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Community Member

dhcp

hi all,

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

any help highly appreciated

regards

2 REPLIES
Silver

Re: dhcp

The connection from a DHCP client to a DHCP scope is the default router that sends the request on behalf of the client to the DHCP server (first via broadcast or to address of IP helper).

If you have 6 scopes, you likely have 6 vlan interfaces which send the requests to the server on behalf of the client.

Take a look at the DHCP config, it should look something similar to:

ip dhcp pool scope name

network 10.1.1.0 255.255.255.0

option 150 ip 10.2.1.1

default-router 10.1.1.1

The default router entry is what allows the dhcp server to allocate the appropriate address to the client.

Community Member

Re: dhcp

Assume:

DHCP server at 10.1.2.3

Source VLAN is 192.168.1.0 / 24

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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