Loops in network

Unanswered Question
Sep 21st, 2007
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Hello,


Can anyone pls guide how to find loops in the network and remove that frm a general scenario view point when there is high utilisation found and network is unstable for all.Router part is found to be fine.

One of the way is block one of redundant port seen with high utilisation .

but how further to chk and nail out the problem.


Thanks in Advance.

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paul.matthews Fri, 09/21/2007 - 06:15
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I think you need to be a little clearer - are you talking Layer two (spanning tree) or layer three (routing) loops.


In both cases the "headline" method is the same - find out where the loop is, find out why you have the loop, fix the problem. Sometimes step 2 is omitted and a link simply shut down.


Routing loops, travert would be a primary tool in finding where the loop it.

for L2 loops, the various spanning tree shows would be key.

foxbatreco Fri, 09/21/2007 - 07:55
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Hello Paul,


Thnks for ur inputs.Yes, am referring to a combination of L3/L2 loops but more specifically loop occuring on an access L2 switch.

Would be helpful if u can just dictate few steps/commands. viz. how to identify which port is causing loops with cisco CLI ,how to mitigate tht in a mission critical environment , to carry out the method said by you above.


Thnks in advance!!!


JORGE RODRIGUEZ Fri, 09/21/2007 - 08:21
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Hi,


here are couple of links for understanding bridging loops, since you are asking how to troubleshoot and have not indicated your network being down I assume you do not have a current issue with l2 loops.. look into spanning tree layer 2 archtecture .


http://www.cisco.com/en/US/partner/tech/tk389/tk621/technologies_tech_note09186a00800951ac.shtml


STP

http://www.cisco.com/en/US/partner/tech/tk389/tk621/technologies_tech_note09186a00800951ac.shtml#brid_loop




http://www.cisco.com/en/US/partner/tech/tk389/tk621/technologies_tech_note09186a00801c137e.shtml




As for routing loops you would normally see a behaviour of not geting to a destination network from withing your network, it all depends on how your logical architecture is lay out and how you are implementing statics or dynamic routing.


Normaly a routing "loop" is where a route loops at one router hop not knwing where or how to get to a destination network sending the request back and forth from router to router.. a traceroute will show which router is the one not knowing the destination, hence the routing loop can be corrected at that router point.


the trace route command can be issue from any network device, on cisco routers is " trace " and you may well know from a PC would be tracert.



Trace routes are among troubleshooting techniques in identifing routing loops.


http://www.cisco.com/en/US/partner/tech/tk827/tk831/technologies_tech_note09186a00800cdf2e.shtml



Good luck


Jorge

foxbatreco Sat, 09/22/2007 - 06:43
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Hello guys..


Thnks for all ur help/suggestion on the query.

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