Zero Config: Error “LAN Port disconnected. Please connect LAN port to discover again.” in Grandstream IP PBX UCM62XX and UCM6510

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Description

When utilizing Zero Config to detect devices across subnets, users might encounter the error “LAN port is disconnected. Please connect LAN port to discover again.” in the Grandstream IP PBX UCM62XX series.

Network Setup

  • UCM IP Address: 172.26.28.250
  • Phone IP Address: 192.168.20.101
Figure 1 Network diagram of UCM62XX and IP Phones
Figure 2 shows the error of “LAN port is disconnected. Please connect LAN port to discover again.” during the auto discovery

Troubleshooting steps

  1. Verified that the UCM63XX can ping the endpoints (eg, IP Phones).
  2. Confirmed that the endpoints can access the UCM63XX.
  3. Ensured that the manual registration of the devices works.
  4. Checked that the Firewall has all relevant ports open to allow seamless SIP communication.
  5. Verified that the UCM’s LAN port is properly connected.
  6. Confirmed that no NAT configurations on the phones or extensions.

Workaround

To resolve the issue and enable Zero Config to auto-detect devices across subnets, try the following steps:

  1. Navigate to Network Settings on the UCM and set the device to Switch Mode.
  2. Connect the network cable to the WAN port instead of the LAN port.
  3. Restart the UCM by powering it up using the PSU (Power Supply Unit).
  4. Once the system is powered on, reattempt auto-discovery. Devices should now be detectable.

Root Cause Analysis

This behavior is a known limitation in older UCM models, including the UCM62XX and UCM6510. Unlike the UCM63XX series, the older models distinctly identify WAN and LAN ports. When a cable is connected to the LAN port, and Zero Config attempts to detect external devices from other subnets, the UCM routes this discovery through the WAN port. This misconfiguration results in the error message.

Understanding Auto-Discovery Behavior

The Zero Config feature is influenced by certain network conditions:

  1. Scanning Subnets: Larger subnets require more time for the UCM to scan and discover connected devices, leading to delays in detection. Divide the networks into smaller subnets, such as /24, to optimize discovery time and allow broadcast searches to complete more efficiently.
  2. Inter-subnet challenges: Devices in a different subnet but within the UCM’s network range cannot be auto-discovered. Add specific subnets to the Subnet Whitelist
    in Zero Config Settings to enable the UCM to discover devices in designated subnets more effectively.