HP A5120 EI Series Configuration Manual page 67

Device Comparison process
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NOTE:
In Table 11, each configuration BPDU contains the following fields: root bridge ID, root path cost,
designated bridge ID, and designated port ID.
After the comparison processes described in
established, and the topology is shown in
Device C compares the configuration BPDUs of all its
ports, decides that the configuration BPDU of Port C1 is
the optimum, and selects Port C1 as the root port with
the configuration BPDU unchanged.
Based on the configuration BPDU and path cost of the
root port, Device C calculates the configuration BPDU
of Port C2 {0, 10, 2, Port C2}, and compares it with
the existing configuration BPDU of Port C2 {1, 0, 1, Port
B2}. Device C finds that the calculated configuration
BPDU is superior to the existing one, selects Port C2 as
the designated port, and replaces the configuration
BPDU of Port C2 with the calculated one.
Port C2 receives the updated configuration BPDU of
Port B2 {0, 5, 1, Port B2}, finds that the received
configuration BPDU is superior to its existing
configuration BPDU {0, 10, 2, Port C2}, and updates
its configuration BPDU.
Port C1 receives a periodic configuration BPDU {0, 0,
0, Port A2} from Port A2, finds that it is the same as the
existing configuration BPDU, and discards the received
one.
Device C finds that the root path cost of Port C1 (10)
(root path cost of the received configuration BPDU (0)
plus path cost of Port C1 (10)) is larger than that of Port
C2 (9) (root path cost of the received configuration
BPDU (5) plus path cost of Port C2 (4)), decides that
the configuration BPDU of Port C2 is the optimum, and
selects Port C2 as the root port with the configuration
BPDU unchanged.
Based on the configuration BPDU and path cost of the
root port, Device C calculates a designated port
configuration BPDU for Port C1 {0, 9, 2, Port C1} and
compares it with the existing configuration BPDU of
Port C1 {0, 0, 0, Port A2}. Device C finds that the
existing configuration BPDU is superior to the
calculated one and blocks Port C1 with the
configuration BPDU unchanged. Then Port C1 does not
forward data until a new event triggers a spanning tree
calculation process, for example, the link between
Device B and Device C is down.
Table 1
Figure
60
1, a spanning tree with Device A as the root bridge is
17.
Configuration BPDU on
ports after comparison
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Root port (Port C1): {0,
0, 0, Port A2}
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Designated port (Port
C2): {0, 10, 2, Port
C2}
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Port C1: {0, 0, 0, Port
A2}
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Port C2: {0, 5, 1, Port
B2}
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Blocked port (Port C1):
{0, 0, 0, Port A2}
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Root port (Port C2):
{0, 5, 1, Port B2}