Tackle MTU Misalignment on Xiaomi Mi Router 4C with Dual‑Band Access

Understanding MTU Misalignment

When you connect two networks, the data you send travels in packets. Each packet carries a header and a payload. The Maximum Transmission Unit (MTU) is the largest packet size the network will forward without fragmentation. If a packet is too large for one hop, it must be split into smaller fragments. Fragmentation can hurt performance and sometimes cause packets to be dropped, leading to slow streams, intermittent Wi‑Fi drops, or outright connectivity problems.

On home routers, the MTU is normally set to 1500 bytes for Ethernet and 1492 bytes for PPPoE connections. The Xiaomi Mi Router 4C defaults to 1500 on both its 2.4 GHz and 5 GHz interfaces, which is fine in most cases. However, when the router sits behind a VPN, or when the upstream ISP uses a different MTU, packets may need to be re‑segmented, and the router may not handle this correctly. The result is MTU misalignment, a subtle but crippling problem that shows up as slow video, laggy online gaming, or occasional “timeout” errors.

Below is a full, step‑by‑step guide to detecting, diagnosing, and fixing MTU misalignment on the Xiaomi Mi Router 4C while keeping both Wi‑Fi bands operational.

Why Dual‑Band Access Matters

The Mi 4C ships with a 2.4 GHz band (up to 300 Mbps) and a 5 GHz band (up to 867 Mbps). Each band has its own MTU setting in the router’s firmware. If you change only one, the other may still be misaligned. Users often enable the 5 GHz band for high‑bandwidth tasks like gaming or 4K streaming, while keeping the 2.4 GHz band for IoT devices. A misconfigured MTU on the 5 GHz band can silently poison traffic for all devices connected to it, while leaving the 2.4 GHz band seemingly fine.

Detecting MTU Misalignment

Before you can fix anything, you need to confirm that misalignment exists. The easiest way is to use the ping command with the do not fragment flag (-M do on Linux/macOS, -f on Windows).

1. Open a command prompt or terminal on a device connected to the router.
2. Ping a public IP (Google’s DNS works well): 8.8.8.8.
3. Incrementally increase the packet size until you see packet loss or fragmentation.

Example Session (Linux)

ping -c 10 -M do -s 1472 8.8.8.8   # 1472 bytes payload + 28 bytes header = 1500
ping: sendmsg: message too long

The message too long indicates the MTU is lower than 1500 on the path. Reduce the size:

ping -c 10 -M do -s 1460 8.8.8.8   # 1460 + 28 = 1488

If this succeeds without errors, the MTU is likely 1480 or 1482 bytes.
A similar procedure on Windows uses ping -f -l 1472 8.8.8.8.

Tip: Perform the test on both the 2.4 GHz and 5 GHz connections to compare results.

Checking the Router’s Current MTU

The Xiaomi Mi Router 4C offers two ways to view its MTU:

1. Web Administration Interface

1. Open a browser and navigate to http://192.168.31.1 or http://miwifi.
2. Log in with your admin username/password.
3. Go to Advanced Settings → Wireless Settings.
4. Under each band, look for an MTU field. If it says 1500, that’s the default.
   • The UI may not display the value directly; you’ll need to enable Advanced Mode by checking a box at the bottom of the page.

2. Telnet/SSH (If enabled)

If you have enabled telnet or SSH on the router, log in and run:

uci get network.lan.mtu
uci get wireless.@wifi-iface[0].mtu   # 2.4 GHz
uci get wireless.@wifi-iface[1].mtu   # 5 GHz

If any of these values are 1500, you’re probably not ready to modify them until you’ve confirmed misalignment.

Preparing to Change MTU

Changing MTU on a router can cause temporary disconnections. To minimize disruptions:

• Backup your router configuration.
• Schedule the change during low‑usage hours (late evening or early morning).
• Document current settings so you can revert if something goes wrong.

