<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">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.<div class=""><br class=""></div><div class="">The only exception I know of where a loop is created was from a bug that was quickly fixed (CSCut26167). <br class=""><br class="">Joe <div class=""><br class=""><div><div class="">On Nov 10, 2016, at 8:56 AM, Ryan Huff <<a href="mailto:ryanhuff@outlook.com" class="">ryanhuff@outlook.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div id="divtagdefaultwrapper" dir="ltr" style="font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; font-size: 12pt; font-family: Calibri, Arial, Helvetica, sans-serif;" class=""><div style="margin-top: 0px; margin-bottom: 0px;" class="">Steven ... correct, by plugging in the PC Port to the switch as well, you would be sending BPDUs back into the switch.</div><div style="margin-top: 0px; margin-bottom: 0px;" class=""><br class=""></div><div style="margin-top: 0px; margin-bottom: 0px;" class="">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.</div><div style="margin-top: 0px; margin-bottom: 0px;" class=""><br class=""></div><div style="margin-top: 0px; margin-bottom: 0px;" class="">= Ryan =<br class=""></div><div style="margin-top: 0px; margin-bottom: 0px;" class=""><br class=""></div><div style="margin-top: 0px; margin-bottom: 0px;" class=""><br class=""></div><div style="" class=""><hr tabindex="-1" style="display: inline-block; width: 1067.21875px;" class=""><div id="divRplyFwdMsg" dir="ltr" class=""><font face="Calibri, sans-serif" style="font-size: 11pt;" class=""><b class="">From:</b><span class="Apple-converted-space"> </span>cisco-voip <<a href="mailto:cisco-voip-bounces@puck.nether.net" class="">cisco-voip-bounces@puck.nether.net</a>> on behalf of Stephen Welsh <<a href="mailto:stephen.welsh@unifiedfx.com" class="">stephen.welsh@unifiedfx.com</a>><br class=""><b class="">Sent:</b><span class="Apple-converted-space"> </span>Thursday, November 10, 2016 8:43 AM<br class=""><b class="">To:</b><span class="Apple-converted-space"> </span>Ahmed Elnagar<br class=""><b class="">Cc:</b><span class="Apple-converted-space"> </span><a href="mailto:cisco-voip@puck.nether.net" class="">cisco-voip@puck.nether.net</a><br class=""><b class="">Subject:</b><span class="Apple-converted-space"> </span>Re: [cisco-voip] IP Phone Ports</font><div class=""> </div></div><div class=""><div class="">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?</div><div class=""><br class=""></div><div class="">Kind Regards</div><br class=""><div class=""><div class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-ligatures: normal; font-variant-position: normal; font-variant-caps: normal; font-variant-numeric: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px;">Stephen Welsh</div><div class="" style="font-family: Helvetica; font-style: normal; font-variant-ligatures: normal; font-variant-position: normal; font-variant-caps: normal; font-variant-numeric: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px;">CTO</div><br class="Apple-interchange-newline" style="font-family: Helvetica; font-style: normal; font-variant-ligatures: normal; font-variant-position: normal; font-variant-caps: normal; font-variant-numeric: normal; font-variant-alternates: normal; font-variant-east-asian: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px;"><span class=""><span id="cid:CBBF0493-235C-43D2-A874-9FB3D95AF598@b2.unifiedfx.com"><image003.png></span></span></div><br class=""><div class=""><blockquote type="cite" class=""><div class="">On 10 Nov 2016, at 13:35, Ahmed Elnagar <<a href="mailto:ahmed_elnagar@hotmail.com" class="">ahmed_elnagar@hotmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="WordSection1" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px;"><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"><span class="" style="color: rgb(31, 73, 125);">That is not nice at all<span class="Apple-converted-space"> </span></span><span class="" style="font-family: Wingdings; color: rgb(31, 73, 125);">L</span><span class="" style="color: rgb(31, 73, 125);"></span></div><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"><span class="" style="color: rgb(31, 73, 125);"> </span></div><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"><span class="" style="color: rgb(31, 73, 125);">Did you figure out a way to prevent it from switch side?</span></div><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"><span class="" style="color: rgb(31, 73, 125);"> </span></div><div class=""><div class="" style="border-style: solid none none; border-top-color: rgb(225, 225, 225); border-top-width: 1pt; padding: 3pt 0in 0in;"><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"><b class="">From:</b><span class="Apple-converted-space"> </span>Matthew Loraditch [<a href="mailto:MLoraditch@heliontechnologies.com" class="">mailto:MLoraditch@heliontechnologies.com</a>]<span class="Apple-converted-space"> </span><br class=""><b class="">Sent:</b><span class="Apple-converted-space"> </span>Thursday, November 10, 2016 3:34 PM<br class=""><b class="">To:</b><span class="Apple-converted-space"> </span>Ahmed Elnagar <<a href="mailto:ahmed_elnagar@hotmail.com" class="">ahmed_elnagar@hotmail.com</a>>; '<a href="mailto:cisco-voip@puck.nether.net" class="">cisco-voip@puck.nether.net</a>' <<a href="mailto:cisco-voip@puck.nether.net" class="">cisco-voip@puck.nether.net</a>><br class=""><b class="">Subject:</b><span class="Apple-converted-space"> </span>RE: IP Phone Ports</div></div></div><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"> </div><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"><span class="" style="color: rgb(31, 73, 125);">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.