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OBi-200 always tenaciously clings to old IP address

Started by JaskHill, June 06, 2020, 07:02:03 AM

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JaskHill

Hi,

I found OBi-200, or any OBi unit for that matter, has an intentional and a bit annoying design behavior.

I use DHCP to assign an IP to OBi-200, using fixed MAC address mapping. So OBi-200 will always get the same IP address for this MAC.

Some days ago, I changed the mapping, to map its MAC to another IP address. I restarted the router, and OBi-200 (by clicking the "Reboot" button in its web management UI).

But I found OBi-200 always clings to old IP address, no matter what.  After two days, still the same IP address, not the new IP I assigned for its MAC.

This behavior is annoying. Does someone have a clue?

RFC3261

Quote from: JaskHill on June 06, 2020, 07:02:03 AM
Does someone have a clue?

It is somewhat common that a device will request the same address is had before a reboot as part of start-up, and then it is up to the DHCP server to decide to reject that request, or accept it.  And depending on your DHCP server even if it would assign a new IP using its tables for a brand new (DHCPDISCOVER) request, it will not do so in reply to a DHCPREQUEST to resume using the same IP for the same MAC if that address in is the pool and available to reissue.

Depending you your DHCP server you may be able to remove the existing lease entry manually.

And you should be able see what is actually happening by looking at the logs of your DHCP server (you may need to enable debug output).

It is possible that if you go into the OBi configuration, and set it to a fixed IP, and reboot, and then set it back to DHCP and reboot it will issue an initial DHCPDISCOVER in that case (it would depend on exactly how the OBi stored past DHCP addresses).

JaskHill

But the most common device, a Windows-OS computer, will definitely not have this kind of strange behaviour.

After a restart, a DHCP-enabled Windows-OS computer will always get the IP address assigned to it based on the latest MAC mapping ( If the router sets up MAC->IP mapping ).  So why does OBi200 decide not to follow standard practice?

SteveInWA

It's not just OBiTALK devices, as RFC3261 pointed out.  I have observed this behavior for decades, with all sorts of devices.