We have recently installed a Cisco SG300-52MP as a backbone router with DHCP server and voice VLAN. We have also discovered that when current DHCP lease is expired, the router issues a completely new DHCP address to the device, breaking active connections to it, if any. This also has a possibility for a Cisco IP phone to reset its network, potentially dropping it out of voice VLAN for an unknown reason, so I'd like to alter this behavior, and should a DHCP request would come hitting an expired lease, the router would re-allocate the same IP address to the device. How can I do this? I am primarily configuring the SG300 with browser, however I can perform CLI configuration if needed.

SG300 firmware is the latest, or at least one of the latest,

1 Answer 1


There's 2 different mechanism that comes in play:

  • allocation of a new DHCP lease.

  • renewal of an existing DHCP lease

Normally a new client will first request a DHCP lease, and get a random IP address, and prior to the expiration of the lease it will ask for a renewal of the existing lease and keep the same IP.

I see two things that could explain your issue:

  • the lease duration configured on the DHCP server is too small, and the lease expire before the client try to renew it, so from the DHCP server point of vue, this is no more a renewal but a new lease
  • a bug in the switch software

What you can try:

  • check the lease duration and the client operating systme behavior regarding DHCP leases
  • on a windows machine, force a renewal with ipconfig /renew and see if it actually renew the existing lease or change it's IP

  • inspect the traffic between the client and the server with wireshark/tcpdump to see what really happens

this said, except for very small network, I would not rely on a DHCP server on this kind of device but rather use the DHCP relay feature to forward DHCP request to an external DHCP server, generally on a server

I'm not fond of Microsoft, but used their DHCP for many years and never had any issue with it, while I had some trouble with Cisco equipment's DHCP servers.

  • The main trouble is that there is no server in that network, but many phones and desktops, so we can only use Cisco device as DHCP. Using ASA5506 ended up a disaster, using SG300 works to a degree, so we stuck with SG300 being DHCP server. That leaves wireshark stuff out of reach :( wish I'd have a peek at network layer.
    – Vesper
    Nov 15, 2017 at 13:01
  • From what I have derived - DHCP lease time is 10h, no entry to configure the window when the client should renew lease. Also it's possible that I'm missing something in that config, and it already behaves as such (at least one problem client did get its lease renewed to the same IP address), but I also see a lot of expired DHCP leases in the console in the PC network (there are wireless devices in there as well, so it's kind of normal, but they grow...). Maybe I'm mistaking this as a problem. Thanks for help anyway.
    – Vesper
    Nov 15, 2017 at 13:14
  • @Vesper, "so we can only use Cisco device as DHCP. Using ASA5506 ended up a disaster, using SG300 works to a degree, so we stuck with SG300 being DHCP server." Well, you could install a real DHCP server. The DHCP servers built into network devices are usually not very good.
    – Ron Maupin
    Nov 15, 2017 at 15:16
  • @RonMaupin If we could, we'd install a server that would do DHCP, admin/troubleshooting activities etc, but the client doesn't want to pay more.
    – Vesper
    Nov 16, 2017 at 10:45
  • @Vesper So how many time will you spent debugging this and who will pay for it ? (rhetorical question, no need to answer).
    – JFL
    Nov 16, 2017 at 12:08

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