DHCP Option 176

DHCP Option 176
Just the basics of DHCP option 176 are covered here. See the “4600 Series IP Telephone LAN
Administrator's Guide” for more details.
The DHCP specification has what are called options, numbered from 0 through 255. Each option is
associated with a specific bit of information to be sent by the DHCP server to the DHCP client. For
example, option 1 is the subnet mask option and is used to send the subnet mask to the client. Option 3 is
the router option and is used to send the default gateway address and other gateway addresses to the
client. Some options are defined – such as options 1 and 3 – and others are not. The defined options are
found in RFC 2132.
Options 128 through 254 are site-specific options. They are standard options that are not defined, and
vendors may use these options and define them to be whatever is necessary for a specific application.
Avaya IP telephones use site-specific option 176 as one of the methods to receive certain parameters from
the DHCP server.

For the Avaya application of option 176, it is defined as a string. The string contains parameters and
values separated by commas, as illustrated after the following table. The most prevalent parameters and
values are as follows.
Parameter Value
MCIPADD Address(es) of gatekeeper(s) – at least one required
MCPORT The UDP port used for registration – 1719 default
TFTPSRVR Address(es) of TFTP server(s) – at least one required
L2QVLAN 802.1Q VLAN ID – 0 default
L2QAUD L2 audio priority value.
L2QSIG L2 signaling priority value.
VLANTEST The number of seconds a phone will attempt to return to the
previously known voice VLAN
Table 8: DHCP option 176 parameters and values
The typical option 176 string for a single-VLAN environment looks like this.
MCIPADD=addr1,addr2,addr3, … ,MCPORT=1719,TFTPSRVR=addr
At least one gatekeeper (C-LAN or S8300) address must be present after MCIPADD to point the phones
to a call server. MCPORT specifies which UDP port to use for RAS registration. IP telephone firmware
1.6.1 and later already have 1719 as the default port, but it is prudent to include it. A TFTP server
address is necessary so that phones know where to go to download the necessary script files and binary
codes (see “Boot-up Sequence” heading below). L2QVLAN and VLANTEST would be included if
802.1Q tagging were required, such as in a dual-VLAN environment (see section 4.2). Other parameters
may be added, such as L2QAUD and L2QSIG, which are used to specify the L2 priority values for audio
and signaling. If these values are not specified in option 176, the default values (6/6) are used.
Note: The L3 priority values (DSCP) are received from the call server, as configured on the SAT ipnetwork-
region form. The reason L3 values are received from the call server and L2 values are not is
because an IP phone accepts all L2 values from one source. The preferred and recommended method is
via DHCP option 176. An alternative method is described in section 3.5, heading “ip-network-map,”
which utilizes the L2 values administered on the SAT ip-network-region form.
An administrator must create option 176 on the DHCP server and administer a properly formatted string
with the appropriate values. Option 176 could be applied globally or on a per scope basis. The
recommendation is to configure option 176 on a per scope basis, because the values themselves or the
order of the values could change on a per scope basis. As part of the DHCP process at boot-up, the IP
telephone requests option 176 from the DHCP server.
DHCP Lease Duration
A DHCP server gives out an IP address with a finite or infinite lease, and the Avaya recommended lease
duration for IP phones is 2 to 4 weeks. The DHCP specification calls for the client to renew the lease at
determined intervals, typically beginning at half-life of the lease. If the first renewal attempt fails, there
are allowances in the specification for further renewal attempts, dependent on the length of the lease. Too
short a lease requires too many renewals, which not only taxes the DHCP server but can also disrupt
service to the IP phones if renewals cannot be accomplished for whatever reason. On the other hand, too
long a lease can result in IP address exhaustion if hosts are unplugged from the network without properly
shutting them down to invoke a release of the IP address lease.

Ref: http://www.vocalcom.fr/download/VOIP-AVAYA-IP-GUIDE-3-0.pdf

DHCP for multiple VLANs

1. Normal way of doing it is to have trunk link between router and switch and setup DHCP server on the router based on the subinterfaces

2. If no trunk link available, IP helper on the switch can also be considered.