How to check the loop in Cisco router?

5 pts.
Tags:
Cisco
Cisco Routers
Network administration
Network loop
Routers
i want to know to how to check whether is a loop occurred in the network? because load in tx and rx in increasing...
ASKED: April 8, 2009  5:30 AM
UPDATED: December 24, 2011  9:39 PM

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Are you thinking that there is a routing loop, or a layer 2 loop ?

For both you really need a good diagram of the network to help you (it is a good thing to have anyway). It must show ALL items that are in the network and all the connections between them.

For a routing loop, you need to understand how the traffic is expected to flow, and try using the traceroute command to see if the packets visit the same router more than once.

If you think there is a layer 2 loop, then turn on spanning-tree on the switches, and see if the traffic levels drop, after a few minutes, and monitor for a while to see if this is now fixed. The diagram should help you detect where the loop is located.

A packet sniffer, such as WireShark can also help you to identify if the loop. Look for duplicate packets, with increased TTL. Look in several places around the network. If possible get a 10/100Mbps hub, and put it in-line with the network, and connect the PC with WireShark to the hub (only 3 conections to the hub), which allows you to easily see the traffic, with no config changes to the network.

There is no ‘magic bullet’ to fix a loop. It needs some investigation, and a good knowledge of the network topology. I hope these tips help to get you started.

You may like to provide a bit more information here, then it may be easier to give more directed assistance.

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  • Alilink
    Hello , well , now in my work i have some problem by this Loop , i search for killing this function for ever by the killing the cause , but yet not i know how can i do it , i know the STP function , yes maybe good but not the bester soul. my friends if you have more search about this problem i can very very thank you . regards anonymous
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