I have a similar problem. When we factory reset a phone, it will not automatically switch back to the correct vlan even though dhcpd.conf option 132 is set. The Grandstream DHCP_Options_Guide PDF file (page 17) has some packet captures showing the phone should do a request for option 132 - but GXP2160 does not do this.
I added to my dhcpd.conf:
option vlan-id code 132 = text;
option vlan-id "34";
I did similar captures on my GXP21XX phones and these phones do not request option 132 so even though the server has option 132 configured so it does not send it.
I’ve also tried forcing a send with
option dhcp-parameter-request-list = concat(option dhcp-parameter-request-list,84);
This forces dhcpd server to send the vlan-id, wireshark shows it is sent, but the phone does nothing with option 132.
Here is the wireshark capture for the DHCP Discover coming from the GXP2160 - note NO request for option 132:
Please add this feature to the GXP21XX phones so they auto-provision onto the correct vlan after a factory reset. Thanks.