Short-circuiting two ethernet outlets

October 20, 2011 12:40 AM

Why is short-circuiting two ethernet outlets such a big deal?

Today we got an angry mail from our IT -Department, they were very upset about somebody connecting two ethernet wall outlets with one cat5 cable. Apparently it caused a massive disturbance in our intranet and was difficult to track down. So, why does this cause such a big problem?

posted by SweetLiesOfBokonon to Computers & Internet (10 answers total)

Yep that'll annoy an IT department.
I believe the problem is called a switching loop.
posted by Packed Lunch at 1:12 AM on October 20, 2011 [1 favorite]

Best answer: Yes. So a broadcast packet enters a switch and is sent out every port. It goes out portA and portB (that you have just connected) along with portC, portD, (all the rest of the ports on the switch). Now that packet going out portA is received on portB, and the packet sent out portB is received on portA and presto! the switch now has received *two* incoming broadcast packets which it dutifully sends out each port yet again and BOOM two more copies of the packet come back in and dammit it floods those out again, and again, and again, and *BOOM*.
posted by zengargoyle at 1:13 AM on October 20, 2011 [3 favorites]

It sounds like it is causing a loop
posted by zemblamatic at 1:13 AM on October 20, 2011

Best answer: By connecting two Ethernet end-points together with a cable your colleague created a switching loop. Its likely that the trouble your intranet subsequently experienced was a broadcast storm. During a broadcast storm the network is flooded by packets being broadcast across the entire network, driven by one or more broadcast packets repeatedly bouncing back and forth between the switches on the loop. A well engineered network moderates the effect of a loop by either running Spanning Tree Protocol (which detects loops and breaks one of the redundant legs of the loop), dividing the network into separate broadcast domains, or both.
posted by RichardP at 1:18 AM on October 20, 2011 [4 favorites]

You know how short circuits in electric circuits cause power to build up way too fast?

Like that, but with data.
posted by LogicalDash at 3:56 AM on October 20, 2011 [1 favorite]

While it was a dumb thing to do, the real question is why your IT department has a L2 switched network in which doing this results in a broadcast storm. A well-designed modern network generally doesn't need to rely too much on spanning tree to keep things straight, but it's still a safety net that prevents things like this from happening. Not to mention that there are a ton of other safeguards to avoid switching loops (i.e., Cisco's bpduguard feature and other vendors' equivalents). What should have happened is that one or both of the switches involved should have shut down the affected ports as soon as they realized that there was a switch on the other side that wasn't supposed to be there.

The fact that it happened suggests to me that either A) your IT department switched off the safety features on the switches because they were inconvenient, or B) they built a network using cheap unmanaged consumer-grade switches. Someone deserves to be smacked around for this, but it's not the person who connected two switch ports.
posted by McCoy Pauley at 8:30 AM on October 20, 2011 [3 favorites]

Out of curiosity, why would anyone take an ethernet cable and connect two ethernet outlets? Was there a point to doing that, or was it just a mistake?
posted by exphysicist345 at 5:55 PM on October 20, 2011

It's an inexpensive ddos attack, probably a prank I suppose.
posted by LogicalDash at 7:26 PM on October 20, 2011

(eh, not exactly distributed, whatever)
posted by LogicalDash at 7:26 PM on October 20, 2011

Frankly, if you're trying to do a DoS attack that way, you might as well just go whole hog and build yourself an etherkiller.

(Extra points if you deliver a line like "Now you got power over ethernet, baby!" in a Schwarzenegger voice as you plug it in.)
posted by McCoy Pauley at 8:11 PM on October 20, 2011