DHCP option 128 for phones

Dragon0x40
Dragon0x40 used Ask the Experts™
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How do ip phone and computers get the correct ip address from the DHCP server?

I know that a string can be put into option 128 and that string tells the phone where the TFTP server with the phone configuration is.

The dhcp server knows what scope to use because of the ip address of the default gateway with the ip helper address assigned? The dhcp packet has the default gateway address so it knows which ip scope to use?

What if the port is a trunk and the phone and data are on different vlans?

The phone and the computer don't know what vlan they belong to so how can you put the phone and data traffic seperate vlans.
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Expert Office
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Director of Solutions Consulting
Commented:
The phone should tag it's packets with the vlan you setup manually on the phone. The computer will have untagged packets.

Author

Commented:
If you relocate a voip phone then you need to configure the phone to use the correct vlan tag?

The vlan tag is set with the buttons on the phone?
Aaron TomoskyDirector of Solutions Consulting

Commented:
It is on my avaya ones and my polycom sip conference phones. I can't speak for every brand. Usually you press star or something as it's turning on to setup vlans and static or dhcp and such.
The VLANs settings are usually made at the switch level...
This is how packets get tagged.
Aaron TomoskyDirector of Solutions Consulting

Commented:
Unless you are usin the phone as a switch for the computer because the office only has one port. Not the best or easiest setup but many Offices have this requirement for lack of wiring

Author

Commented:
is option 128 used for all brands of IP phones or only some brands?
Even if the phone uses the same DHCP option number, the content will be different.

I manage several Avaya and Mitel systems, and they are completely different, on Avaya systems, they can even be different between different phone types. I also manage some Open Source systems using Cisco phones, and they are also different.

Author

Commented:
So if you have a trunk interface with a phone and a computer then the computer uses the native vlan and the phone uses a different vlan because it has been setup to tag the frames with a predetermined vlan tag?
Aaron TomoskyDirector of Solutions Consulting
Commented:
That's how I've set it up yes. I you know a better way I'm open to it. I recently relabels a bunch of offices to not have to do this anymore. Also the phones were 10/100 and the computers are 10/100/1000 so they were being limited. Much faster now and I can just do port based vlan.
You can get VoIP phones with gigabit ports...

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