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DHCP
Issue 4 August 2006 65
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:
Note: Examples of good DNS administration include:
- Option 6: “aaa.aaa.aaa.aaa”
- Option 15: “dnsexample.yourco.com”
- Option 66: “tftpserver.yourco.com,zzz.zzz.zzz.zzz”
- Option 176: “MCIPADD=xxx.xxx.xxx.xxx”
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 lease of the first two
telephones expires, the third telephone cannot get a lease until the reservation period
expires. Even if you remove the other two telephones from the network, the third
telephone remains without a lease until the reservation period expires.
In Table 4
, the 4600 Series IP Telephone sets the system values to the DHCPACK message
field values shown.
The rest of this section describes some common DHCP servers.
Table 4: 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.