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 <[email protected]> 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. <[email protected]> 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 >> _______________________________________________ >> PLUG mailing list >> [email protected] >> http://lists.pdxlinux.org/mailman/listinfo/plug >> > _______________________________________________ PLUG mailing list [email protected] http://lists.pdxlinux.org/mailman/listinfo/plug
