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I'm wondering if having a short DHCP lease is likely to cause any issues, such as:

  • Dropped VoIP calls
  • Dropped VPN connections

If so, could you explain why?

Dan
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Tobias Tromm
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  • Anecdotal: We had a Windows DHCP server migration last year. Prior to migration, we lowered the lease time on all scopes to 5 minutes. We restored the old DHCP config onto the new server, including the 5 minute lease. Several weeks later, we noticed we'd forgot to change the lease times back. With ~5000 clients (including guest networks) there were no perceptible issues. – jscott Apr 11 '13 at 12:21
  • @jscott Just for fun, I'll mention that a short lease time can cause problems when streaming images. For example, Citrix recommend a lease time of >1 day for their Provisioning Services. That's rather an edge case, though! – Dan Apr 11 '13 at 12:27

4 Answers4

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None of the above - you're confusing Lease Time with a DHCP client releasing the address.

The client should always renew the IP address well before the lease is up, meaning that there are no problems so long as the client remains active.

The only real problem with a short lease from an ISP is that you have no guarantee that you'll get the same IP back if your equipment goes down for a period of time. But this isn't a problem, because you shouldn't ever be relying on getting the same IP back as a DHCP client - regardless of lease time.

Dan
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  • My ISP force renew of IP every 48 hours. – Tobias Tromm Apr 11 '13 at 12:20
  • @TobiasTromm Well, if your IP is *really* changing during uptime (Which I doubt) then yes, of course you'll see problems. It's no different to pulling the cable. However, that has nothing to do with DHCP leases and is not expected behaviour. You should speak to your ISP about this. Why do you need a list if you're having problems? – Dan Apr 11 '13 at 12:22
  • Yes, they change during uptime. Exactly because of this I need a list to enforce my argument. I need more examples. – Tobias Tromm Apr 11 '13 at 12:45
  • @TobiasTromm I'm really not sure what you need. Changing IP addresses is bad for communication - you don't need a list of possible symptoms to prove that, it's a stated fact. Speak to your helpdesk. – Dan Apr 11 '13 at 12:46
  • I just need more arguments, I'm from Brazil, my ISP is GVT (Global Village Telecom). – Tobias Tromm Apr 11 '13 at 12:51
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    @TobiasTromm They're not arguments, and we're not here to make lists for you. Please read our [FAQ]. – Dan Apr 11 '13 at 12:51
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No, because those examples are wrong.

Normal operation would be like this:

  1. Client asks for a lease.
  2. DHCP server answer with an IP, the period it is valid and optionally some more information (e.g. DNS server name, search domain, a suggested hostname, ...)
  3. After half the lease time is up the client asks to renew it, keeping the same IP
  4. DHCP server grants this and we forever loop around points 3 and 4.

Now if you put the lease time very short then:

  1. You are getting a lot more lease renewals.
  2. If you turn the client off for a period longer then the remaining lease you migt get another IP.
    Note that getting another IP is fine. If that is not desired than the client should have a fixed IP and not use DHCP at all, or the DHCP server needs an address reservation.
Joel Coel
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Hennes
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2

I have (anecdotally) seen problems with a large and poorly set-up and poorly maintained environment when that environment's admins (whom I supported from afar, geographically-speaking) changed their DHCP lease to 5 or 10 minutes. As Hennes points out, at half of that time, EACH client sends an IP renewal request to the DHCP server(s), and the surge of traffic ground the environment to it's knees.

If your network architecture is sound and sane (and primarily gigabit or better), you should be fine.

But WHY? Why do you feel the need to keep a short lease? Even if you have more clients in your private IP space than available IPs, you can just go to a larger IP space (like all of 10.x.x.x)!

gWaldo
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1

Actually, Yes it can.

Wikipedia does a pretty good job explaining DHCP.

  1. DHCPDISCOVER -- Client Broadcasts to discover available DHCP servers.
  2. DHCPOFFER -- available DHCP Server responds with lease offer
  3. DHCPREQUEST -- Client requests a lease from an available server.
  4. DHCPACK -- Server acknowledges the request.
  5. Client binds to ip address.
  6. DHCPREQUEST -- Once lease is half over, re-request the same lease.
  7. DHCPACK --- Server acknowledges the request.
  8. Go to 6

If the lease time is short, then the time available to renew the lease is short. If the DHCP server is overloaded or the network not stable, it's quite easy for this time to elapse.

When this happens, the client will release the ip address and cause a small outage while it starts the process from the beginning.

  1. DHCPDISCOVER -- Client Broadcasts to discover available DHCP servers.
  2. DHCPOFFER -- available DHCP Server responds with lease offer
  3. DHCPREQUEST -- Client requests a lease from an available server.
  4. DHCPACK -- Server acknowledges the request.
  5. Client binds to ip address.
  6. DHCPREQUEST -- Once lease is half over, re-request the same lease.
  7. DHCPREQUEST -- No response. Try again..
  8. DHCPREQUEST -- No response. Try again..
  9. DHCPREQUEST -- No response. Try again..
  10. Lease expired. Relinquish IP Address. Go to 1.

My university had a lease time of 14 minutes. While this worked fine most of time, every so often the dhcp would time out. If the DHCP client is unable to obtain an IP address before the lease expires, it will relinquish the IP address causing a small outage.

user606723
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