Loop on a network

I got a call today saying that there is a some kind of loop that's causing an issue in one of my classrooms. They told me that had to disable the port until i found out what was causing this issue. i was told that it could be a switch being plugged back into itself or a bad network card or some other things.

The loop ended up being caused because a switch was being plugged back into itself. My question is that, is there any software that i can use in the future that would make this easier, so i wouldnt have to go to each computer just in case it was a bad network card?

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Comments

rsutton

What kind of switches are these? Cisco switches have features you can enable to prevent loops, actually I think for most switches STP is enabled by default.

RobertKaucher

I know of no software that could track down a bad NIC causing a broadcast storm. But I can tell you that 9 times out of 10 a broad cast storm is going to be caused by something like what you found.

Any decent managed switch should be able to give additional information.

kriscamaro68

STP on a switch is suppose to prevent network loop backs or broadcast storms if i remember correctly. I would log into the management int on the switches throughout the building and make sure this is setup. If they are cisco switches I believe you can also set them up to disable a port that is getting stormed automatically.

EmpoweredBizTech

The best way I found to find a NIC that's is broadcasting way too much is to download Wireshark and sniff the line under promiscuous mode. as For the switch like the other guys said if you are using a managed switch STP or Spanning Tree Protocol is your friend in that arena

MAC_Addy

C'mon man. I know we work in IT... but you have to get up off your butt once in a while!!

haha. I'm just playing with you.

ULWiz

If you looking for software then Wireshark is also my recomendation. Should be able to see it there.

ColbyG

Spanning Tree weee.

Hyper-Me

As others have said, STP.

Unfortunately, the Network Admins where I work are ignorant so they turn STP OFF!! on all our switches because they dont know how to configure it right, and thus we have to manually troubleshoot loops.

Kaminsky

portfast ?

rwwest7

Hyper-Me wrote: »

As others have said, STP.

Unfortunately, the Network Admins where I work are ignorant so they turn STP OFF!! on all our switches because they dont know how to configure it right, and thus we have to manually troubleshoot loops.

Please enlighten us ignorant folk. My understanding is that with STP enabled, each time something is plugged in (aka a computer turned on in the morning) that it takes the port 30 seconds to finish it's STP stuff and become ready. This is a problem because computers boot faster then that, so when Johnny goes to log in he gets a domain not found error. So turning on portfast allows the port to be ready when the computer is and people can log on.

I think it's easier to track down a loop once a year maybe than to deal with 20 "can't log ins" a day.

ColbyG

rwwest7 wrote: »

Please enlighten us ignorant folk. My understanding is that with STP enabled, each time something is plugged in (aka a computer turned on in the morning) that it takes the port 30 seconds to finish it's STP stuff and become ready. This is a problem because computers boot faster then that, so when Johnny goes to log in he gets a domain not found error. So turning on portfast allows the port to be ready when the computer is and people can log on.

I think it's easier to track down a loop once a year maybe than to deal with 20 "can't log ins" a day.

Spanning Tree PortFast BPDU Guard Enhancement - Cisco Systems

networker050184

rwwest7 wrote: »

Please enlighten us ignorant folk. My understanding is that with STP enabled, each time something is plugged in (aka a computer turned on in the morning) that it takes the port 30 seconds to finish it's STP stuff and become ready. This is a problem because computers boot faster then that, so when Johnny goes to log in he gets a domain not found error. So turning on portfast allows the port to be ready when the computer is and people can log on.

I think it's easier to track down a loop once a year maybe than to deal with 20 "can't log ins" a day.

Using portfast takes care of the login delay issues. You don't have to turn off STP for that. Also depending on the size and importance of your network tracking down a loop can be very difficult and time consuming. If its bad enough you will have to track down and unplug cables or console into switches or power cycle. Sometimes there may not even be someone on site at the time which causes even more down time. Turning STP off is not worth the consequences IMO.

rwwest7

ColbyNA wrote: »

Spanning Tree PortFast BPDU Guard Enhancement - Cisco Systems

I see. Hadn't gotten to BPDU gaurd yet in my studies. Actually, I think I have just hadn't absorbed it yet.