H3C Technologies H3C S5560 Series Switches - Manual (Page 101)

67
Device Comparison process
Configuration BPDU on
ports after comparison
Device C determines 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)). Device C determines 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 performs the following tasks:
35. Calculates a designated port configuration BPDU for Port
C1 {0, 9, 2, Port C1}.
36. Compares it with the existing configuration BPDU of Port
C1 {0, 0, 0, Port A2}.
37. Determines that the existing configuration BPDU is
superior to the calculated one and blocks Port C1 with
the configuration BPDU unchanged.
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.
• Blocked port (Port C1):
{0, 0, 0, Port A2}
• Root port (Port C2): {0,
5, 1, Port B2}
After the comparison processes described in Table 8 , a spanning tree with Device A as the root bridge
is established, as shown in Figure 22 .
Figure 22 The final calculated spanning tree
The configuration BPDU forwarding mechanism of STP
The configuration BPDUs of STP are forwarded according to these guidelines:
• Upon network initiation, every device regards itself as the root bridge and generates configuration
BPDUs with itself as the root. Then it sends the configuration BPDUs at a regular hello interval.
• If the root port received a configuration BPDU superior to the configuration BPDU of the port, the
device performs the following tasks:
{
Increases the message age carried in the configuration BPDU.
{
Starts a timer to time the configuration BPDU.
{
Sends this configuration BPDU through the designated port.