Topic Last Modified: 2011-04-04

Devices acquire an IP address from DHCP as soon as they have a VLAN ID. The device will cache the IP address, however this is cleared out every time the device restarts.

The device uses DHCP to obtain an IP address. Upon connecting to the network for the first time, the device will send a DHCPDiscover request (including the Lync class ID ms-uc-client). As per standard DHCP practice, DHCP will respond with a range of available IP addresses, and should also include the DNS server and gateway addresses. The device then sends a DHCPRequest for a specific IP address, and if it is still available the DHCP server sends a DHCPack that the device has leased that address.

If the device upon restarting already has an IP address and the lease for that address is at 50 percent, the device sends a DHCPRequest to renew the lease. The DHCP server will either allow or deny. If this is denied the device restarts the IP address acquisition process by sending a DHCPDiscover broadcast.

If the device upon restarting has an IP address and the lease for that address is at 87 percent, the device sends a DHCPRequest to rebind to that IP. Again, the DHCP will either allow or deny this. If this is denied the device restarts the IP acquisition process from the beginning.

No response from DHCP server

Issue: The device fails to get an IP address and shows "Cannot get IP address." Check the device’s System Information. The device will either have a IP address, or a 169.* IP address, or some other address known in your deployment to not valid.

Resolution: First, reset the device. This restarts the connectivity process. This should resolve intermittent problems, such as DHCP failures due to network drop outs.

If this is not successful, determine that the DHCP server(s) for the scope this device is on are running and contactable. For example, send a ping request to the server(s) hosting the DHCP server(s). To do this, open a command prompt and type:

ping <ip address of the DHCP server host machine(s)>

If this times out, the server hosting the DHCP server may be having network problems, and these will need to be resolved. If there are a number of DHCP servers, you should check each server. If only some of the servers do not respond in a timely manner there may be a routing problem for the DHCP servers. In this situation, a network administrator needs to investigate the issue.

If the DHCP server(s) is running and can be contacted, check that the DHCP server has not exhausted its scope of IP addresses, meaning that it cannot hand out addresses. Other potential problems include the switch between the device and the DHCP server caching responses.

Additional Information

To check whether the DHCP server is failing to hand out IP addresses, plug a laptop running Windows 7 into the network port the device was plugged into. Next, do the following:

  1. Click Start, click All Programs, click Accessories, right-click Command Prompt and then click Run as administrator.

  2. Type the following commands to release the network adapter and see if DHCP will provide the computer with a new address:

    Copy Code
    ipconfig /release <connection name>
    ipconfig flushdns
    ipconfig /registerdns

Check the system log using Event Viewer after 15 minutes for any DHCP errors. You can also check if your computer has an IP address by trying to connect to a website or other network locations.