DHCP, IP Phones and Multiple VLANs

by ScanSource Catalyst Tech Support on August 15, 2008 · 1 comment

in Catalyst Tech Support,ScanSource Catalyst

Scenario:  DHCP is needed to assign IP phones IP address.  The ports the phones will be plugged into will connect the IP phone and the PC, which will be plugged in to the back of the IP phone.

The switch will need to have 2 VLANs on it, one for voice and one for data.  The port the phone is plugged into will need to be assigned to the data VLAN as untagged and the voice VLAN as tagged.

The call server (IP phone system) will be plugged into a port assigned only to the voice VLAN.  There will also need to be a DHCP server on the data VLAN and the voice VLAN.

For Avaya IP phones there is a need to create a custom option in the DHCP server named SSON, the option number will be 176.  The string value of this option is:
“MCIPADD=#.#.#.#,MCPORT=1719,TFTPSRVR=#.#.#.#,L2Q=1,L2QVLAN=0”
The VLAN is set by the L2QVLAN parameter.  L2Q=1 enables VLAN tagging.

The option 176 will have to reside in the data VLAN DHCP server as well as the voice VLAN DHCP server.  The reason is as follows:  When a phone is plugged into the port that is untagged for data and tagged for voice it will speak on the untagged network. This is because the IP phone does not know what VLAN the voice resides on.  If there is a DHCP server on the data VLAN that tells the phone (with option 176) to use the voice VLAN tag the phone will accept this reboot and come up on the voice VLAN.  At that time the IP phone will accept the DHCP offer from the voice VLAN DHCP server.

If there is not a DHCP server on the data VLAN the IP phones will have to have the VLAN ID manually assigned.  This can be done by press * when the phone is waiting for a DHCP offer, this is identified on the IP phone screen with DHCP # of seconds and * to program.  Make certain the 802.1Q = Auto or on.  Next set the VLAN ID= to the voice VLAN ID.  VLANTEST can be left at default.  When asked to save the new values say yes (#) and reboot.

Learn more about this topic at scansourcecatalyst.com >

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