dhcp for voip phones

Unanswered Question
Jan 31st, 2008

Hi all

I was told the other day that there is an option in dhcp server i think its 43, where is gives the ip phone the correct ip address for the vlan, is this correct ?

also how does the switch know that the phone is a tagged packet, does the 802.1p tag sit inside the 802.1q tag ?

I have this problem too.
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dominic.caron Thu, 01/31/2008 - 06:10

The IPphone does receive a lease from a DHCP server. In the lease, you use option 150 and return the adresse of one(or many) TFTP server. The phone then connect to that tftp to download it's config.

Option 43 is used with wireless LWAPP to return the addresse of the master controler to a access point.

The phone know what tag to use by looking inside cdp packet from the switch. The voice vlan is configure on the switch...

andrew.butterworth Thu, 01/31/2008 - 07:44

I assume you are talking about Cisco IP Phones? If thats the case the reply from Dominic is correct - the IP Phones request DHCP Option 150; the DHCP server should reply with the IP address (or array of IP addresses) for the TFTP server. This is usually the CallManager Publisher, however it could be one of the subscribers or even an external TFTP server.

The IP Phones learn what the Voice VLAN tag is via CDP - the switch interface configuration 'switchport voice vlan X' command tells the switch to advertise via CDP the 802.1q tag of the voice VLAN for that specific interface (plus it enables a single Tagged VLAN to exist on the switchport in addition to the untagged access VLAN).

If you are using a different vendors IP Phones (Ericsson springs to mind....) then DHCP Option 43 could be used. Ericsson IP Phones request DHCP option 43 to discover extra information - software server IP address, H.323 Telephony Domain Name, Voice VLAN Tag(s) etc. Ericsson IP Phones don't support CDP so they initially boot on the native (untagged, access) VLAN and once the VLAN Tag information is learned via DHCP Option 43 they then release the access VLAN IP address and begin using the received Voice VLAN Tag, sending a new DHCP Discover etc.

This presents a bit of a conflict if you are deploying Ericsson IP Phones & Cisco LWAPP Access Points as they both request DHCP Option 43, however they require different information to be returned. Luckily both vendors support DHCP Vendor Classes so it is easily worked around.

Other vendors (Nortel, Mitel, Avaya) use different DHCP Options to inform the IP Phones of server/booting information - it does vary though and you should refer to the relevant vendors documentation......



carl_townshend Thu, 01/31/2008 - 09:03

so when the phone say siemens intially boots up does it boot into the default untagged vlan, then does the dhcp server see that its a phone requesting an address, does it then get a different address than the normal pc's and put into the proper vlan.

I have seen in some occasions where you put the phone in the correct vlan first, is this normal ?

andrew.butterworth Thu, 01/31/2008 - 09:14

It depends on the Vendors implementation. I have never worked with any Siemens IP Phones so I have no idea how they operate. Have a look at their documentation.

Your comment:

'then does the dhcp server see that its a phone requesting an address, does it then get a different address than the normal pc's and put into the proper vlan'

The IP Phone can signal to the DHCP server what it is via DHCP Option 60 (this is the Vendor Class stuff I mentioned). If the DHCP Server is configured appropriately it can provide specific information to that type of client - for example Ericsson IP Phones send 'Ericsson IP-Phone' in DHCP Option 60 in their DHCP Discover & Request packets. The DHCP Server doesn't 'put' the device in the relevant VLAN, this is a function of the IP Phone - i.e. it decides to start 'talking' with the Voice VLAN 802.1q Tag. What usually happens is the IP Phone starts untagged, receives its DHCP Vendor Specific information - usually part of this is the VLAN Tag, it then releases the IP Address and starts again using the VLAN Tag it received.

Not all DHCP Servers support Vendor Classes - in fact other than Windows 2000/2003 I don't know of any others (there must be?).


carl_townshend Tue, 04/15/2008 - 08:13

so to clarify

phone boots up, gets ip in normal data vlan

dhcp server passes vlan info to phone

phone then reboots

phone then tags packets with new vlan id

is this correct ?

andrew.butterworth Tue, 04/15/2008 - 08:23

Is this correct? It might be, it depends on the vendor. If its Cisco then no, the Voice VLAN Tag is learned via CDP; unless the Cisco IP Phone didn't see the CDP packet it never appears on the access VLAN. If its Ericsson or Nortel then yes it operates more or less like this - the phones don't actually reboot, they release the IP address in the untagged VLAN and then send a new DHCP request out using the learned Voice VLAN Tag. Mitel IP Phones can use either as they support CDP and also have a DHCP vendor option.

I don't know about other vendors, you should check with the documentation.


jgross Tue, 04/15/2008 - 08:33


for Siemes Phones it looks like this for example.

