Discovery can carry network policy
LLDP advertises link-local information between neighbors. LLDP-MED adds capabilities useful for media endpoints, including network policy. A voice policy can tell a supporting phone which VLAN and priority markings to use. Advertising a VLAN does not create that VLAN's forwarding path. The switch still needs the intended VLAN membership, uplink configuration and services. A correctly advertised policy can therefore move a phone into a subnet where DHCP or the PBX is unreachable.
Diagnose the boot transition
Observe the port's neighbor information and the policy actually advertised. Then check the phone's selected VLAN, resulting DHCP lease and access to DNS, NTP, provisioning and SIP services. Compare the phone's PC passthrough port separately; its untagged data traffic may follow a different policy. When static phone settings, DHCP options and LLDP-MED disagree, use the exact model's precedence rules. Repeated changes between VLANs can appear as random registration loss. Capture the transition rather than inspecting only the final failed state.
Treat policy as configuration, not authentication
Do not grant broad network trust merely because a device advertises itself as a phone. Define access controls independently and verify what a non-phone client can reach from the same port under the intended authentication design. Pilot changes on one port with recovery access. Confirm voice tagging, data passthrough and call quality after a reboot. Keep the advertised policy and actual switch configuration together in the deployment record so the next administrator can compare intent with forwarding behavior.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.