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How Spanning Tree Protocol prevents loops – retro video

19/02/2017

This is a hand drawn retro video on STP, which explains STP in great yet simple detail, in a pratical way.

 

 

How to stop a switching loop

 

  1. Discover the topology (scope) of the loop.

    Once it has been established that the reason for the network outage is a forwarding loop, the highest priority is to stop the loop and restore the network operation. In order to stop the loop, you must know which ports are involved in the loop: look at the ports with the highest link utilization (packets per second). The show interface Cisco IOS software command displays the utilization for each interface.

    In order to display only the utilization information and the interface name (for a quick analysis), you might use Cisco IOS software regular expression output filtering. Issue the show interface | include line|\/sec command to display only the packet per second statistics and the interface name:

    cat# show interface | include line|\/sec
    
    GigabitEthernet2/1 is up, line protocol is down
      5 minute input rate 0 bits/sec, 0 packets/sec
      5 minute output rate 0 bits/sec, 0 packets/sec
    GigabitEthernet2/2 is up, line protocol is down
      5 minute input rate 0 bits/sec, 0 packets/sec
      5 minute output rate 0 bits/sec, 0 packets/sec
    GigabitEthernet2/3 is up, line protocol is up
      5 minute input rate 99765230 bits/sec, 24912 packets/sec
      5 minute output rate 0 bits/sec, 0 packets/sec
    GigabitEthernet2/4 is up, line protocol is up
      5 minute input rate 1000 bits/sec, 27 packets/sec
      5 minute output rate 101002134 bits/sec, 25043 packets/sec
    GigabitEthernet2/5 is administratively down, line protocol is down
      5 minute input rate 0 bits/sec, 0 packets/sec
      5 minute output rate 0 bits/sec, 0 packets/sec
    GigabitEthernet2/6 is administratively down, line protocol is down
      5 minute input rate 0 bits/sec, 0 packets/sec
      5 minute output rate 0 bits/sec, 0 packets/sec
    GigabitEthernet2/7 is up, line protocol is down
      5 minute input rate 0 bits/sec, 0 packets/sec
      5 minute output rate 0 bits/sec, 0 packets/sec
    GigabitEthernet2/8 is up, line protocol is up
      5 minute input rate 2000 bits/sec, 41 packets/sec
      5 minute output rate 99552940 bits/sec, 24892 packets/sec
    

    Pay particular attention to the interfaces with the highest link utilization. In this example, these are interfaces g2/3, g2/4, and g2/8; they are probably the ports that are involved in the loop.

  2. If you see maxed out input traffic on a trunk (meaning the loop traffic is coming into that device from somewhere else), go to the device on the other end of the trunk and issue a “show interfaces” command on that device. Keep doing this until you reach a device that only has maxed out output traffic on the trunks. This means that the culprit is directly connected to the device you are currently logged into and the loop traffic is originating on the device you are currently logged into,
  3. Break the loop.

    To break the loop, you must shut down or disconnect the involved ports.

    It is very important to not only stop the loop but to also find and fix the root cause of the loop. It is relatively easier to break the loop

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