In TIA Portal, the expected topology can be downloaded into the PROFINET PLC; at start-up the PLC compares the actual topology (discovered via LLDP) against the expected one and raises an alarm if they differ, which makes cabling errors automatically detectable.

Topology Download acts as a re-cabling safety net: the Configured Topology captures what the integrator drew (port X of device A connects to port Y of device B), the Actual Topology is what LLDP discovers at runtime, and any mismatch raises a PLC alarm. This avoids the classic failure mode where maintenance reconnects a cable to the wrong port, the plant keeps running, and the error is only discovered weeks later when the next device replacement silently fails. It is now standard practice in modern industrial and tertiary deployments.

Re-download the topology after each commissioning campaign — a configured topology that lags behind reality by two ports is worse than no topology check at all, because it produces nuisance alarms that operators learn to ignore.