Step‑by‑Step Fix for the Mi 4C

A. Using the Web UI

1. Navigate to the Wireless Settings page for the band that needs adjustment.
2. Enable Advanced Mode if it isn’t already.
3. Locate the MTU field – it may be hidden under an “Advanced” dropdown.
4. Enter the corrected MTU value.
   • Commonly, a safe value for most residential ISPs is 1480.
   • For PPPoE connections, 1492 is typical.
   • For VPN connections, 1400 often works best.
5. Save the settings. The router will reboot automatically.

After reboot, run your ping test again to confirm the new MTU is effective.

B. Using Telnet/SSH

If you prefer command‑line or if the UI is broken, use Telnet/SSH:

# Set 2.4 GHz MTU to 1480
uci set wireless.@wifi-iface[0].mtu='1480'
uci commit wireless
# Set 5 GHz MTU to 1480
uci set wireless.@wifi-iface[1].mtu='1480'
uci commit wireless
# Apply changes
wifi reload

The wifi reload command restarts the Wi‑Fi drivers without rebooting the whole router, which is quicker.

Verifying the Fix

After making the change, confirm that both bands operate without packet loss:

1. Ping the router from a client on each band: ping -c 10 192.168.31.1.
2. Check the MTU using ip (Linux) or netsh (Windows) on the client to confirm the OS is negotiating the correct size.
3. Stress‑test with a high‑bandwidth activity: stream 4K video, run a large file download, or play an online game.
4. Monitor latency with tools like ping or mtr. You should see no “fragmentation needed” messages.

Common Pitfalls and How to Avoid Them

Advanced: Using MPTCP or VPN Optimisation

If you still experience issues after adjusting MTU, consider the following:

• Multi‑Path TCP (MPTCP): Allows devices to use both 2.4 GHz and 5 GHz simultaneously. Some devices support it, but the Mi 4C firmware does not. However, some third‑party firmware (e.g., OpenWrt) does, and you can install it on the Mi 4C.
• VPN MTU Negotiation: Many VPN clients allow you to specify an MTU value or auto‑detect it. Ensure the client MTU matches the router’s MTU to avoid fragmentation.
• QoS Rules: Set Quality of Service to prioritize latency‑sensitive traffic, which can mask minor MTU problems.

Restoring Default Settings

If the new MTU causes unexpected problems, revert to the defaults:

• Web UI: Set MTU back to 1500 on both bands and save.
• Telnet/SSH:

  uci set wireless.@wifi-iface[0].mtu='1500'
  uci set wireless.@wifi-iface[1].mtu='1500'
  uci commit wireless
  wifi reload
  

Always re‑run the ping tests to confirm.

Keeping Your Router Firmware Updated

The Mi 4C firmware occasionally includes bug fixes for MTU handling and dual‑band stability. To check for updates:

1. Open the admin page (http://192.168.31.1).
2. Go to System → Firmware Update.
3. Follow prompts to download and install any available updates.

A recent firmware version may already correct hidden MTU alignment bugs, reducing the need for manual tweaking.

Summary Checklist

• [ ] Run ping tests on both Wi‑Fi bands to confirm misalignment.
• [ ] Note the lowest packet size that works without fragmentation.
• [ ] Log in to the router and view current MTU settings.
• [ ] Adjust MTU to a standard value that matches your ISP/VPN requirements.
• [ ] Save changes and reboot/reload Wi‑Fi.
• [ ] Re‑run ping tests to ensure stability.
• [ ] Perform real‑world usage tests (streaming, gaming).
• [ ] Update firmware if a newer version is available.
• [ ] Document the final MTU values for future reference.

Final Thought

MTU misalignment is a quiet villain in home networking. By systematically testing, diagnosing, and adjusting MTU on the Xiaomi Mi Router 4C’s dual‑band interface, you can restore smooth connectivity for every device in your home. With the router properly tuned, your 5 GHz band will finally deliver the high‑speed performance you expect, while the 2.4 GHz band remains stable for legacy devices. Enjoy seamless streaming, lag‑free gaming, and reliable VPN sessions—all thanks to a few careful tweaks to an otherwise overlooked setting.