration BPDU of Device A
{0, 0, 0, AP2}. Device C finds that the received
configuration BPDU is superior to the configuration
BPDU of the local port {2, 0, 2, CP1}, and updates the
configuration BPDU of CP1.
■ Port CP2 receives the configuration BPDU of port BP2
of Device B {1, 0, 1, BP2} before the message was
updated. Device C finds that the received
configuration BPDU is superior to the configuration
BPDU of the local port {2, 0, 2, CP2}, and updates the
configuration BPDU of CP2.
CP1: {0, 0, 0, AP2}
CP2: {1, 0, 1, BP2}
By comparison:
■ The configuration BPDUs of CP1 is elected as the
optimum configuration BPDU, so CP1 is identified as
the root port, the configuration BPDUs of which will
not be changed.
■ Device C compares the calculated designated port
configuration BPDU {0, 10, 2, CP2} with the
configuration BPDU of CP2, and CP2 becomes the
designated port, and the configuration BPDU of this
port will be replaced with the calculated
configuration BPDU.
Root port CP1:
{0, 0, 0, AP2}
Designated port CP2:
{0, 10, 2, CP2}
■ Next, port CP2 receives the updated configuration
BPDU of Device B {0, 5, 1, BP2}. Because the received
configuration BPDU is superior to its old one, Device
C launches a BPDU update process.
■ At the same time, port CP1 receives configuration
BPDUs periodically from Device A. Device C does not
launch an update process after comparison.
CP1: {0, 0, 0, AP2}
CP2: {0, 5, 1, BP2}
By comparison:
■ Because the root path cost of CP2 (9) (root path cost
of the BPDU (5) + path cost corresponding to CP2 (4))
is smaller than the root path cost of CP1 (10) (root
path cost of the BPDU (0) + path cost corresponding
to CP2 (10)), the BPDU of CP2 is elected as the
optimum BPDU, and CP2 is elected as the root port,
the messages of which will not be changed.
■ After comparison between the configuration BPDU of
CP1 and the calculated designated port configuration
BPDU, port CP1 is blocked, with the configuration
BPDU of the port remaining unchanged, and the port
will not receive data from Device A until a spanning
tree calculation process is triggered by a new
condition, for example, the link from Device B to
Device C becomes down.
Blocked port CP2:
{0, 0, 0, AP2}
Root port CP2:
{0, 5, 1, BP2}
Table 99 Comparison process and result on each device
Device Comparison process
BPDU of port after
comparison
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