[PLUG] Lan loops - follow on PoE injector

Russell Senior russell at personaltelco.net
Wed Dec 25 09:32:22 UTC 2019


I happened to have a netgear FS105 nearby. Plugging in a laptop to a switch
port, and plugging a patch cable between two other switch ports and pinging
a random ip address from the laptop set off the broadcast storm. Running
tcpdump from the laptop showed a bunch of "MPCP, Opcode Pause, length 46"
packets. Unplugging the loop, the packets stop immediately. Note, this
mostly won't be a problem on a pre-auto-mdix switch[1] unless you are
taking particularly good aim at your foot.

[1] https://en.wikipedia.org/wiki/Medium-dependent_interface#Auto_MDI-X

Happy Holidays!


On Tue, Dec 24, 2019 at 3:08 PM Russell Senior <russell at personaltelco.net>
wrote:

> 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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>> PLUG at pdxlinux.org
>> http://lists.pdxlinux.org/mailman/listinfo/plug
>>
>



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