What to do if devices in the same VLAN fail to communicate with each other when connected to TP-Link switch

Scenario:

As is shown, two PCs with static IP addresses in the VLAN 10 cannot communicate with each other. The data packets need to pass three switches to be transmitted between the two PCs.

Troubleshooting Steps:

Note: This guide takes the Web GUI of SG2210MP as an example.

Step 1: Check the port status. Go to SYSTEM > System Info > System Summary where you can check the port status. Here both the Port 1 connected to the PC and the uplink Port 9 are in Link Up status.

If the port status is unstable or the link speed is not satisfying, go to MAINTENANCE > Device Diagnostics, choose the corresponding port, and click Apply. As is shown below, four pairs of cables are all shown as Normal, indicating that the networking cables function well.

Step 2: Check the status of spanning tree and loop detection. Both spanning tree and loop detection are disabled by default in the Standalone mode, while in the Controller mode, spanning tree is disabled and loop detection is enabled by default.

If STP is enabled, go to L2 FEATURES > Spanning Tree > Port Config where you can check the port status. Here the status of the uplink Port 9 configured with STP is Forwarding without being blocked.

If loopback detection is enabled, go to L2 FEATURES > Switching > Port > Loopback Detection where you can check the loop status and block status of the corresponding port. Here the Port 1 connected to the PC with loop detection enabled is not blocked.

Method 1: Check packet discards of the port via CLI commands. As is shown below, Tx Discards and Rx Discards are 0, indicating that no network congestion or packet discards occur.

Method 2: Go to MAINTENANCE > Traffic Monitor and monitor the Octets Rx and Tx statistics of the port over a period of time (e.g. 10 seconds). These statistics can be used to see whether there is congestion on the port by deducing the average throughput over this period and comparing it with the port bandwidth.

Step 3: Check the creation of VLAN. Go to L2 FEATURES > VLAN > 802.1Q VLAN. Here VLAN 10 has been created.

Step 4: Check the VLAN configuration of the corresponding port. Go to L2 FEATURES > VLAN > 802.1Q VLAN > VLAN Config and click Edit. Here the Port 1 connected to the PC and the uplink Port 9 have been added to VLAN 10. The PC ports are usually untagged while the uplink ports are tagged in the VLAN.

Step 5: Check the PVID configuration of the corresponding port. Go to L2 FEATURES > VLAN > 802.1Q VLAN > Port Config where you can check the PVID configuration of all the ports. The PVID of the Port 1 connected to the PC should be set as 10, while that of the uplink port is kept as 1 by default.

Step 6: Check the MAC address table. Go to L2 FEATURES > Switching > MAC Address > Address Table where you can check the mappings between MAC addresses, VLAN IDs, and ports. Mapping errors can affect data forwarding, leading to devices’ failure to communicate with each other.

Step 7: Check the filtering policies (e.g. ACL, IMPB). Go to SECURITY > ACL > ACL Config. Here no ACL has been configured for VLAN 10.

Go to SECURITY > IPv4 IMPB > ARP Detection. ARP detection is disabled by default. Here the ARP detection is not enabled for VLAN 10.

Go to SECURITY > IPv4 IMPB > IPv4 Source Guard. IPv4 Source Guard is disabled by default. Here IPv4 Source Guard is not enabled for the corresponding ports.

The checking procedure for IPv6 IMPB is similar and will not be repeated here.