Once IP phone is connected to PoE Ethernet Switch, it will get the required power through Cisco-proprietary PoE or 802.3af PoE.
In this phase, the phone will be waiting for the response of CDP broadcast to get the voice vlan. If the phone is connected to a device that does not support CDP, the CDP query times out, and the phone moves to the next step in the registration sequence. The registration might if you have voice and data vlan
- The IP phone will send DHCP broadcast on its voice vlan asking for IP address
- DHCP server will respond with DHCP offer. This offer will include IP address, subnet mask, default gateway, domain name, dns server, etc. It will include TFTP server details under option 150
Note: By default Cisco IP Phones will have DHCP client enabled. You can disable DHCP client from the phone (Navigate to Settings > Network Configuration) and configure the network settings manually.
At this stage, the phone will contact the TFTP server to get its configuration file. Configuration file corresponds to the device name. This file contains information about CUCM IP/Name and Port.
- If phone was manually added to CUCM, TFTP server will locate the configuration file and transfer it to the phone
- If the TFTP server didn't find the file, it will respond to the phone with 'File Not Found'. The phone will send another request asking for default configuration file named XMLDefault.cnf.xml.
- If auto-registration is enabled, TFTP server will transfer the file to the phone. If disabled, TFTP server will respond with 'File Not Found'.
Note: Phones are the only device type which can use auto-registrations
Upgrading Firmware (optional)
In this phase, the phone will try to establish a TCP connection with the first node in the priority list provided in the config file. The phone will receive the firmware load ID and will compare it with the running firmware. If different, the phone will request/download the new firmware from the TFTP server. It will also download the RingList.xml from the TFTP server.
Note: You can verify firmware mismatch (configured load ID vs. Active load ID) from 'Unified Reporting' tab.
This is the final step where the phone send registration request to CUCM (SCCP or SIP). Based on the response, if successful, the phone will display the configure DN, button template, softkey template, etc. If failed, the error cause will be displayed on the phone screen and the phone will retry.