[cisco-voip] IP Phone Ports

[Joe Martini]

Cisco IP phones are designed to forward BPDUs between the network and PC port to prevent loops and let spanning tree block ports when a switch receives a BPDU it sent out.  So if a user connects both ports to the same switch one port will go into blocking state to prevent a loop.  If BPDU guard is used like others said the port goes into err-disable, again preventing a loop.

The only exception I know of where a loop is created was from a bug that was quickly fixed (CSCut26167).  

Joe  

On Nov 10, 2016, at 8:56 AM, Ryan Huff <ryanhuff at outlook.com> wrote:

Steven ... correct, by plugging in the PC Port to the switch as well, you would be sending BPDUs back into the switch.

So as long as BPDU guard is enabled on the switch ports, then doing this would err-disable both switch ports (the one the PC Port and Network Port is plugged into). Pretty easy at that point to tell who the culprit is remotely, and in person.

= Ryan =


From: cisco-voip <cisco-voip-bounces at puck.nether.net <mailto:cisco-voip-bounces at puck.nether.net>> on behalf of Stephen Welsh <stephen.welsh at unifiedfx.com <mailto:stephen.welsh at unifiedfx.com>>
Sent: Thursday, November 10, 2016 8:43 AM
To: Ahmed Elnagar
Cc: cisco-voip at puck.nether.net <mailto:cisco-voip at puck.nether.net>
Subject: Re: [cisco-voip] IP Phone Ports
 
I thought that this would be prevented by default because of spanning tree/BPDU, but it’s common practice to disable spanning tree for access ports (i.e. using portfast) so that may allow the loop to occur, so maybe just disable port-fast?

Kind Regards

Stephen Welsh
CTO

<image003.png>

> On 10 Nov 2016, at 13:35, Ahmed Elnagar <ahmed_elnagar at hotmail.com <mailto:ahmed_elnagar at hotmail.com>> wrote:
> 
> That is not nice at all L
>  
> Did you figure out a way to prevent it from switch side?
>  
> From: Matthew Loraditch [mailto:MLoraditch at heliontechnologies.com <mailto:MLoraditch at heliontechnologies.com>] 
> Sent: Thursday, November 10, 2016 3:34 PM
> To: Ahmed Elnagar <ahmed_elnagar at hotmail.com <mailto:ahmed_elnagar at hotmail.com>>; 'cisco-voip at puck.nether.net <mailto:cisco-voip at puck.nether.net>' <cisco-voip at puck.nether.net <mailto:cisco-voip at puck.nether.net>>
> Subject: RE: IP Phone Ports
>  
> Yes it will create a loop. Not a fun time trying to track these things down. I’ve had some non-Cisco IP phones completely down a LAN.
>  
> Matthew G. Loraditch – CCNP-Voice, CCNA-R&S, CCDA
> Network Engineer
> Direct Voice: 443.541.1518
> 
> Facebook <https://www.facebook.com/heliontech?ref=hl> | Twitter <https://twitter.com/HelionTech> | LinkedIn <https://www.linkedin.com/company/helion-technologies?trk=top_nav_home> | G+ <https://plus.google.com/+Heliontechnologies/posts>
>  
> From: cisco-voip [mailto:cisco-voip-bounces at puck.nether.net <mailto:cisco-voip-bounces at puck.nether.net>] On Behalf Of Ahmed Elnagar
> Sent: Thursday, November 10, 2016 8:28 AM
> To: 'cisco-voip at puck.nether.net <mailto:cisco-voip at puck.nether.net>' <cisco-voip at puck.nether.net <mailto:cisco-voip at puck.nether.net>>
> Subject: [cisco-voip] IP Phone Ports
>  
> Dear all;
>  
> I want to know what will happen if a user connect “by mistake” both the PC port and Switch port to the network switch? Will this create a loop? Anyone tried it before?
>  
>  
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