On Wednesday, 5 December 2018 19:07:37 UTC, The Natural Philosopher wrote:

I have to say that is the motrs utter ******** I have read today.
Broadcast packets curculkate around thje network?
Crap.
What ARE you talking about?

OK. I'll spell it out. A lot of VoIP phones have two ethernet ports
so that one port can be connected to the network socket on the wall
or in the floor and the other can have a PC plugged into it.
This is useful when there are not enough network connections for all
the phones and computers. It is done a lot.

Sometimes people do silly things with network cables. On several
occasions in my experience people have plugged BOTH network ports
on a phone into a nearby network switch or into a pair of network
sockets on the wall. It may not be logical or sensible, but it
happens.

The network now has a loop. Broadcast packets can circulate
round that loop indefinitely as the time to live header is not
decremented by switches. Those circulating packets get broadcast
to every device on the network, using up bandwidth and processing
time in the network stack of the connected devices.

The symptoms vary a lot. Sometimes just the phones stop working
properly as they have relatively slow processors. Other times the
whole network grinds to a halt. It does not always happen
immediately - it can take time for enough packets to be in
circulation to cause a problem. Many switches try to minimise
the harm caused by network loops. They limit the maximum throughput
of broadcast packets, usually to around 10% of the connection
speed. This converts a complete disaster into "just" a very
sluggish network. Other switches send probe packets to detect
loops and switch off the affected ports if a loop is detected.
This still causes problems, especially if one of the ports involved
is the uplink port of the switch.

None of this is ******** or crap. I have seen it happen and have
packet dumps to prove it.

Look up "broadcast storm". It is a well known effect.

John