LLDP (Link Layer Discovery Protocol, IEEE 802.1AB) lets PROFINET devices identify each other and automatically discover the network topology, which is exploited by diagnostic tools such as PRONETA or the TIA Portal Topology Editor.

LLDP is a standard Ethernet protocol natively embedded in PROFINET: each device periodically emits (typically every 5 s) an LLDP frame containing its identity, ports and role; neighbours receive and store this information in the LLDP-MIB; tools such as PRONETA or TIA Portal then query the MIBs and rebuild a visual topology. LLDP also enables the automatic detection of cabling anomalies, for instance a device patched into the wrong port.

If LLDP shows a topology that diverges from the engineered one, fix the wiring before commissioning: a swapped port pair can pass cyclic data yet later defeat any topology-based diagnostics or expected port-based features.