How the L2 loops are found ?

Answered Question
Apr 29th, 2010
User Badges:

First of all, i want to say Hello, im new at this community.


I want to know how the layer 2 loops are discovered. I'm working at big enteprise network which consists of lot of devices and they are connected to catalyst 65xx switches. There was a  few times, when our network was almost down, because of network loops. I know that these loops commonly occurs when the vlans are bad configured. as the senior engineers said it can be found with looking at mac-address-table. So just wanted to know base info, how loops can be found ? I would see allot mac addresses, when in normal condition we will not ? How the loop cause (vlan on wrong interface ?) are found.


Any help or links to litterature are welcome. Thanks in advance.

Correct Answer by Reza Sharifi about 7 years 1 month ago

Hi,


Have a look at this document for understanding spanning tree protocol and how it prevents loops


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


HTH

Reza

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 4.3 (3 ratings)
Loading.
rahurao Thu, 04/29/2010 - 18:01
User Badges:

Hi Team,


For troubleshooting spanning tree you always need to have a good understanding of your network. Issue the spanning tree bpdu filter on the ports which you do not want to participate in spanning tree.


The Link provided for the design considerations and troubleshhtoing spanning tree was made by Francois Tallet and he have always recommed to use the following commads to check the basic changes due to spanning tree:



show interfaces
    *
      show spanning-tree
    *
      show bridge
    *
      show processes cpu
    *
      debug spanning-tree
    *
      logging buffered

++ These commands can at least give you an idea about the changes which have happened in the topology. The mac address table lookup needs to checked when you are facing mac-flapping error messages which are caused due to spanning tree only.

Ganesh Hariharan Fri, 04/30/2010 - 01:28
User Badges:
  • Purple, 4500 points or more
  • Community Spotlight Award,

    Member's Choice, February 2016

First of all, i want to say Hello, im new at this community.


I want to know how the layer 2 loops are discovered. I'm working at big enteprise network which consists of lot of devices and they are connected to catalyst 65xx switches. There was a  few times, when our network was almost down, because of network loops. I know that these loops commonly occurs when the vlans are bad configured. as the senior engineers said it can be found with looking at mac-address-table. So just wanted to know base info, how loops can be found ? I would see allot mac addresses, when in normal condition we will not ? How the loop cause (vlan on wrong interface ?) are found.


Any help or links to litterature are welcome. Thanks in advance.


Hi,


The most common sign in network is broadcast storms.In this case switch will cycle from zero to one hundred percent (in some cases) or go to one hundred percent and stay there.The impact for the above will end up the slowness in network traffic to dead end or halts altogether as the switch process overloads and vapor-locks.


Most of the Cisco switches I've seen, the LEDs start off, blink at an increasing rate, lock on solid, then go out (protection kicks in and shuts down the ports) and it repeats till it not solved.


As suggested STP mechanism to prevent the layer 2 loops in given links also be the reasons.


Hope to Help !!


Ganesh.H


Remember to rate the helpful post

rahurao Fri, 04/30/2010 - 01:34
User Badges:

Hi,


The  most common sign in network is broadcast storms.In this case switch  will cycle from zero to one hundred percent (in some cases) or go to one  hundred percent and stay there.The impact for the above will end up the  slowness in network traffic to dead end or halts altogether as the  switch process overloads and vapor-locks.


Most of the Cisco switches I've  seen, the LEDs start off, blink at an increasing rate, lock on solid,  then go out (protection kicks in and shuts down the ports) and it  repeats till it not solved.


As suggested STP mechanism to prevent the layer 2  loops in given links also be the reasons.


Hope to Help !!


Ganesh.H


Remember to  rate the helpful post






Hi Ganesh,


We are trying to check for troubleshooting spanning tree and not the broadcast strom and high CPU on a switch i believe.


So instead of looking at the LED's we would definately like to check the show spannig tree details in order to check the BPDU's generated more in a particuler vlan.


I am really not sure why are we trying to check broadcast storm or effects of the same.

Ganesh Hariharan Fri, 04/30/2010 - 01:44
User Badges:
  • Purple, 4500 points or more
  • Community Spotlight Award,

    Member's Choice, February 2016

Hi,


The most common sign in network is broadcast storms.In this case switch will cycle from zero to one hundred percent (in some cases) or go to one  hundred percent and stay there.The impact for the above will end up the  slowness in network traffic to dead end or halts altogether as the  switch process overloads and vapor-locks.


Most of the Cisco switches I've  seen, the LEDs start off, blink at an increasing rate, lock on solid,  then go out (protection kicks in and shuts down the ports) and it  repeats till it not solved.


As suggested STP mechanism to prevent the layer 2  loops in given links also be the reasons.


Hope to Help !!


Ganesh.H


Remember to  rate the helpful post






Hi Ganesh,


We are trying to check for troubleshooting spanning tree and not the broadcast strom and high CPU on a switch i believe.


So instead of looking at the LED's we would definately like to check the show spannig tree details in order to check the BPDU's generated more in a particuler vlan.


I am really not sure why are we trying to check broadcast storm or effects of the same.

Hi Rahul,


Thanks for pointing out the right track !!


Ganesh.H

Actions

This Discussion