Refusing DHCP offer


I mostly work with Mikrotik routers.

I have noticed, from time to time, that a workstation will refuse the DHCP reservation that has been assigned to it

If I allow free DHCP access it works fine i.e. the same workstation properly obtains an IP address

The issue only seems to be linked to reserved IP addresses.

any ideas ?
Yann ShukorOwnerAsked:
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bbaoIT ConsultantCommented:
if the DHCP service is provided by one of your Mikrotik routers, and you believe the configuration is correct, the first thing i would do is to update its firmware to the latest version as commonly this kind of bugs are patched by the vendor regularly, especially for some popular models.
Can you tell us what is your DHCP service?
Yann ShukorOwnerAuthor Commented:
thanks bbao, someone also suggested that the address might already be active on the network; I'll have to test that theory

vivigatt: Mikrotik
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bbaoIT ConsultantCommented:
if you guess the address has already been in use on the network, then you can confirm this by unplugging the problematic workstation then PINGing that address from another host on the network. you may also check / search the firewall's log to see any DHCP error indicating duplicate IP.

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The latest DHCP implementations suggest to try to ping an address before assigning it to some host.
Yet I don't know how that applies to reservations. RFC 2131 §2.2 mentions this: "As a consistency check, the allocating   server SHOULD probe the reused address before allocating the address,    e.g., with an ICMP echo request (aka ping), and the client SHOULD probe the  newly received address, e.g., with ARP."
You may want to check if there is an updated dhcp/firmware for your dhcp server(s)...
Yann ShukorOwnerAuthor Commented:
Well I discovered a second device, on a different site, that is refusing the reserved IP address; in both cases the routers were RB951G

The first device was a PC, this time it was a VoIP phone

The phone was installed six months ago, and it has worked fine up till now
I haven't recently updated ROS (currently 6.27)

If I disable the reservation, the phone properly obtains a new IP address

The IP address isn't already assigned anywhere on the network

very confusing
Have you upgraded your router's) firmware?
Yann ShukorOwnerAuthor Commented:
After reverification, it was in fact because the IP address was already in use
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