Addressing MTU Mismatch on Huawei B618 LTE Router

Understanding MTU on the Huawei B618

The Maximum Transmission Unit, or MTU, is the largest size of a data packet that can travel across a network link without needing to be broken into smaller pieces. When the MTU is set incorrectly, packets may be fragmented or dropped, leading to slower speeds, connection instability, and a poor overall user experience. For users who rely on the Huawei B618 LTE router as the central hub for their home office, even a minor MTU mismatch can disrupt video calls, cloud backups, or simple web browsing.

What is an MTU mismatch?

A mismatch occurs when the MTU value configured on the router differs from the MTU required by a downstream device or the upstream Internet Service Provider. For example, the B618 may be set to the default 1500 bytes, while the ISP’s network path requires 1492 bytes due to PPPoE encapsulation. If a packet larger than 1492 bytes is sent, it will be dropped or fragmented, causing noticeable latency spikes.

Why it matters for home office users

• Video conferencing – Small packet losses can trigger jitter buffers that degrade audio and video quality.
• Cloud sync – Large file uploads may stall or time out if packets exceed the MTU.
• VPN tunnels – Many VPN protocols add overhead; an MTU that is too high will cause packets to exceed the underlying link.
• Remote desktop – High latency packets can make screen updates feel sluggish.

By addressing an MTU mismatch early, users can enjoy a stable and efficient network connection.

Assessing the current MTU

1. Log into the B618’s web interface
   Open a browser and type 192.168.8.1 or the router’s IP address. Enter the admin username and password. The default is often “admin”/“admin” or “admin”/“password”.

2. Navigate to the LAN or Wireless settings
   The MTU setting is typically located under the LAN configuration tab.

3. Note the current MTU value
   The router usually displays “MTU” or “Maximum Transmission Unit” with a numeric value such as 1500.

4. Check the upstream link
   Use a command line tool to identify the path MTU to a reliable server, such as Google’s public DNS at 8.8.8.8.

   ping -f -l 1472 8.8.8.8
   

   If the ping reports “Packet needs to be fragmented but DF set” at 1472 bytes, the path MTU is 1472.

5. Compare the two values
   If the B618’s MTU is higher than the path MTU, you have a mismatch.

Identifying the root cause

• PPPoE encapsulation – Many ISP connections use PPPoE, which adds 8 bytes of overhead. If the ISP uses PPPoE, the path MTU is 1492 instead of 1500.
• VPN or tunneling – If the home office uses a VPN, the effective MTU decreases further. A common rule of thumb is to subtract 50 bytes for common VPN protocols.
• Network equipment – Older switches or firewalls might have a hard‑coded MTU.

Adjusting the MTU on the Huawei B618

The B618 offers several ways to modify the MTU: via the web interface, via the command line (SSH), or by editing the WAN interface configuration. Below are the safest methods.

Method 1 – Web interface

1. Access the WAN settings
   In the web GUI, select the WAN or Internet tab.

2. Locate the MTU field
   It may be labeled “MTU”, “Packet Size”, or “Maximum Packet Size”.

3. Enter the new MTU
   Input a value that is one or two bytes lower than the path MTU you discovered earlier. If the path MTU is 1492, set the router to 1492 or 1490 to provide a buffer.

4. Save and reboot
   Confirm changes and allow the router to reboot.

Method 2 – SSH and the command line

1. Enable SSH on the router
   In the web GUI, go to System > Administration > SSH Access. Turn it on and note the port (default 22).

2. Connect from a terminal

   ssh admin@192.168.8.1
   

   If prompted, accept the key and enter the password.

3. Enter privileged mode

   enable
   

   and provide the enable password if set.

4. Navigate to the WAN interface

   configure terminal
   interface wan
   

5. Set the MTU

   mtu 1492
   

6. Exit and save

   exit
   write memory
   

   Reboot the router to apply changes.

Method 3 – Using the WAN interface configuration file

Some users prefer to edit the config file directly. This method is more advanced and carries risk if the file is malformed.

1. Backup the current configuration

   copy running-config startup-config
   

2. Open the config file

   configure terminal
   interface wan
   

3. Modify the MTU line
   Search for “mtu” and change the value.
4. Save and exit

   end
   write memory
   

5. Reboot

   reload
   

Testing after adjustment

Once the MTU has been altered, confirm that the mismatch has been resolved:

1. Ping test
   Use ping with the packet size just below the new MTU value:

   ping -f -l 1480 8.8.8.8
   

   The ping should succeed without fragmentation.

2. Speed test
   Run a throughput test to verify that upload and download speeds are stable and within expected ranges.

3. Check application performance
   Re‑open video calls, resume cloud sync, and confirm that latency and packet loss have diminished.

Common pitfalls and troubleshooting

• MTU too low – Setting the MTU far below the path MTU can unnecessarily fragment packets. Keep the value within a few bytes of the path MTU.
• ISP constraints – Some ISPs enforce a fixed MTU and may reset your router to their default if it diverges. If you encounter persistent resets, contact the ISP to confirm the correct MTU.
• Dual WAN or failover – If the B618 is configured for dual WAN or failover, adjust the MTU on each WAN interface separately.
• Firmware updates – Occasionally, a firmware update can change default settings or introduce bugs. After updating firmware, re‑check the MTU.

Advanced configuration: Path MTU Discovery (PMTUD)

The B618 can be configured to enable Path MTU Discovery, which allows the router to learn the lowest MTU along the path automatically. This feature is usually on by default, but can be verified:

1. Check PMTUD status
   In the web GUI, look for “Path MTU Discovery” or “PMTUD” under WAN settings.

2. Enable if disabled
   Toggle the setting to “Enabled”.

When PMTUD is active, the router will send ICMP “Fragmentation Needed” messages to discover the optimal MTU. This reduces manual tuning and adapts to changes in the network path.

Impact on VPN and VPN tunneling

If the home office uses a VPN, the MTU on the B618 should account for additional overhead. A typical VPN overhead is around 40-50 bytes. Therefore, if the ISP requires 1492 bytes, set the router’s MTU to 1440 or lower:

• VPN only – MTU = 1492 – 50 = 1442 (round down to 1440).
• VPN + PPPoE – MTU = 1492 – 8 – 50 = 1434 (round down to 1432).

The VPN client may also allow its own MTU setting. Coordinate the two values to avoid fragmentation on either side.

Monitoring long‑term performance

Once the MTU mismatch is resolved, users should keep an eye on network performance:

• Regular speed tests – A weekly or monthly test will catch any regressions.
• Log packet loss – Many routers log ICMP fragmentation messages; review logs for any new errors.
• Router firmware – Stay updated with the latest firmware from Huawei, as patches may improve MTU handling.

If a new ISP plan or device is added to the network, re‑evaluate the MTU again.

Final checklist for a stable MTU setup

• Confirm the ISP’s path MTU using ping or traceroute.
• Adjust the B618 MTU to match or slightly below the path MTU.
• Enable Path MTU Discovery if available.
• Account for VPN overhead by reducing the MTU further.
• Save settings and reboot the router.
• Verify by ping, speed test, and application usage.
• Monitor logs for fragmentation warnings.

By following these steps, home office users can eliminate MTU mismatches, reduce latency, and enjoy a more reliable and faster network on their Huawei B618 router.

The diagram above illustrates the typical placement of the MTU setting in the B618’s web interface, helping users locate the correct field quickly.

Understanding the flow of packets through the network and where fragmentation can occur is essential for diagnosing MTU issues.

With a proper MTU configuration, the Huawei B618 becomes a robust foundation for any home office setup, ensuring that remote work, video conferencing, and cloud-based workflows run smoothly.