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DHCP Generic Setup

DHCP server setup involves the following phases:

  1. Installing the DHCP server software according to vendor instructions.
  2. Configuring the DHCP server with the following information:

NOTE: The DHCP standard states that when a DHCP lease expires, the device should immediately cease using its assigned IP Address. This is not necessarily the desired behavior for telephones, especially if a user is on a call. If the network has problems and the only DHCP server is centralized, the server is not accessible to the given telephone. In this case the telephone is not usable until the server can be reached.

Preferably, once assigned an IP Address, the telephone continues using that address after the DHCP lease expires, until a conflict with another device is detected. As Table 7:  4600 Series IP Telephone Customizable System Parameters indicates, the system parameter DHCPSTD allows an administrator to specify that the telephone will either:

NOTE: Microsoft DHCP servers support only dotted-decimal format for file server addresses, not symbolic names. Option 66 need not be used if the TFTP server is identified in the Site Specific Option Number string (Option 176), or if HTTP is to be used exclusively instead of TFTP. However, to simplify configuration, we recommend that you use Option 66 if you are using TFTP. If you use both Option 66 and Option 176 to identify TFTP servers, the value(s) in Option 176 overrides the value(s) in Option 66.

MCIPADD=xxx.xxx.xxx.xxx,MCPORT=yyyy,TFTPDIR=<path>,TFTPSRVR=
zzz.zzz.zzz.zzz

OR

MCIPADD={list of DNS names}, MCPORT=yyyy, TFTPDIR=<path>, TFTPSRVR= {list of DNS names}

As of Release 2.4, also use Option 176 to identify the VLAN IDs used for the Ethernet line interface and the secondary Ethernet interface for VLAN separation. In this instance, if your voice VLAN ID is xxx and your data VLAN ID is yyy, at a minimum, add the following string to Option 176:

L2Q=1,L2QVLAN=xxx,PHY2VLAN=yyy

The following is the recommended format for VLAN separation on dynamically programmed IP telephones. In this example,

L2Q=1,L2QVLAN=xxx,VLANSEP=1,PHY2VLAN=yyy,PHY2PRIO=z

To enable VLAN separation, also set related parameters in the 46xxsettings.txt file, as described in 4600 Series IP Telephone Scripts and Application Files.

NOTE: The total length of the DHCP packet cannot exceed 576 bytes.

List the TFTPDIR value before the TFTPSRVR value, if the latter is specified in the SSON.

Some DHCP applications limit the length of Option 176 to 247 characters.

You can have SIP-specific, H.323-specific, or common system parameters in the same SSON Option. All such parameters are downloaded to all IP telephones, but only the relevant parameters for a given telephone are acted upon.

The 4600 Series IP Telephones do not support Regular Expression Matching, and therefore, do not use wildcards. See Administering Options for the 4600 Series IP Telephones.

In configurations where the upgrade script and application files are in the default directory, do not use the TFTPDIR=<path>.

You do not have to use Option 176. If you do not use this option, you must ensure that the key information, especially TFTPSRVR, MCIPADD, and MCPORT, is administered appropriately elsewhere. For example, when you specify the DNS server in Option 6, and the Domain Name in Option 15, you can use the configured names “AvayaTFTPServer” and “AvayaCallServer” for TFTPSRVR and MCIPADD, respectively. Upgrading from IP telephone Releases prior to R1.60 requires Option 176 to be minimally administered with MCIPADD.

Administer DHCP servers to deliver only the options specified in this document. Administering additional, unexpected options might have unexpected consequences, including possibly causing the IP telephone to ignore the DHCP server.

The media server name and TFTP server name must each be no more than 32 characters in length.

NOTE: Examples of good DNS administration include:

Depending on the DHCP application you choose, be aware that the application most likely will not immediately recycle expired DHCP leases. An expired lease might remain reserved for the original client for a day or more. For example, Windows NT DHCP reserves expired leases for about one day. This reservation period protects a client’s lease for a short time. If the client and the DHCP server are in two different time zones, the computers’ clocks are not in synch, or the client is not on the network when the lease expires, there is time to correct the situation.

The following example shows the implication of having a reservation period: Assume two IP Addresses, therefore two possible DHCP leases. Assume three IP telephones, two of which are using the two available IP Addresses. When the first two telephones’ lease expires, the third telephone cannot get a lease until the reservation period expires. Even if the other two telephones are removed from the network, the third phone remains without a lease until the reservation period expires.

In Table 3, the 4600 Series IP Telephone sets the system values to the DHCPACK message field values shown.

Table 3:  DHCPACK Setting of System Values 
System Value
Set to
IPADD
The yiaddr field.
NETMASK
Option #1 (if received).
GIPADD
The first four octets of Option #3 (if received).
TFTPSRVR
The first four octets of the siaddr field.

The rest of this section describes some common DHCP servers.


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