3Com MSR 50 Series Configuration Manual page 450

450 C 23: MSTP C
HAPTER ONFIGURATION
Table 14 Comparison process and result on each device
Device Comparison process
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.
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 computed 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 computed configuration BPDU.
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.
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 computed 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 computing
process is triggered by a new condition, for example, the
link from Device B to Device C becomes down.
After the comparison processes described in the table above, a spanning tree with
Device A as the root bridge is stabilized, as shown in
BPDU of port
after
comparison
CP1: {0, 0, 0,
AP2}
CP2: {1, 0, 1,
BP2}
Root port CP1:
{0, 0, 0, AP2}
Designated port
CP2:
{0, 10, 2, CP2}
CP1: {0, 0, 0,
AP2}
CP2: {0, 5, 1,
BP2}
Blocked port
CP2:
{0, 0, 0, AP2}
Root port CP2:
{0, 5, 1, BP2}
Figure 129.