Configuration Bpdu Forwarding Mechanism In Stp - 3Com Switch 4500 26-Port Configuration Manual

Configuration BPDU
Forwarding Mechanism
in STP
CP2 will receive the updated configuration BPDU, {0, 5, 1, BP2}, from Switch B.
Since this configuration BPDU is better then the old one, the old BPDU will be
updated to {0, 5, 1, BP2}.
Meanwhile, CP1 receives the configuration BPDU from Switch A but its
configuration BPDU will not be updated and retain {0, 0, 0, AP2}.
By comparison, {0, 5, 1, BP2}, the configuration BPDU of CP2, is elected as the
optimum one, CP2 is elected as the root port, whose BPDU will not change,
while CP1 will be blocked and retain its BPDU, but it will not receive the data
forwarded from Switch A until spanning tree calculation is triggered again by
some changes. For example, the link from Switch B to C as down.
Thus the spanning tree is stabilized. The tree with the root bridge A is
illustrated in Figure
Figure 52 The Final Stabilized Spanning Tree
Switch B
with priority 1
To facilitate the descriptions, the description of the example is simplified. For
example, the root ID and the designated bridge ID in actual calculation should
comprise both Switch priority and Switch MAC address. Designated port ID
should comprise port priority and port MAC address. In the updating process
of a configuration BPDU, other configuration BPDUs besides the first four items
will make modifications according to certain rules. The basic calculation
process is described below:
Upon the initiation of the network, all the Switches regard themselves as the
roots. The designated ports send the configuration BPDUs of local ports at a
regular interval of HelloTime. If it is the root port that receives the configuration
BPDU, the Switch will enable a timer to time the configuration BPDU as well as
increase MessageAge carried in the configuration BPDU by certain rules. If a path
goes wrong, the root port on this path will not receive configuration BPDUs any
more and the old configuration BPDUs will be discarded due to timeout. Hence,
recalculation of the spanning tree will be initiated to generate a new path to
replace the failed one and thus restore the network connectivity.
However, the new configuration BPDU as now recalculated will not be propagated
throughout the network right away, so the old root ports and designated ports
that have not detected the topology change will still forward the data through the
old path. If the new root port and designated port begin to forward data
immediately after they are elected, an occasional loop may still occur. In RSTP, a
transitional state mechanism is thus adopted to ensure the new configuration
BPDU has been propagated throughout the network before the root port and
52.
Switch A
with priority 0
AP1
5
BP1
BP2
4
Switch C
with priority 2
STP Overview
CP2
171