DHCP Server Administration
Issue 1 June 2010 53
remain reserved for the original client a day or more. For example, Windows NT
®
DHCP reserves expired leases for about one day. This reservation period protects a
lease for a short time. If the client and the DHCP server are in two different time
zones, the clocks of the computers are not in sync, 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 for the first two
telephones 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
telephone remains without a lease until the reservation period expires.
In Table 9
, the 1603SW-I SIP IP Deskphone sets the system values to the DHCPACK message
field values shown.
Table 9: DHCPACK Setting of System Values
System Value Set to
DHCP lease time Option #51 (if received).
DHCP lease renew time Option #58 (if received).
DHCP lease rebind time Option #59 (if received).
DOMAIN Option #15 (if received).
DNSSRVR Option #6 (if received, which might be a list of IP Addresses).
HTTPSRVR The siaddr field, if that field is non-zero.
IPADD The yiaddr field.
LOGSRVR Option #7 (if received).
MTU_SIZE Option #26.
NETMASK Option #1 (if received).
ROUTER Option #3 (if received, which might be a list of IP Addresses).
SNTPSRVR Option #42.
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