Consider a large network with many desktops, laptops, and networked printers. Wh

Question

Consider a large network with many desktops, laptops, and networked printers. What are the advantages and disadvantages of short versus long lease times? Consider if the DHCP server is unavailable for a short time – what are the impacts if lease time is short, or if lease time is long? What do you expect is a typical lease time for most large organizations with desktops, servers, wireless devices, networked printers and other networked devices (e.g., network-connected HVAC controllers, IP camera systems)?

Explanation / Answer

Guidelines for Lease Times

To define appropriate values for lease times, consider these events on your network:

Frequency of changes to DHCP options and default values.
Number of available IP addresses compared to clients requesting them.
Number of network interface failures.
Frequency at which computers are added to and removed from the network.
Frequency of subnet changes by users.
All these events can cause clients to release IP addresses or the leases to expire at the DHCP server. Consequently, the addresses may return to the free-address pool for reuse. If many changes occur on your network, Cisco recommends a lease time between one and three days for active networks, and between four and ten days for inactive networks. Assigning such a lease time reassigns IP addresses more quickly as clients leave the subnet.

Another important factor is the ratio of available addresses to connected computers. For example, the demand for reusing addresses is low in a class C network having 254 available addresses, of which only 40 are used. A long lease time, such as two months, might be appropriate in such a situation. The demand would be much higher if there were 240 to 260 clients trying to connect at one time. In this situation, you should try to configure more address space. Until you do, keep the DHCP lease time to under a hour.

Tip   Short lease periods increase the demand that the DHCP server be continuously available, because clients will be renewing their leases more frequently. The DHCP failover functionality can help guarantee such levels of availability.
Be careful when creating policies that have permanent leases. A certain amount of turnover among clients occurs, even in a stable environment. Portable hosts might be added and removed, desktop hosts moved, and network adapter cards replaced. If you remove a client with a permanent lease, it requires manual intervention in the server configuration to reclaim the IP address. It would be better to create a long lease, such as six months, to ensure that addresses are ultimately recovered without administrator intervention.

Recommendations for lease durations include:

Set cable modem lease times to seven days (604800 seconds). The leases should come from private address space, and the cable modems should seldom move around.
Leases for customer premises equipment (CPE) or laptops should come from public address space and should match the habits of the user population, with as long a lease as possible to reduce load on the server.
Shorter lease times require more DHCP request and response buffers. Set the request and response buffers for optimal throughput (see the “Setting DHCP Request and Response Packet Buffers” section).
Allow the server to determine the lease period, by ensuring that the allow-lease-time-override policy attribute is disabled, which is its normal default. Even if enabled, clients can only request lease times that are shorter than you configure for the server. Some clients always request a fixed lease time (such as an hour) or the same one they had previously. These kinds of requests can cause problems in that the client never gets the full lease time, thereby generating more traffic for the server.
Defer any lease extensions for clients trying to renew leases before the halfway mark in the lease. For details, see the “Deferring Lease Extensions” section.

In a home setup, with the very limited options that most soho routers dhcp support not much.. if you have lots of machines that use up most of your available scope.. you might want a shorter lease, to free up addresses as machines go off the network.
Or if you dns, or other options that can be handed out with dhcp change often - then a short lease time might be better to speed up how fast these changes would get pushed out, etc..
If you have lots of machines and do not make changes to your options, and your not worried about address space.. Or you want machines to be able to be off the network for awhile and when they come back still get the same address they had last time, and your looking to lower the network traffic for dhcp -- then a longer lease would make more sense.
But for the typical home setup.. whatever the default is should be fine, which quite often is 24 hours.
BTW -- which lease are we talking about, the one the cable modem gets from your ISP.. or the one your routers dhcp hands out to your clients? Most of the time you do not have an option to request a longer lease from your ISP.. and even if you could request a specific length -- they most likely just hand out whater they want to hand out - and could care less if you request a longer one or not, etc..

With a very low lease time you will see an increase of network traffic, particularly broadcast traffic as the "discover" and "offer" phases of DHCP are layer 2 broadcasts. How much of an issue this is depends on many factors such as the size and complexity of the network, latency, performance of the DHCP server, etc. Keep in mind DHCP clients do not wait until their lease is expired to try to renew it. So if you gave me a 60-second lease I'll be talking to the DHCP server (potentially) every 30 seconds to renew it.

As for "weird" issues, anything goes. Different DHCP clients will behave differently. Some may handle it fine, some may have problems renewing so often and fail. Perhaps there are clients which get a lease and simply sleep for a certain period of time then check if they need to renew or toss the address if it expired. If the sleep is longer than the lease then the system will keep the IP longer than it is allowed to. I haven't seen that issue before but I have seen things like the IP a client requests in the "request" phase being different than the one the server gave it in the "offer" phase but the server actually gave the client the "request" IP, which was already in use. Never under-estimate how poorly software can be written.

THANK YOU

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