Troubleshooting guide for ARP Learning Failure

Contents

Objective

Requirements

Introduction

Troubleshooting Steps

Conclusion

Objective

This article introduces a few troubleshooting steps for solving ARP learning failure problem.

Requirements

• Omada Smart, L2+ and L3 switches.

Introduction

In the realm of network engineering, ARP is a critical function that enables communication between devices on a local network. ARP plays a pivotal role in mapping a network device’s IP address to its physical MAC address, allowing seamless data transfer within an Ethernet network. This process, known as ARP learning, is fundamental to the proper operation of network communications.

ARP learning is essential for several reasons:

• Efficient communication: It allows devices to quickly resolve IP addresses to MAC addresses, reducing the need for repetitive ARP request and replies.
• Network Performance: By caching ARP entries, devices minimize the overhead associated with frequent ARP broadcasts, thus improving overall network performance.
• Seamless Connectivity: ARP learning ensures that devices can discover and communicate with each other without manual intervention, fostering a dynamic and scalable network environment.

As ARP learning is important in network communication, when ARP learning fails, the network can experience significant disruptions. Devices may be unable to resolve IP addresses to MAC addresses, leading to failed data transmissions and connectivity issues. Also, while ARP cannot be learned, the ARP requests will be sent repeatedly in the network, increasing network traffic and degrading the user experience. What’s more, failure to properly lean and cache ARP entries can expose the network to ARP spoofing attacks, where malicious actors intercept or manipulate data.

Troubleshooting Steps

Step 1. Check if the status and line protocol status of the interface is up.

After logging in to the web GUI of a switch, go to L3 FEATURES – Interface, click Detail on the target interfaces, and check if the protocol of the interfaces is up.

If the interface status is normal, the interface status and line protocol status should be both up, like below:

If the status are down like below, which means this interface is not working normally, you will need to check the status of cables and ports and ensure they are working normally, until the interface status go up.

Step 2. Check if the ARP reply packet has reached the CPU.

If all the interfaces and physical ports, cables are working normally, the next thing to do is to check if the ARP reply packet has really reached the CPU. To do this check, port mirroring is needed. Go to MAINTENANCE – Mirroring, click Edit on the port mirroring session.

We need to mirror the ingress of CPU to a port in order to monitor if the ARP reply has reached the CPU. So in Destination Port Config, select a port where the monitoring PC is connected. In Source Interfaces Config, go to CPU, select the CPU and change the Ingress status to Enabled. After selected, click Apply to save the port mirroring configuration.

After configuring, check the Port Mirroring Session List to see if the session has been correctly set.

After configured, use Wireshark or other monitor tools and check if the ARP reply could be captured from CPU ingress. If no ARP reply is captured, please check if your device has correctly sent the ARP reply.

Step 3. Check if your current ARP entries has exceeds the maximum number of ARP entry of this device.

If the ARP reply is confirmed to be received by the CPU, the next thing to check is the current ARP entries quantity. Go to L3 FEATURES – ARP – ARP Table, and you can see how many ARP entries are learned by the device in total.

Then go to the product page on our official website, in Specification – Software Features – L2 and L2+ Features/L3 Features, you can see the maximum ARP entries supported on this device, if the current ARP entries has already reached the limit, no new ARP could be learnt.

Conclusion

Till here, we have introduced a few steps for the troubleshooting of ARP learning failure, if the problem still exists, please collect the relative information and contact TP-Link Support.

Get to know more details of each function and configuration please go to Download Center to download the manual of your product.