HP A6600 Configuration Manual page 54

Device Comparison process
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In Table 11, each configuration BPDU contains four fields: {root bridge ID, root path cost, designated
bridge ID, designated port ID}.
After the comparison processes described in the table above, a spanning tree with Device A as the root
bridge is established, and the topology is shown in
Figure 15 Topology of the final calculated spanning tree
The spanning tree calculation process in this example is only simplified process.
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 spanning tree calculation
process is triggered by a new event, for example, the
link between Device B and Device C is down.
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Figure 15.
Configuration BPDU on
ports after comparison
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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}