Network Loop causing a broadcast

Hello, I hope someone can help reasure me that I have resolved this problem.

I am supporting a large network supporting over 4500 client PC and 2000 peripherals.

Today I noticed a segment of the network had failed so I quickly responded by going down to the distribution nodes to replace a know intermittently faulty hub.

Unfortunately because the cabling in the cabinet is (soon not to be) very messy when I connected the new intelligent device I did not notice a rogue patch cable connecting to the next switch in line.

Once I reconnected all the patch cables I had removed as well as the uplink, and returned back to the office I started to get a complete new network outage. I believe this to be a broadcast so I went back to the cabinet and pulled out each cable one by one until the network settled down. The patch cable in question was connected back to another switch. This switch in question is able to auto negotiate and determine if the patch cable connected is an x over cable (or at least I think this is the case).

So my question is did the rogue patch cable cause the fault? Considering this device should be intelligent enough handle this type of fault.

Many Thanks

 

This does sound very much like a broadcast storm. A bog standard switch is not intelligent enough to handle loops.

For example where switch one is connected to switch two, which is connected to switch three, which is itself connected back to switch one. When you get a broadcast sent out from switch one, it goes to switch two. Switch two sees that it comes from switch one and passes it on to switch three. Switch three receives the broadcast. It won't send it back to switch two, as that is where it sees it coming from, but it will send it on to switch one as it does not know that that is where the broadcast originated. Switch one now receives the broadcast it originally sent out, but sees it coming from switch three and passes it on to switch two. The result: every broadcast starts looping round the three switches until it times out. The effect is magnified by the fact that there may well be a loop coming back the other way (where the initial broadcast goes from 1 to switch 3, then 2, and then back to 1)

There is a system called spanning tree that detects that there is more that one path to the same location and switches off one of the connections to break the loop. This is available on medium to high end switches.

I expect that either you broke the loop by moving the connection to another switch, or you connected a spanning tree enabled switch into the loop and that that switch broke the loop by shutting down the link.