Network loops

[chakradhar bolla]

          Hi,

Our lan is looped freequently,,because users by mistakely connected two IO ports with one network cable.

how avoide loops

[InayathUlla Sharieff]

Hi Chakradhar,

Loops are usually prevented by running Spanning Tree Protocol.

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_configuration_example09186a008009467c.shtml

Please feel free to let me know if yu required any further clarification on this.

Regards

Inayath

[chakradhar bolla]

Hi inayath,

Thanq for ur  info

But,

STP is by default enabled on cisco switches.

STP will work only switching loops.how to disable IO port level loops?

[InayathUlla Sharieff]

Hi Chakradhar,

Sorry  forgot to mentioned that to you.

The feature that prevents the loops on your network is STP, spanning tree protocol

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat2960/12225see/scg/sws

tp.htm

However is important to understand that this works automatically on the switch and you do

not have to perform any configuration because it is enable by default

Now there is some auxiliary port commands for STP that will help you to improve loop

prevention. Those commands are described on the following link

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat2960/12225see/scg/sws

tpopt.htm

2-Other features you can use to further protect the network include:

storm-control

and port security with action error-disable and a low max MAC addresses on port.

Adding these two provide you further protection.

see

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_40_se/configuration/guide/swtrafc.html

HTH

Regards

Inayath

[Steve Fuller]

Hi,

From the available spanning tree enhancements, the Spanning Tree Portfast BPDU Guard feature is probably the one that will be most useful to protect against what you describe.

From the above:

At the reception of BPDUs, the BPDU guard operation disables the port that has PortFast configured. The BPDU guard transitions the port into errdisable state, and a message appears on the console.

If a user connects two ports configured with this feature with a single cable, then the port that receives a BPDU from the other port first will become down, and so break the loop.

We would typically use this on all access facing ports as there should be no reason to receive Spanning Tree BPDU on ports that connect to end systems.

Regards