[PLUG] Lan loops - follow on PoE injector

Mike C. mconnors1 at gmail.com
Tue Dec 24 21:46:34 UTC 2019


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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