[PLUG] Lan loops - follow on PoE injector

Russell Senior russell at personaltelco.net
Tue Dec 24 23:08:12 UTC 2019


Also possibly helpful explanation here:

  https://en.wikipedia.org/wiki/Switching_loop

Probably the best way to demonstrate this would be with a >=4 port
unmanaged switch with two client devices and a loop:

 A----port1 port2----port3 port4----B

where A and B are client devices and ---- are ethernet connections. ping
between A and B. Connect and disconnect the ethernet between port2 and
port3. Observe packet loss and/or a bunch of furiously flashing activity
LEDs on the switch.

On Tue, Dec 24, 2019 at 1:52 PM Mike C. <mconnors1 at gmail.com> wrote:

> The router/switch looks this way:
> - WAN (eth0)
>     +- LAN1 (eth1)
>     +- LAN2 (eth2)
>     +- WLAN3 (wlan0)
>
> The easiest way is to get an old hub / bridge from Free Geek, Goodwill, etc
> > for a $1 and connect it via to Eth1 & Eth2 of the sw/rtr. Viola you have
> a
> > LAN loop!
> >
>
> I suspect the Openwrt sw/rtr will disable one of the eth ports down pretty
> quickly via some version of spanning tree protocol and put a quick end to
> your lil' science experiment.
>
> LAN loops were far more common back in the day of hubs & bridges and the
> more haphazard way LANs were thrown together. Also, all the ports on hubs &
> bridges share the same mac address and as you connect them together not
> only do you expand the collision & broadcast domain but you potentially
> create more than 1 communication path to end devices. FUN!
>
> Probably the most famous story of LAN loop is the spanning tree protocol
> network failure at Beth Israel Deconess hospital. I know about his story
> because when it happened I was a "young" Network Engineer newly hired by
> Nortel Networks attending training in Billerica, MA. I learned all about
> this problem and Nortel's network architecture and technologies to avoid
> this type of catastrophic failure.
>
> For your reading pleasure and edification.
> https://www.computerworld.com/article/2581420/all-systems-down.html
>
> Happy Holidays!,
>
> -- Mike
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