One Unmanaged Switch, Two IP Phones

Unanswered Question
Jun 1st, 2010

If I plug an unmanaged switch into the UC500, 1 phone and a laptop both work when plugged directly into the switch.  When I plug a 2nd phone into the switch, the 2nd phone returns a busy signal when you dial it or when you from it.

Can anyone explain why this is?


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David Trad Tue, 06/01/2010 - 15:00

Hi Jason,

What brand is this unmanaged switch?

I am not sure if the problem you are experiencing is related to the security on the port, if it the UC detects more then one MAC address it might lock it down, however if it is a Cisco switch this shouldn't happen.

Does the first phone still work when the second one is plugged in? or does it exhibit the engadged signal as well?




Sorry one last question, does this unmanaged switch support CDP or Vlan tagging?

jcarter@mungeri... Wed, 06/02/2010 - 05:17

The switch is an 8 port SMC cheap switch.  No CDP.  No VLAN tagging.

After further testing, it appears the phones work some of time together, and sometimes only the first registered phone works.  The first registered phone always works.  The second works maybe one out of five times you try it.  One of five is an average.  It will ring two times in a row then not work ten times in a row.

David Trad Wed, 06/02/2010 - 16:10


Are the PC's plugged into the back of the phones?

Can you do the following please:

  • Make sure the phones are stand alone and nothing is plugged into them
  • On the CLI run the following: ROUTER# sh ephone reg    and then    ROUTER# sh ephone unreg
  • On the CLI again run the following: ROUTER# debug tftp event    and then ROUTER# term mon
  • Capture any information you can post them into a txt file and then post it up here so it can be looked at

Outside of a Cisco switch I have only ever seen Cisco phones run on a HP Procurve as it supports CDP and VLAN tagging, if it is a dummy switch I have this gut feeling that you are going to get continual issues as the phones may have issues registering to the correct VLAN's (Just a thought).

Look forward to your next update.



Tiberie Kirijas Sat, 03/10/2012 - 04:05

Hi David,

I would really appreciate if you or someone on the forum would relieve my worries in confirming that this setup would work:

For a prospect client, in the process of the creation of the Bill Of Materials for a UC540W and 9 SPA PoE IP Phones and one PoE camera (PVC2300-EU), without thinking, I excluded the extra switch (SRW208P-K9-EU) so we would be price competitive, and in that moment I though I could rely on the clients Cisco SF 100-24 unmanaged switch for the 2 extra switchports that we needed to accommodate all of the equipment. At that moment I thought I would just need two power adapters for the two extra IP Phones, and ta daaa, everyone is happy.

Now I'm worried if this setup would work! The Cisco SF 100-24 switch doesn't have voice vlan, CDP... How I'm going to connect the SPA phones behind this switch and have the same switch connected to the expansion port on the UC540W?

Would appreciate any quick suggestion or guidance.



David Trad Sat, 03/10/2012 - 16:20

Hi Tiberie,

I think you have gone and dug yourself a hole with this one

In the many years I have been doing this a lesson keeps repeating itself,

don't discount a deal to beat a competitor, when that discount will lead to

costing you more after the sale has completed, than what you can make from

it in the long run... This is a typical scenario of that where an SF-200

would have been ample if you needed to save that $100 to win that deal.

The only way to get a Cisco IP phone to work on an unmanaged switch is to

use a power pack for the phone, and then manually program in the settings

into the phone... I.E VLAN ID for the voice-vlan, call manager IP address,

tftf source address.

Personally I believe you are creating a headache and a long term management

issue of the system, but unless you have the budget to change that you are

stuck using the above stated method, and even then I can not give you a

100% guarantee that it will work smoothly for you.

I would point out that the smart port role on the UC trunk port needs to be

set as switch so that all VLAN's pass through, it may not do this

automatically because no CDP information will be exchanged.

Let us know if you get stuck and I will try to help out as much as I can



"Sent from my Acer Iconia A500"

On Mar 10, 2012 10:05 PM, "tiberiekirijas" <

Tiberie Kirijas Sun, 03/11/2012 - 05:29

Hi David,

I really appreciate your answer. I know that I've shoot my self in the leg without reason

I plan to test the scenario tomorrow with a Cisco IP phone behind a Cisco manageable layer 2 switch set with all ports in VLAN 1 (this would act as the unmanaged switch) & CDP shutdown "no cdp run", have on the other side switchport with VLAN 1 and VLAN 100 -voice, and hope that with the manual configuration on the phone (VLAN ID for the voice-vlan, call manager IP address,tftf source address) it would register to our CUCM and be able to make calls. It's a long shot...

Thank you for the suggestion for manually adding the UC540 extension port in "switch" role for the interconnection with the unmanaged switch. You are right, it won't automatically negotiate in "trunk" role.

Two more questions:

  1. I know it's not a good design, but what if I assign only one VLAN for the data and the IP phones, set option 150 on the data DHCP scope so the PC and the IP Phones would be on the same vlan and subnet? The broadcast traffic should not interfere that much for 9 PC, 1 SMB server and 9 IP Phones... or am I mistaking?

  2. I'm thinking, out of 9 IP SPA phones, 8 would be connected to the integrated switch, and the 9th could it be connected to the expansion port (interface FastEthernet0/1/0), and use the WAN port as switchport to connect to the rest of the unmanaged switched network?
    The UC540 would be used just as IP PBX server, the Internet connection it's going to be provided through CPE ADSL gateway.

Thank you very much for you assistance.



Tiberie Kirijas Thu, 03/22/2012 - 03:15

Ok, here is an update.

Strike of luck, the client asked just as much phones as there are switchports on the UC540W

When I was configuring the UC540W couple of days ago, I noticed on the CCA that the expansion port can be assigned as well for IP phone.

Only left thing is,

1) to figure out how to properly edit the aa_sbcs_v04.aef script so there is no 5 seconds pause before the call is transferred

    to the operator

2) if Dial-by-Extension is used, how to remove the English prompt that says "Transferring...".

Do you have any good guides to suggest?

All the best,


David Trad Thu, 03/22/2012 - 13:36

Hi There,

Sadly I can't help you with questions one and 2

I always left things as default with every build, I was a pain-in-the butt

engineer, if the client requested things like that I would just tell them

to stop being picky lol... I got away with it every time too



On Mar 22, 2012 8:15 PM, "tiberiekirijas" <

Tiberie Kirijas Fri, 03/23/2012 - 01:22

hahhaha sleek solution to the problem

Cheers David and thanks a lot for your sincere assistance.




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