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Much like our switches and routers perform those checks and procedures while booting, so do Cisco IP Phones – but, of course, the phone and the router boot processes have a significant difference!
This is the fundamental Cisco IP Phone boot procedure:
The first step, you’d think, is to connect your phone to a switch port physically. If the Phone and Switch agree on the PoE system to be used, their power can be obtained by Power Over Ethernet (PoE). Originally known as “inline power,” PoE can transmit power to the connected computer through a copper cable.
Cisco Discovery Protocol, our longstanding buddy, is now in the image as its CDP, which shows the phone how Voice should use VLAN. Make sure that CDP works on the IP Phone linked port, as disabled CDP is popular.
Now the phone has an IP address – after all, without an IP phone, it won’t be so nice! The same way a “normal” host device is gained – via DHCP – from that address and other relevant information. The IP Phone sends a DHCP application to the DHCP server via the router.
This transmission involved configuration of the router with the ip helpers’ address command; routers do not transmit broadcasts by design.
The method is like a PC getting an IP address – the DHCP server answers the normal offer and the phone acknowledges the initial offer.
However, this procedure is not exactly like a PC demanding a DHCP address. In DHCP – Choice 150, there is a small additional option for IP Phones to be accurate. This Option informs the IP Phone that the TFTP Server is a crucial part of the IP Phone service, which contains the configuration file of the phone.
The Phone would then contact the TFTP registry and retrieve its setup file. The phone will learn the locations of the call processing agents it should be using from the contents of the configuration file and then try to reach them in the order they’re in the file.
In the next CCNA Voice tutorial in the following series, as well as via a video accessible from your preferred video sharing platform, I’m sure you set up a Cisco router as a DHCP Server.