Untagged Subnet is and tagged (with VLAN ID 20) it is

OptiPoint 410 should get the VLAN ID 20 via VLAN Discovery:

Please make sure that the untagged packets of the first DHCP Discover cycle reach the DHCP server that provides the vendor specific options for the optiPoint. Most likely you will have to use Relay Agents/DHCP Helper.

The options for the subnet have to contain the option 43.

The data format of the option 43 is following:

Option Code for the first encapsulated option:

Always: 01

Length of the first data field:

Always: 0A (10 Byte)

Encapsulated data for option code 1:

Always: 53 69 65 6D 65 6E 73 00 00 00 (Siemens plus 3 Fill bytes)

Option Code for the second encapsulated option:

Always: 02

Length of the first data field:

Always: 04 (4 Bytes)

Encapsulated data for option code 2 (VLAN ID):

Example: for VLAN ID 20: 00 00 00 14

Ending with 5 Padding Bytes and the end option:

Release Note page 9 (of 10) [optiPoint 410 advanced]

Always: 00 00 00 00 00 FF

If this data format for option 43 is not matching or not send by the DHCP server the phone will start with the manual

configured VLAN ID, if Layer 2 QoS is enabled. If Layer 2 QoS is not enabled the phone will start without using a




carl_townshend Wed, 04/16/2008 - 01:39

Hi there igross

can you confirm the phone first gets a ip address in the data vlan, then it gets passed the options with the new vlan id, then phone requests new ip in voice vlan ?

jgross Mon, 04/21/2008 - 11:09

Hello Carl,

yes it works in this way. Be sure that you have enough free ip addresses in the data vlan. You can speed up the freeing of unused ip addresses in the data vlan by telling the dhcp server to try to ping adresses and set them free is nobody answers.



Darren Ramsey Mon, 04/21/2008 - 15:09

You might try LLDP to set the VVID, if your phone supports it. We are moving our Nortel IP phones over to LLDP and had good luck so far.

jgross Tue, 04/22/2008 - 08:05


I tried Siemens Phones with an IP8K with LLDP and had no luck, but I don't now the sw version of the IP8K.



jgross Tue, 04/22/2008 - 09:17

Hello Carl,

LLDP means Link Layer Discovery Protocol (LLDP) and LLDP Media Endpoint Discovery (LLDP-MED). This is a standard for CDP.

Its Supported on Cisco Switches starting with 12.2(37) afaik.



Darren Ramsey Tue, 04/22/2008 - 10:07

Use 12.2(44) code as ealier versions had a bug. Configure your phone to support LLDP-MED, and enable LLDP "LLDP RUN".

jppdavies999 Mon, 03/30/2009 - 04:30

Can someone help with a problem I have with siemens SIP phone. Phone works as described earlier in this thread - DHCP request on native vlan and then using option 43 go into IP Telephony Vlan.

the problem:-

if I use a 3750PE and do a trace I can see the DHCP release going back to the DHCP server thus releasing the IP address on the native Vlan.

3750 software - c3750-ipbase-mz.122-35-SE5

If I use a 4503 with 48 port PE card then I dont see the DHCP release. Thus the IP addresses on the native vlan are still on lease and fill up the used IP addresses.

C4503 software - cat4500-ipbase-mz.122-31.SGA8.bin

Same phone, same subnet, just different switch???

Config on both switches is pretty straightforward:-

switchport access vlan 10

switchport voice vlan 52

Ok other enries but I dont see any relevance to this

I have also heard that there is a mechanism with Microsoft DHCP server to regularly ping end points and release if no response. Cant find where this is done - can anyone help??

John Davies South Wales Police

etmarcof Tue, 11/10/2009 - 15:33

Hi Andrew,

I'mt trying to config DHCP option 43 for voice vlan for Ericsson IP Phones (now Aastra)with cisco switches

My DHCP server is cisco switch cat 3750.

What should be phone config file regarding vlan settings?

My Switch port config:

switchport access vlan 20

switchport mode access

switchport voice vlan 10

for example my voice vlan = 10

vlan for PC connected to phone =20

Best regards,


fab5freddy Fri, 07/30/2010 - 04:00


I also got the requets to configure option 43 for Aastra phones.

Could somebody help me with the hex string?

Following options have to be configured:

Vendor Class = Aamadeus IP Phone):

SIP port pbx: 5060

SIP port phone: 5060

SIP port  backup PABX: 5060

PABX adr=


Vlan_prio= 6

PABX adr Back Up=

There is more info in attach" Aastra Phones

The whole manual is als in attach, called "Aastra Aamadeus phone dhcp option 43"



Wanderson Pereira Mon, 03/25/2013 - 07:56


I have the same issue with Aastra phones, the documentation aastra ask me to set up the options 43 and 60, but I dont know how I can do that.

Can anybody help me?

Shoud I set up only that?

option 60 AastraIPPhone6731i

option 60 AastraIPPhone6739i

option 43 number vlan



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