</span></div><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"><span class="" style="color: rgb(31, 73, 125);"> </span></div><div class=""><p class="MsoNormal" style="margin: 0in 0in 12pt; font-size: 11pt; font-family: Calibri, sans-serif;"><span class="" style="font-size: 10pt; color: rgb(31, 73, 125);">Matthew G. Loraditch – CCNP-Voice, CCNA-R&S, CCDA<br class="">Network Engineer<br class="">Direct Voice: 443.541.1518</span><span class="" style="color: rgb(31, 73, 125);"></span></p><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"><span class="" style="color: rgb(31, 73, 125);"><a href="https://www.facebook.com/heliontech?ref=hl" class="" style="color: rgb(149, 79, 114); text-decoration: underline;"><span class="" style="font-size: 8pt; color: blue;">Facebook</span></a></span><span class="" style="font-size: 8pt; color: rgb(31, 73, 125);"><span class="Apple-converted-space"> </span>|<span class="Apple-converted-space"> </span></span><span class="" style="color: rgb(31, 73, 125);"><a href="https://twitter.com/HelionTech" class="" style="color: rgb(149, 79, 114); text-decoration: underline;"><span class="" style="font-size: 8pt; color: blue;">Twitter</span></a></span><span class="" style="font-size: 8pt; color: rgb(31, 73, 125);"><span class="Apple-converted-space"> </span>|<span class="Apple-converted-space"> </span></span><span class="" style="color: rgb(31, 73, 125);"><a href="https://www.linkedin.com/company/helion-technologies?trk=top_nav_home" class="" style="color: rgb(149, 79, 114); text-decoration: underline;"><span class="" style="font-size: 8pt; color: blue;">LinkedIn</span></a></span><span class="" style="font-size: 8pt; color: rgb(31, 73, 125);"><span class="Apple-converted-space"> </span>|<span class="Apple-converted-space"> </span></span><span class="" style="color: rgb(31, 73, 125);"><a href="https://plus.google.com/+Heliontechnologies/posts" class="" style="color: rgb(149, 79, 114); text-decoration: underline;"><span class="" style="font-size: 8pt; color: blue;">G+</span></a></span></div></div><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"><span class="" style="color: rgb(31, 73, 125);"> </span></div><div class=""><div class="" style="border-style: solid none none; border-top-color: rgb(225, 225, 225); border-top-width: 1pt; padding: 3pt 0in 0in;"><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"><b class="">From:</b><span class="Apple-converted-space"> </span>cisco-voip [<a href="mailto:cisco-voip-bounces@puck.nether.net" class="" style="color: rgb(149, 79, 114); text-decoration: underline;">mailto:cisco-voip-bounces@puck.nether.net</a>]<span class="Apple-converted-space"> </span><b class="">On Behalf Of<span class="Apple-converted-space"> </span></b>Ahmed Elnagar<br class=""><b class="">Sent:</b><span class="Apple-converted-space"> </span>Thursday, November 10, 2016 8:28 AM<br class=""><b class="">To:</b><span class="Apple-converted-space"> </span>'<a href="mailto:cisco-voip@puck.nether.net" class="">cisco-voip@puck.nether.net</a>' <<a href="mailto:cisco-voip@puck.nether.net" class="" style="color: rgb(149, 79, 114); text-decoration: underline;">cisco-voip@puck.nether.net</a>><br class=""><b class="">Subject:</b><span class="Apple-converted-space"> </span>[cisco-voip] IP Phone Ports</div></div></div><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"> </div><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;">Dear all;</div><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"> </div><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;">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?</div><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"> </div><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"> </div></div><span class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; float: none; display: inline !important;">_______________________________________________</span><br class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px;"><span class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; 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white-space: normal; widows: auto; word-spacing: 0px;"><span class="" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; float: none; display: inline !important;"><a href="https://puck.nether.net/mailman/listinfo/cisco-voip" class="">https://puck.nether.net/mailman/listinfo/cisco-voip</a></span></div></blockquote></div><br class=""></div></div></div><span style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; float: none; display: inline !important;" class="">_______________________________________________</span><br style="font-family: Helvetica; font-size: 12px; 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