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ethernet level loops

Posted on 2002-04-04
Medium Priority
Last Modified: 2012-05-04

Do loops do exist at ethernet level. Are there ways to detect such loops. I want to write a program that would attempt to find out if such loops exist,  but I don't know how to go about it. I would appreciate some pointers.

Question by:bsd_linux
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LVL 40

Expert Comment

ID: 6920005
Im not positive that I know what you mean by loops, but I assume you mean a case where data on the wire is circulating and never reaching it's destination. If that's what you mean I don't think it's possible for hardware that is operating properly. At the transport level the data is being send from one NIC to some other NIC on the network, addressed by MAC. That seems to me to mean that the data is going to be accepted by the target NIC or it isn't, so either way the data will make a one-way trip.
LVL 10

Expert Comment

ID: 6922415
loops can occur on switched/bridged lans where u can connect the network physically in a loop. but i think the higher levels employ techniques to prevent this from creating a big problem (for a long time). they are TTL (in ip) or s/w timeouts


Expert Comment

ID: 6936476
A loop in an ethernet will instantly create a broadcast storm, therefore the way to detect a loop is that the whole network is down ;-)
To experience it yourself:
Put Machine A on Hub A. Put Machine B on Hub B. Ping machine B from machine A. No reply.
Connect the two hubs. Everything is working fine.
Make a second connecting between the two hubs. If the hub has collision lamps they should be constantly on now, as the packets are being sent back and forth between the two hubs.
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LVL 40

Expert Comment

ID: 6936745
Well yes, if the network is physically or logically via a wrong configuration connected if a loop then you've got a problem. But, then I'd consider that to be a case where the hardware wasn't working properly (the network certainly wouldn't be usable for normal traffic). My asumption was that the question related to a normally operating network and in that case I don't believe a loop is possible at the transport (ehternet) layer.

It is easy to have loops at higher layers by mis-configured routing tables.

Accepted Solution

svindler earned 900 total points
ID: 6936778
I have yet to see an ethernet loop that wasn't created by either physically doing doing something like mentioned above or disabling spanning tree on a switch port.
In every case it was discovered almost instantaneously. Signs to look for:
Excessive broadcasts indicated by LEDs on switches and hubs
No servers accessible from workstations
Lynch mob marching down the hall
LVL 40

Expert Comment

ID: 6936918
Don't for get the notification that occurs when the mob of peasants appears outside of your door with pitchforks and torches.


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