I have several locations with managed network switches - for example, one being a stack of 3com 4500's. On occasion we have an issue where a user somewhere in the building decides to plug in their own consumer grade switch, which is fine until they accidentally create a loop back by plugging a cable from one port to the other!

This ends up causing all kinds of havoc on the network, basically effectively taking it down in most cases.

Is there a way I can prevent this (and hopefully detect it if it happens)?

I believe that's what Spanning Tree Protocol (STP) is for, correct?

I see the device has a configuration screen for "MSTP", and by port and device I can configure it. Here is an example port MSTP status currently:

 Port Protocol       :enabled
 Port Role           :CIST Designated Port
 Port Priority       :128
 Port Cost(Dot1T)    :Config=auto / Active=200000
 Desg. Bridge/Port   :32768.0022-5782-5900 / 128.22
 Port Edged          :Config=enabled / Active=enabled  
 Point-to-point      :Config=auto / Active=true
 Transmit Limit      :10 packets/hello-time
 Protection Type     :None
 MSTP BPDU format    :Config=auto / Active=legacy
 Port Config
 Digest Snooping     :disabled
 Rapid Fwd State     :Rapid Forwarding 
 Num of Vlans Mapped :2 
 PortTimes           :Hello 2s MaxAge 20s FwDly 15s MsgAge 0s RemHop 20
 BPDU Sent           :426
          TCN: 0, Config: 0, RST: 0, MST: 426
 BPDU Received       :0
          TCN: 0, Config: 0, RST: 0, MST: 0

And the device status:

-------[CIST Global Info][Mode MSTP]-------
CIST Bridge         :32768.0022-5782-5900
Bridge Times        :Hello 2s MaxAge 20s FwDly 15s MaxHop 20
CIST Root/ERPC      :32768.0022-5782-5900 / 0
CIST RegRoot/IRPC   :32768.0022-5782-5900 / 0
CIST RootPortId     :0.0
BPDU-Protection     :disabled
TC-Protection       :enabled / Threshold=6
Bridge Config
Digest Snooping     :disabled
TC or TCN received  :0
Time since last TC  :0 days 16h:52m:12s

Device ports status:

MSTID     Port                   Role  STP State    Protection 
   0     Ethernet1/0/2            DESI  FORWARDING     NONE  
   0     Ethernet1/0/3            DESI  FORWARDING     NONE  
   0     Ethernet1/0/4            DESI  FORWARDING     NONE  
   0     Ethernet1/0/5            DESI  FORWARDING     NONE  
... etc ...
  • see related link on the right, "what does spanning tree do?" – SpacemanSpiff Feb 29 '12 at 14:31

That's correct. You should turn on spanning tree for any port that may potentially have another bridge(switch) plugged into it. The only negative effect this will have is a reconvergence of spanning tree any time you plug a new device into a VLAN. This means that when you plug in a new device the port will be placed into a blocking state and you won't be able to use that port until spanning tree figures out that it's safe to use. This usually takes anywhere from 10 seconds (rapid stp) to a minute (per vlan stp). If you have any ports that you absolutely need to come up immediately or that you know will never have another bridge plugged into them you can disable spanning tree on those ports. Also with spanning tree if you're using multiple vendors on your switches be sure to use a version of spanning tree that is compatible throughout your network.

  • So it looks like it is enabled on my ports (see above where it shows "port protocol : enabled", but looks disabled in the device MSTP config. Even though it looks like it's on for the ports, does that mean it's NOT because it's disabled at the device level? – Scott Szretter Feb 29 '12 at 15:47
  • If you're having problems with loops in the network then your spanning tree is not configured correctly. Unfortunately I've never worked with 3com switches so I can't help you there. – resmon6 Feb 29 '12 at 15:53

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