| As previously shown, STP implements a
series of timers to prevent bridging loops from occurring within the
network. It can also take 30-50 seconds for a network to converge to
a new topology. While the network is converging, physical addresses
that can no longer be reached are still listed in the switch table.
Because these addresses are in the table, the switch will attempt to
forward frames to devices it cannot reach. Fortunately, the STP
change process requires the switch to clear the table faster in
order to get rid of unreachable physical addresses.
The Figure illustrates a link failure
between Switches D and E, which in turn triggers a topology change
condition. This topology change condition triggers a topology change
BPDU to be generated toward the root bridge. In order for this BPDU
to reach the root bridge, each switch forwards the update out the
root port to the designated bridge.
A topology change in a Spanning-Tree
network triggers the following steps:
- A bridge (Switch D) notices that a
change has occurred to a link.
- The bridge (Switch D) sends a
topology change BPDU out the root port destined ultimately for
the root bridge. The topology-change BPDU is indicated by a
change to the 1-byte flag of the BPDU. The bridge will send out
the topology change BPDU until the designated bridge for that
segment responds with a topology-change acknowledgement (also in
the 1-byte flag field).
- The designated bridge (Switch B)
for that segment sends out a topology-change acknowledgement to
the originating bridge (Switch D). The designated bridge for
that segment also sends a topology-change BPDU out its root port
destined for either the designated bridge of the other segment
or the root bridge.
- When the root bridge receives the
topology-change message, the root bridge will change its
configuration message to indicate that a topology change is
occurring. The root bridge sets the topology change in the
configuration for a period of time equal to the sum of the
FwdDelay and MaxAge parameters.
- A bridge receiving the
topology-change configuration message from the root bridge uses
the FwdDelay timer to age out entries in the address table. This
allows the device to age out entries faster than the normal
5-minute default so that stations that are no longer available will be aged out faster. The bridge
continues this process until it no longer receives
topology-change configuration messages from the root bridge.
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