The length of the DHCP lease governs the amount of time a host “owns” the address. To continue
using the address, the host must renew with the server before the lease expires. Designers
must consider the overhead of this renewal traffic and the impact of failed or unavailable DHCP
servers. In general, long leases are appropriate for fixed environments, and short leases are
applicable in more dynamic installations.
Consider a fully functioning network with 100 workstations and a lease length of five minutes.
This is an extreme example (that no self-respecting engineer would install) because DHCP will
send a renewal request at an interval equal to one-half the lease period. The overhead for just IP
address leases would be 2,400 requests per hour, not including any DNS queries and the multiple
packets involved in each request (see Figure 24.6). This is a high amount of overhead for information
that should not change under normal circumstances.
In addition, when a lease expires, the host must release its IP address. Without a DHCP
server, it will be unable to communicate on the network because it has no IP address.
The alternative to a short lease is to make the lease very long. Consider the impact of a lease
equal to 60 days. Should the hosts remain on a local subnet with very few changes, this would
substantially reduce the volume of traffic.
However, this would not be appropriate for a hotelling installation. Hotelling is a concept
introduced years ago in which notebook users would check into a cubicle for a day or even a
week. DHCP is a great solution for such an installation because the MAC addresses are constantly
changing, but a long lease time would be inappropriate here. Consider a scenario in
which each visitor connects once per quarter, or every 90 days. And, for this example, presume
that there are 800 users of the service, and the pool is a standard Class C network of 254 host
addresses. If the lease were long—90 days for this example—only the first 254 users would be
able to obtain an address. Clearly, this is not appropriate for this type of installation, which is
an important consideration for the network designer.
As mentioned earlier, the default DHCP lease renewal interval (on Windows NT) is 72 hours.
DHCP attempts to renew the lease after one-half the lease duration, or 36 hours in the case of
default Windows NT.
The default lease on Cisco IOS-based DHCP servers is 24 hours.
For reference, the mechanism by which DHCP obtains an address is illustrated in
Figure 24.6. Note that DHCP uses a system of discovery to locate the DHCP server—a
phase that uses the helper function. After the DHCP server is found, the offer is returned
to the workstation, and the request is positively or negatively acknowledged. One way to
remember the DHCP process is with the mnemonic DORA, which stands for Discover,
Offer, Request, and Acknowledgment.