we are running dhcp server in core switch for cisco phones , we define 6 dhcp scopes for 6 floor for cisco phones , i need to understand the logic connection between floor phones , how they communicate to the right dhcp scope , i tried with sniffer , but could not find much information , can somebody help
When the ip phone boots, it sends a DHCP DISCOVER message containing its MAC address to ip address 255.255.255.255. By default, the router interconnecting the VLANs blocks these messages. However, by configuring "ip helper address 10.1.2.3" you forward these messages to the dhcp server with a source address of 192.168.1.255.
The DHCP server replies with a DHCP OFFER message to 192.168.1.255 detailing the allocated IP address and the originating MAC address.
There are other transactions for lease renewal etc.
You should limit the protocols forwarded by "ip helper-address" to "bootp" as by default it forwards all sorts of rubbish.
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