Auto MDI/MDI-X + conference rooms + bored == loop

Steven King sking at
Fri Mar 26 23:16:17 CDT 2010

Along with bpduguard, Cisco switches also continue to look for loops
with loopguard. They continuously look for the Keepalive packets that
they send out each port. So as long as you have not turned off STP all
together on the port, you will be fine.

On 3/26/10 6:21 PM, Matthew Huff wrote:
> Bpduguard if running cisco.
> set all the switch ports to bpduguard or enable it globally
> -----Original Message-----
> From: Chuck Anderson [mailto:cra at WPI.EDU] 
> Sent: Friday, March 26, 2010 6:09 PM
> To: nanog at
> Subject: Auto MDI/MDI-X + conference rooms + bored == loop
> Anyone have suggestions on Ethernet LAN loop-prevention?  With the 
> advent of Auto MDI/MDI-X ports on switches, it seems way too easy to 
> accidentally or maliciously create loops between network jacks.  We 
> have bored or inattentive people plugging in patch cords between 
> adjacent network jacks.  STP for loop-prevention isn't working so well 
> for us.
> STP "edge" or "portfast" or "faststart" modes are required for 
> end-station ports (with normal STP, DHCP often times out after 30+ 
> seconds it takes to go into Forwarding state).  Since the "edge" STP 
> mode goes into Forwarding state immediately, there is a period when 
> loops will form, causing havok with upstream gear until STP blocks the 
> port (if it ever does see below).
> "Desktop" switches.  You know, those 4 or 5 port Gigabit Ethernet 
> switches.  Apparently, many of them don't do any kind of STP at all.  
> Recommendations on ones that do STP?
> RSTP: is it any better than traditional STP in regards to "edge" ports 
> and blocking before a loop gets out of hand?  Or perhaps blocking for 
> 5-10 seconds before going into Forwarding state, hopefully preventing 
> loops before they happen but also allowing DHCP clients to get an 
> address without timeouts?  Recommendations on "Desktop" switches that 
> do RSTP?
> Thanks for your suggestions/discussion.

Steve KingSenior

Senior Linux Engineer - Advance Internet, Inc.
Cisco Certified Network Associate
CompTIA Linux+ Certified Professional
CompTIA A+ Certified Professional

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