Somewhere in the plant, a construction vehicle accidentally cuts the connection between
unmanaged switch #1 and unmanaged switch #2. The managed switch in the network
notices (typically around 6 seconds when connected to an unmanaged switch) that the port in
blocking mode is not receiving configuration messages and transitions through the listening,
learning, and forwarding states (Figure below).
Device C
d
Switch #3
Unmanaged
Switch #2
Device B
Unmanaged
Switch #1
Switch
Device A
Port is
forwarding
Port is
blocking
Link is
broken
This would seem to have solved the problem as both ports in the managed switch are in
forwarding mode, but it is not the case. Due to the fact that the other three switches are
unmanaged, they do not have the intelligence to know that there has been a change in the
network topology. switch #1 still points to switch #2 when device A is trying to talk to device
B (across the broken Ethernet link). The bottleneck has been discovered, as we have to wait
until the MAC table in switch #1 ages out its entries of device A and device B. The same
applies for devices connected to switch #2 (B talking to A) and switch #3 (C talking to A).
As a result of this “money saving” configuration, the network redundancy performance is
traded off and left at the mercy of the time it takes to age out MAC table entries in switches
1, 2, and 3. Depending on the model of unmanaged Ethernet switch, entries in the MAC
table are usually aged out in a time period of 5 minutes or more.
This introduces at least 5 minutes of downtime for the plant, which could have a very
detrimental cost with respect to the operation of the plant. By replacing switches 1, 2, and 3
with managed switches, the network convergence time is reduced to a less than a second. An
additional benefit is that the network is not limited to only one redundant loop and can have
a “mesh” of connections for a truly redundant network scheme at all points in the network.
4-28
Chapter 4 - Managed Switch Software Setup
Stride Industrial Ethernet Switches User Manual 2nd Ed. Rev. A
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