Operation Manual – MSTP
H3C S5600 Series Ethernet Switches
Chapter 1 MSTP Configuration
1-9
Device
Comparison process
BPDU of port
after
comparison
z
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.
z
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.
CP1: {0, 0, 0,
AP2}
CP2: {1, 0, 1,
BP2}
By comparison:
z
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.
z
Device C compares the calculated 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 calculated
configuration BPDU.
Root port CP1:
{0, 0, 0, AP2}
Designated port
CP2:
{0, 10, 2, CP2}
z
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.
z
At the same time, port CP1 receives configuration
BPDUs periodically from Device A. Device C does
not launch an update process after comparison.
CP1: {0, 0, 0,
AP2}
CP2: {0, 5, 1,
BP2}
Device
C
By comparison:
z
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.
z
After comparison between the configuration BPDU
of CP1 and the calculated 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 calculation process
is triggered by a new condition, for example, the
link from Device B to Device C becomes down.
Blocked port
CP2:
{0, 0, 0, AP2}
Root port CP2:
{0, 5, 1, BP2}
After the comparison processes described in the table above, a spanning tree with
Device A as the root bridge is stabilized, as shown in
Figure 1-3
.
