Fixed assignments on 9300X switches

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Good afternoon

we have recently installed a new network and for the first time have been using Fixed assignments for IP addresses rather than statically assigning to the device and reserving ranges.  This has been working well across a number of VLANs, until yesterday evening two devices on a particular VLAN (the only devices on that VLAN) didn't renew their lease and requested a new address.  There were provided with an address that was not the fixed assignment.  The MAC address has not changed and there is no reason they should have not been able to renew the lease at any point.

We have restarted the devices on a number of occasions since and they keep getting the new non fixed IP address that has been assigned.  When we excluded one of the new addresses to stop it being assigned at restart that device was just allocated the next one in the list, again not the fixed assignment.

We have raised this with TAC and they suggested it could be due to dhcp option 61 but we have a number of similar devices on different VLANs that are still working, and the two failed devices have been working for a few weeks without issue.

Has anyone experienced a similar issue at all?

Thanks

[alemabrahao]

Hi,

Cisco is correct in pointing to DHCP Option 61 as a potential cause.
If a device sends a Client Identifier (Option 61), the DHCP server may use it instead of the MAC address to track leases.
If the identifier changes, the server may treat the device as "new" and assign a different IP.

[PhilipDAth]

@alemabrahao and Meraki are bang on. The MAC address is used in the absence of a client identifier.

You need to ensure that the identifier remains unchanged.

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Thank you @alemabrahao and @PhilipDAth for your replies.  We've been working with TAC as we're getting some odd behaviours around the dhcp reservations and configuration not being saved to the switch.  They might be separate issues or they might be linked, hopefully further investigations from the Cisco internal team will shed some light on this. For now we've fallen back to using static IPs as the issue has spread to access points which needed guaranteed address for ISE authentications.