Unanswered Question
Mar 20th, 2007
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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


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lfulgenzi Tue, 03/20/2007 - 07:12
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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


option 150 ip


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

PETER NEGUS Tue, 03/20/2007 - 08:46
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DHCP server at

Source VLAN is / 24

When the ip phone boots, it sends a DHCP DISCOVER message containing its MAC address to ip address By default, the router interconnecting the VLANs blocks these messages. However, by configuring "ip helper address" you forward these messages to the dhcp server with a source address of

The DHCP server replies with a DHCP OFFER message to 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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