IP-Phones DHCP Option 43 correct String

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atreju
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Posts: 1
Joined: 05 Dec 2017 09:53

IP-Phones DHCP Option 43 correct String

Post by atreju » 05 Dec 2017 11:00

Dear all

We have OXE with OT and Alcatel 8028 and 8068 Phones. We have Windows DHCP with the Option 66 (TFTP Server) configured, we have Cisco Switches witch Access and Voice VLAN defined.

Everything works fine.

Now I have the challenge that I need to set up a Linux - based DHCP Server (Sophos Firewall with built in DHCP functions) and the correct DHCP options.

Now I am really struggling with the DHCP Server because custom vendor-classes is not "possible" on Sophos DHCP like on Windows DHCP.
Configuring the "standard" Option 66, 67, 150 does not work - the phone always says "Bad tftp1 address".

So I guess I need to configure option 43 (which is supported by Sophos DHCP) with the correct HEX value for our environment.
But now the real challenge begins > I am not able to figure out the correct string.

How option 43 works is almost clear to me, but everything I tried did not work.

Could anyone help me with the correct option 43 for Alcatel ipPhone?

We have:

Voice VLAN ID 6
TFTP-Server: 172.16.6.50

Thank you very much
Kind regards
Atreju

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dryhouse
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Posts: 161
Joined: 06 Apr 2010 06:11
Location: Madrid,Spain

Re: IP-Phones DHCP Option 43 correct String

Post by dryhouse » 07 Dec 2017 09:51

may the force be with you....

pluff
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Posts: 19
Joined: 02 Dec 2008 01:40

Re: IP-Phones DHCP Option 43 correct String

Post by pluff » 16 Jul 2018 01:57

I am also unsure as to how this is all supposed to work and there seems to be no definitive explanation of the whole process that is available.I am certainly no expert on DHCP so I could be wrong about some of this but this is what I believe at the moment...

1. AVA will not work if you use "vendor class" entries on the DHCP scope, only the standard option 43 works and a second discovery only happens if the VLAN is not the correct one and the phone remembers the last VLAN. Also the terminating FF in the documentation is not required - for example 3a 02 VV VV FF for VLAN VVVV.

2. Option 66 is not actually the PABX. Option 66 (and you need option 67 with anything in it) This option populates the NextServ option of the DHCP offer so will normally point to the PABX but in my case without it set in the standard options (not Vendor Class) the boot of the phone did not complete.

3. For the operating voice VLAN, set the Vendor Class option 43 to contain sub-options 66 (42h) and 67(43h) for the specific phone type (IP_Touch, IC_Touch, SIP, Mobile etc.) with the specific info required for each phone type. For example - Option 66 would contain 192.168.92.246 for an IP Touch and 192.168.92.246:10443 for an 8082 handset.

4. When entering the data into Option 43 DONOT start the hex string with 2B XX where XX is the length of the full string. This is calculated and added automatically to the offer by the server (at least on WIN2012 DHCP). Entering this will match the documentation but create mal-formed packets in the offer.

Getting my head around these few issues seems to have fixed the problems I had when I followed the documentation but this is by no means a definitive explanation. You IT types may think I am stating the bleeding obvious here but not coming from a server background these issues had to be nutted out with lots of reboots and traces as it is not obvious to the non IT fluent. If someone more experienced than myself would like to add to this and perhaps add a simple excel spreadsheet calculator containing the options required for the different handsets then I think it woul simplify this setup for a lot of people.

And to the person that chose options where 2Bh = 43 and 43h = 67 all in the same document - I hate you....

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