DHCP & Option 176
Avaya IP Phones use DHCP Option 176 to look for information on the default data VLAN on a piece of wire for information on where the Voice VLAN should be.
DHCP (Dynamic Host Configuration Protocol) is a means by which clients can broadcast a request for configuration parameters, which a server will supply. Typically this is used in most infrastructures to provide dynamic IP addresses to clients along with subnet masks, DNS/WINS servers IP addressess. This significantly decreases the administrative burden of adding new hosts to the network.
DHCP contains within its description a mechanism for proprietary options to be passed with the DHCP response. One such option, is option 176 as used by Avaya to supply configuration credentials to IP Phones.
The option 176 parameters are configured as a text string on the DHCP server. A different Option 176 string is added to each DHCP scope depending on its relevance to the IPT solution. In a data VLAN,
Here are the text strings that should be added to the ‘176’ option required on the data/voice Vlans.
This string will tell the phone the IP address of the TFTP server, and also the VLAN ID of the Voice VLAN. The phone will then, after its TFTP download, will reboot and come up on the Voice VLAN.
- MCIPADD=IP address of CLAN/h.323 Gatekeeper
- MCPORT usually=1719
- TFTPSRVR is the IP address of the relevant TFTP server
- L2QVLAN is the Voice VLAN tag
- L2QAUD is the 802.1p QOS value between 0-7 for RTP audio (should be 5)
- L2QSIG is the 802.1p QOS value between 0-7 for IP signaling to the phones (should be 3)
- VLANTEST (recommended set to 30) the number of seconds to wait for a DHCPOFFER when using a non-zero VLAN ID
- PHY1STAT=5 sets Ethernet interface 1 to 100MB full duplex (optional, omit these to allow phones to Autonegotiate)
- PHY1STAT=5 sets Ethernet interface 2 to 100MB full duplex (optional, omit these to allow phones to Autonegotiate)
There are also options for HTTPSRVR for the newer phones which can be included instead or as well as TFTPSRVR.