HP A3100-16 v2 EI, Configuration Manual

Page: 59 / 193

Device Comparison process
Configuration BPDU on
ports after comparison
•
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.
•
Blocked port (Port C1):
{0, 0, 0, Port A2}
•
Root port (Port C2): {0,
5, 1, Port B2}
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 Table 11 , a spanning tree with Device A as the root bridge
is established, and the topology is shown in Figure 13 .
Figure 13 Topology of the final calculated spanning tree
The BPDU forwarding mechanism in STP
STP forwards configuration BPDUs following these guidelines:
• Upon network initiation, every switch regards itself as the root bridge, generates configuration
BPDUs with itself as the root, and sends the configuration BPDUs at a regular hello interval.
•
If the root port received a configuration BPDU and the received configuration BPDU is superior to
the configuration BPDU of the port, the device increases the message age carried in the
configuration BPDU following a certain rule and starts a timer to time the configuration BPDU while
sending this configuration BPDU through the designated port.
52