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Create a Layer Loop Intentionally

Answered Question
Mar 2nd, 2012
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Hi

I Like To Intentionally Create A Layer 2 Loop in My Lab

I have 2960 and 3750 switches and servers with multiple NIC's and also Some PC's and Hubs.


Would You Tell Me Different Ways to Do That ? (Connections and Commands And Features Which Sould Be Disabled or Enabled)


And Of Course Which Ways We Can Use To Notice That A Loop Has Occured (For Example Packet Sniffers or..)


Thanks

Correct Answer by hobbe about 5 years 5 months ago

Yes there are many many ways to make a loop.


scenario 1 would lead to a loop atleast localy in the hub.

add that hub and several other ports to the switch to make a multiloop that will drown the network almost instantly.


Scenario 2 might be considerable but most likely not unless you have a software switch behind them, but there is now way of telling since it is all upt o how you configure them. and there are many parameters so there is not telling what would happen.


No you can not use DHCP for detecting a loop.

but yes the network will go down so it is an indicator but nothing else or more.


well in old switches yes the cpu would go high in a loop, but nowdays most things are done in asics so it would most likely depend on the looped packets if they would cause high cpu.or not.


But I agree with you there is no substitue for experiencing the problems yourself instead of just reading about them.


Please rate if you think this is helpful.


Good luck


HTH

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hobbe Fri, 03/02/2012 - 01:19
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  • Gold, 750 points or more

Hi

The easiest way to form a L2 loop is to turn off spanning-tree and then connect a cable into two ports in the same switch.

send a packet down the link (by fx setting an ip address on the vlan and then try to ping something)


what do you need to se that the löoop have formed ? well just look at the lights or do a show interface and look at the counters

that packet is spinning around in there as fast as the switch can handle it.

set up a span port on the switch (monitor command) and sniff it and you will se the result first hand.


thats the easy way of doing it.


If you want a more realistic scenario then you can connect several switches in a row to make a sling and then do the same or connect them in a mesh and if you look you will find that there are instances where spanning tree will fail without you having to remove it.


Good luck


HTH

mhdganji110 Fri, 03/02/2012 - 01:37
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Thanks

Any Other Way ?


For Example Can These Scenarios Lead to Loop ?


1- Connect Two Ports of Same Switch to a hub (some how it is like the scenario you said)


2- Bridge NIC's on the server and Connect Them to A Switch ?


And For Detecting

Can we use DHCP ? (for dhcp requests are broadcast)

DHCP Broadcasts will travel in the loop and the nic will face trouble in getting ip and also we can see very slow speed or maybe no connection (network goes down)


and what about the show switch and system commands or any command which shows cpu and memory performance ?

do they go high in case of a loop ?

Correct Answer
hobbe Fri, 03/02/2012 - 02:09
User Badges:
  • Gold, 750 points or more

Yes there are many many ways to make a loop.


scenario 1 would lead to a loop atleast localy in the hub.

add that hub and several other ports to the switch to make a multiloop that will drown the network almost instantly.


Scenario 2 might be considerable but most likely not unless you have a software switch behind them, but there is now way of telling since it is all upt o how you configure them. and there are many parameters so there is not telling what would happen.


No you can not use DHCP for detecting a loop.

but yes the network will go down so it is an indicator but nothing else or more.


well in old switches yes the cpu would go high in a loop, but nowdays most things are done in asics so it would most likely depend on the looped packets if they would cause high cpu.or not.


But I agree with you there is no substitue for experiencing the problems yourself instead of just reading about them.


Please rate if you think this is helpful.


Good luck


HTH

davy.timmermans Fri, 03/02/2012 - 03:05
User Badges:
  • Silver, 250 points or more

for lab environment only


int fas0/1

spanning-tree bpdufilter enable

switchport access vlan x

switchport mode access

int fas0/2

spanning-tree bpdufilter enable

switchport access vlan x

switchport mode access

connect these two switchports and you'll have a loop.


If you enable this command on a port, you will disable sending BPDU packets out a port. If you connect two ports configured with bpdufilter- no BPDU packets are sent/received. conclusion: ports will not STP blocked.


Then it's a  matter of waiting. You'll see the green lights go crazy ;-)


To speed up you can connect a pc to fas0/3 - also in vlan x and generate traffic - for example a ping to somewhere

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