Cisco 3850 Switch Issue

[jburleson]

Hello there, I am trying to uplink two switches to extend a network.  I have put in a cisco 3850 Switch and when I plug in a SG350TP switch the port lights up for a couple of seconds then the port becomes unavailable.  I can't even plug a laptop into that port unless I power cycle the switch.  Any help would be appreciated.

JB

[RedNectar]

Hi @jburleson ,

Welcome to the forum

Let me start with a proverb "A picture tells a thousand words".  I'll add some more forum tips at the end.

Plus a step-by-step analysis would help. Did you discover that the laptop wouldn't work BEFORE or AFTER you'd plugged a switch into that port? These things make a difference - especially if the port became err-disabled before plugging the laptop in.

Show us some output - such as a show interface status command from the 3850 switch, indicating which port is giving you the problem. Also, show cdl neighbors and show lldp neighbors might give useful information too, and show apanning-tree blocked-ports

Finally RedNectar's Forum Tips:

• When posting on the forum, add your pictures inline - i.e. PASTE your picture right where you want it, then click on the image and make the image large - like this.
  
  

  

  
  

  This means you pictures are actually SEEN (a) in the email that gets sent to subscribers and (b) anyone who looks at this post in the future. Adding pictures as attachments... puts your submission into the TL;DR category.

• To paste output from a switch/router
  1. Expand the toolbar (by clicking the ellipsis - the 3 dots)
  2. set the paragraph format to Preformatted,
  3. click HTML and find the <pre> </pre> tags
  4. past between the tags. This will preserve spacing

[jburleson]

Chris,

The laptop worked just fine before I plugged the switch in.  After I plugged the switch in I tried to plug the laptop into that port and nothing would come up.  My colleague told me that auto smartport may be enabled on the 3850 and this could be causing the issues.

JB

[RedNectar]

Hi @jburleson ,

Your colleague is likely on the right path.  

I suspect the switch has been configured with BPDU Guard - which means that the port will disable itself (go into a err-disable state) if it receives a spanning tree BPDU on that port.

The purpose of BPDU Guard is to prevent the inadvertent connection of switches to the topology in places where a loop condition could occur.  You can read more about it in this post. https://community.cisco.com/t5/networking-knowledge-base/importance-of-bpdu-guard-and-bpdu-filter/ta-p/3120465

So you are going to have to learn how to connect a console cable and re-configure the switch.  May may even have to begin with a password recovery or factory reset operation if you've never configured the switch before.  But that is a whole new round of searches/AI queries.