[Issue/Resolution] Dell N3200 Series Switches and Inter-VLAN routing Problem

Crossposted using Lemmit.

Original post from /r/sysadmin by /u/InsomniacsDream on 2023-07-07 02:48:59+00:00.

Hey r/sysadmin,

I wanted to share an issue we recently encountered with our Dell N3200 series switches we are deploying and how we managed to solve it. It might be helpful for anyone facing similar challenges in their network setup.

The Problem: We were experiencing difficulties accessing the web interfaces of certain devices on our LAN. After some troubleshooting, we discovered that the root cause was related to the Dell N3200 series switches and their handling of packet fragmentation during inter-VLAN routing affecting mostly clients on the WiFi but also just across Data/Security/Voice VLANs.

The Solution: Thankfully, we were able to resolve the issue by specifying the Maximum Transmission Unit (MTU) of all the switches in our network. By setting the MTU to 1532 using the command #system jumbo mtu 1532, we successfully addressed the problem and regained access to the web interfaces of the affected devices.

Explanation: The MTU defines the maximum size of data packets that can be transmitted over a network. In this case, it appears that the default MTU setting on the Dell N3200 switches was causing problems when routing packets between VLANs. The switches were not fragmenting the packets correctly, leading to issues with accessing web interfaces.

By specifying an MTU value of 1532, we ensured that the switches would correctly handle packet fragmentation during inter-VLAN routing. This adjustment resolved the problem and allowed us to access the web interfaces without any further issues.

Conclusion: If you're experiencing difficulties accessing web interfaces of devices on your LAN, particularly when using Dell N3200 series switches, consider checking the MTU settings. The default MTU might not be suitable for inter-VLAN routing and could cause packet fragmentation problems. By specifying a larger MTU, such as 1532 in our case, you might be able to solve the issue and restore access to the affected devices.

I hope this information proves helpful to those who might encounter a similar problem. If you have any questions or additional insights, please feel free to share them in the comments.