VoIP dhcp option

Can sombody help me to understand the option feature in the dhcp pool configuration for VoIP (see sample below)?

ip dhcp pool Data
   network x.x.x.x
   default-router x.x.x.1
   dns-server x.x.x.x y.y.y.y.y
   option 242 ascii L2Q=1,L2QVLAN=5
   option 176 ascii L2Q=1,L2QVLAN=5
ip dhcp pool VoIP
   network x.x.x.x
   default-router x.x.x.1
   option 242 ascii MCIPADD=x.x.x.x,x,x,x,x,MCPORT=1719,HTTPSRVR=x.x.x.x,TFTPSRVR=x.x.x.x,VLANTEST=0
   option 176 ascii MCIPADD=x.x.x.x,x,x,x,x,MCPORT=1719,HTTPSRVR=x.x.x.x,TFTPSRVR=x.x.x.x,VLANTEST=0
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Ken BooneNetwork ConsultantCommented:
ok so these options 242 and  176 do the same thing but for different types of phones.  The way  it works is that the phone boots up natively in the data vlan.   Avaya 46xx phones use option 176 and 96xx phones use option 242.  

 L2Q=1 is the setting that tells the phone to operate as an 802.1q trunk.  L2QVLAN =5 tells the phone that it should be in vlan 5.  So basically the phone boots up in the data vlan, receives a data IP address, and then looks at these options.  The phone then resets and boots up in the vlan defined by L2QVLAN.  In your case it is vlan 5.  The phone now receives an IP address on vlan 5 and the additional option.  
The difference now is that option has different information for the phone

In your case:
   option 242 ascii MCIPADD=x.x.x.x,x,x,x,x,MCPORT=1719,HTTPSRVR=x.x.x.x,TFTPSRVR=x.x.x.x,

MCIPADD is the ip address of the call server, MCPORT is the port that it will use to communicate on.   HTTPSRVR is the HTTP server that the phone can pull down http files from and TFTPSRVR is the ip address of the tftp server that holds the phone config files.

So basically these additional options tell the phone where to pull down its config from and what call server to register with.

Hope that helps..

Oh btw.. These options are probably used on many other phones.  Here is a doc that will help you maybe:


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leblancAccountingAuthor Commented:
But those options can be configured on the DHCP server like Windows Server for example. Correct?
I will have to specify those options if I configure my router as a DHCP server. Correct?

I will be using NEC VoIP phones itl-8ld-1 dt730. Do you know if they mark the traffic automatically. I could not find any document on how to configure the Cisco switch with NEC VoIP phones. Thanks
Ken BooneNetwork ConsultantCommented:
Yes you can configure those options on the windows DHCP server.  

Are you moving to NEC from Avaya?  The DHCP settings are specific to the phone manufacturer.  They all do different things.  You will need to contact your NEC provider to get the details of how DHCP needs to be setup.

Also, some phones have different methods of getting their vlan.  DHCP with the phone booting up in the data vlan, getting its vlan from DHCP and then resetting itself in the voice vlan and then getting DHCP is kind of an old way of doing it.  Cisco using CDP but this only works with Cisco phones.  Many manufacturers are using LLDP now, which is similar to CDP.  So you will really need to find out from NEC the specifics for how the phone gets its information.   If you can use LLDP the phone only needs to boot up in the voice vlan.  So some information it gets from LLDP and some from DHCP.

It all depends on the phone.
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leblancAccountingAuthor Commented:
NEC phones use 802.1q trunking. But I am not sure if they mark the packet with dscp or cos.
Ken BooneNetwork ConsultantCommented:
That would be a question for NEC, however, you could have the switch mark the traffic on the ingress port.
leblancAccountingAuthor Commented:
I assume that the phone marks the packet and my switch just trust dscp or cos. That's how the Cisco phone works.
Ken BooneNetwork ConsultantCommented:
I would assume the same thing..  I do know several years ago that one of the manufacturers did not have their stuff marked but that was a long time ago.
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