How to debug an aggregation switch

To debug an aggregation switch, systematically verify physical connections, port status, LACP configuration, and switch compatibility to identify and resolve LAG or LACP issues.

Step 1: Verify Physical Connections

Ensure that all member ports of the aggregation group are properly connected and free from physical faults. Check for loose cables, damaged ports, or incorrect connections. Use commands like display interface (H3C) or show interfaces status (TP-Link/Cisco) to confirm that each port is physically up .

Step 2: Check Port and Interface Status

• Confirm that the aggregated interface is not administratively down. If it is, enable it using commands like undo shutdown (H3C) or no shutdown (Cisco/Juniper) .
• Verify that all member ports are in the selected/active state for aggregation. Ports in an unselected state may indicate physical or configuration issues .

Step 3: Validate LACP or LAG Configuration

• Ensure consistent configuration across all member ports, including speed, duplex mode, VLAN settings, and LACP mode (active or passive) .
• Confirm that LACP is enabled on both ends of the link. At least one side should be in active mode to initiate negotiation .
• Check for switch-specific limitations, such as contiguous port requirements or maximum number of aggregated ports .

Step 4: Monitor LACP Activity

• Use vendor-specific commands to view LACP interactions:
  • entstat (IBM AIX) to check LACP status and statistics per port .
  • debugging link-aggregation lacp packet (H3C) to observe LACP PDUs and negotiation .
  • LACP FSM logs (Cisco Nexus) to track state transitions and events .
• Look for key status indicators: Inactive, Negotiating, Aggregated, or Failed. Persistent negotiation or failure often points to configuration mismatches or incompatible port settings .

Step 5: Verify Device Compatibility

Ensure that both ends of the aggregation support the same LACP or static LAG standards. Incompatibility can prevent aggregation from forming correctly .

Step 6: Restart or Reinitialize Interfaces

If all configurations and physical connections are correct but the aggregation still fails, try restarting the aggregated interfaces or the switch to clear temporary faults .

Step 7: Consult Vendor Documentation

Each switch vendor may have specific commands, limitations, or best practices for LAG/LACP. Refer to the official documentation for detailed troubleshooting steps and configuration examples . By following these steps—physical verification, port status checks, configuration validation, LACP monitoring, and device compatibility review—you can systematically identify and resolve aggregation switch issues.