Unmanaged SG350 turns off VoIP phone when plugged into phone PC port

[sulaiman26]

Hello,

I inherited a Cisco SG350-28P switch, which I configured as an unmanaged switch by turning off LLPD, CDP and disabled auto-smart port.  I plugged in the PC port from behind my VoIP CP-8841 phone into port 24 of the Cisco switch.  Immediately after plugging in, the phone turns off.  I did the same with an older Cisco CP-7962G VoIP phone and it gives me the same results.  When I plug in the PC port from either of the two Cisco VoIP phones to any out-of-the-box unmanaged switch (like Netgear GS105), everything works fine.  Can someone suggest how else to configure the Cisco SG350 so that it behaves like a very simple unmanaged switch and doesn't cause VoIP phones to turn off?

Thank you!

-sul.

[balaji.bandi]

post show run

[Giuseppe Larosa · Accepted solution]

Hello @sulaiman26 ,

probaly STP is still running.

However, I would recommend you to connect a switch directly to the main switch and not going via a Cisco IP phone . This is not a correct way to use an IP phone the PC port is intended to connect a single PC host.

Having unmanaged switches in your network can be a nightmare for troublshooting if possible remove all of them.

Of course if it is your home lab you can do these tricks.

Hope to help

Giuseppe

[sulaiman26]

Bingo! Disabled spanning tree and works!

thank you for your quick response.

-s.