DHCP options for VOIP Connections hosted Skype for Business servers

Trying to configure your Polycom phone? Here in this article we would like to share some tips on how to configure DHCP options for your Polycom CX 500 and 700 manually or automatically.

Manual allocation:

This method entails using DHCP to identify the unique hardware address of each network card connected to the network and then continually supplying a constant configuration each time the DHCP client makes a request to the DHCP server using that network device. This ensures that a particular address is assigned automatically to that network card, based on its MAC address.

Automatic allocation:

Using this method, the DHCP automatically assigns an IP address permanently to a device, selecting it from a pool of available addresses. Usually DHCP is used to assign a temporary address to a client, but a DHCP server can allow an infinite lease time.

Polycom phones CX 500 and 700 require DHCP options to configure auto login. DHCP options can be configured manually or automatically via PS scripts.


You will need Skype for Business login credentials. If you don’t have the credentials please contact us at

For manual configuration, follow these steps:

  1. DHCP Vendor Class Options – Option 43
    These settings combine to give the phone all the information it needs to contact the Certificate Provisioning service on the system. The phone will recombine these to be a URL as follows:

    Option Number Option Name ASCII Value Setting Type
    001 Vendor Class (UCIdentifier) MS-UC-Client Common setting
    002 Protocol (URLScheme) https Common setting
    003 Web FQDN (WebServerFQDN) Variable – Set this to match your pool / domain
    004 Web Port (WebServerPort) 443 Common setting
    005 Cert Prov URL (CertProvRelPath) /CertProv/CertProvisioningService.svc Common setting


  2. DHCP Option 120
    Option 120 is a specific option that is described as part of a RFC 3361. The option describes SIP server locations in the form of FQDNs or IP Addresses. Skype for Business and Lync devices have been designed to use this standard option to discover the location of the pool for SIP signalling (registration, authentication, call signalling, etc).

    Option Number Option Name ASCII Value Setting Type
    120 SIP Pool FQDN (UCSipServer) Variable – Set this to match your pool / domain

The encoding of Option 120 is not just a simple ASCII byte conversion for entry into the DHCP server. There are also framing bits around the FQDN that is described as part of RFC 3361.
The RFC described two different types of encodings for Option 120 –

  • FQDN format
  • IP Address format

Only the FQDN formatting option is available for Skype for Business and Lync due to the fact that it uses TLS-based SIP which requires FQDN matching when creating the secure channel connection.

For automatic configuration, you can use the tool from this blog –

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