Influence of vro MTU value on Network Communication (solve the problem that some websites cannot be opened)

Source: Internet
Author: User

About MTU
MTU (maximum transmission unit) is the maximum number of information packets transmitted by network devices. For various routers, the best MTU value is usually the default value. Sometimes, changing the MTU value can improve the performance of the device, which is easy to do, but in fact, this will often lead to other problems. It is best to keep MTU unchanged unless the following situations occur:
When the ISP is not connected or other Internet services are not available, and their technical support staff suggest changing the MTU Value
When Using VPN, you can consider changing MTU in case of performance problems.
To improve some network performance, applications that can optimize MTU value are used, which causes connectivity and other performance problems.
When an information packet is sent to a device with a smaller MTU value, it is divided into several small pieces. Theoretically, the MTU value on all computers, switches, routers, and all devices on the Internet that you can access should be set to the same size. However, you cannot control the MTU value on the Internet. In fact, the optimal MTU value in a LAN depends on hardware, software, and wireless interfaces.

In one case, modifying the MTU size can make the device work well, but it may cause performance and connectivity problems in other aspects.
When devices with different MTU values communicate with each other, the information package will be divided into multiple devices for transmission to devices with the smallest MTU value.
Windows XP automatically sets MTU. In other words, MTU optimizes the overall performance of various applications. Microsoft's article explains why broadband users using Windows XP cannot connect to the ISP
Once a network device breaks down an information package, this information package remains in the state of decomposition until it reaches its destination.
Optimal MTU value for various applications
Setting the MTU size is a process of repeated experiments: the MTU size is reduced from the maximum value of 1500 until the problem is solved. Using one of the following values may solve some problems caused by MTU values:
1500. the maximum value of the Ethernet information package is also the default value. Is a typical setting of network connections without PPPoE and VPN. Is the default setting for various routers, network adapters, and switches.
1492. Optimal PPPoE Value
1472. Use the maximum ping value (information packets greater than this value will be decomposed first)
1468. Optimal DHCP Value
1430. Optimal Values for VPN and PPTP

576. standard value for dialing to ISP
The following describes how to use the Ping command to test the local optimal MTU value in a broadband network.
The test logic is as follows: on the Wide Area Network, packets larger than the ISP's requirements cannot be transmitted, the router will automatically subcontract and transmit the data that does not meet the requirements according to the MTU value of the Local Machine. Therefore, we can send a fixed-length data packet to a host on the Internet, the router is not allowed to change its size to check whether the data packet can be transmitted normally. If the data packet is smaller than or equal to the ISP's MTU value, the data packet can be transmitted normally, if the sent data packet is greater than the MTU value used by the ISP, the transmission will fail, so that we can get the MTU size of the ISP end by continuously sending data packets of different sizes, this data plus 28 bytes of header information is the final MTU value we need.

The test procedure is as follows:
1. dial-up Internet access, and open the "Start" menu → "program → attachment → command prompt ".
2. Enter the command "ping-l 14xx-f". Here we will explain the meaning of the command parameters:
*-L 14xx parameter: sends a fixed-length data packet. 14xx indicates the package size to be tested. As mentioned above, the MTU of ADSL is generally between 1450 and 1492, this value will be changed every time in step 1. In the above formula,-l is L in lower case (not 1 woo ).
*-F parameter: notifies the router not to change the data packet size without authorization.

* Any host on the Internet can also use other host names or IP addresses.
3. Observe the feedback results and determine the MTU value of the ISP.
1) Send the test package for the first time. The packet length is 15001, indicating that the packet size is too large and subcontracting is required. Therefore, if the value (1500) is greater than the MTU of the ISP, the packet size should be reduced.
2) The second test packet sent, with a length of 1490
2. The packet size is reduced to 1490, indicating that the packet is too large. Therefore, the system determines that the value (1460) is greater than the MTU of the ISP and should continue to decrease.
3) Send the test package for the third time. The package length is 1450.
3. The packet size is reduced to 1450, prompting that the packet can be transmitted normally. Therefore, it is determined that the MTU value of the ISP is between 1450 and ~ In the range of 1460.
4) Send the test package for the fourth time. The package length is 1472.
4. The packet size is increased to 1472, prompting that the packet is too large. Therefore, the MTU value is determined to be between 1450 and ~ In the range of 1472.
4. Modify the mtu value of the vro.
The MTU value obtained above cannot be directly used for local settings. It should be added with 28 (28 bytes of data packet header). Therefore, the MTU size will be modified to 1480 on the WAN settings page.
Tested, by modifying the mtu value in the vro, you can successfully resolve the problem of "cannot open web pages on QQ", "cannot open some websites or links", and "cannot log on to MSN" After accessing the Internet through the vro. (set MTU 1480) and "Email attachments cannot be sent.
Disclaimer: some of the documents in this article are from the Internet and are for your reference only. The MTU value is set on tplink: (The following is reprinted from the TPLINK official website) why some websites cannot be opened? Because some websites can be opened, you can determine that the vro is working properly and the connection to the Internet is also normal. This situation is most likely caused by incorrect DNS server settings or automatic retrieval. For example, if the DNS server address provided by a local ISP is obtained, the domain name resolution of some websites cannot be successful, therefore, you cannot access these websites. Another possibility is MTU. Go to the vro Management page-network parameters-WAN port settings (PPPOE is in WAN port settings-Advanced Settings options, change MTU from 1500 (1492 in PPPOE mode) to 1400 to see how it works. If this problem occurs in ubuntu, you do not need to modify the sudo ifconfig ppp0 mtu 1400
If you are using a vro to access the Internet, try again.
Sudo ifconfig eth0 mtu 1400

Anyway, how can we modify the MTU of the local machine through 1400?
The modification method is as follows:
1. Run regedit
2. Open HKEY_LOCAL_MACHINE \ SYSTEM \ CurrentControlSet \ Services \ Tcpip \ Parameters \ Interfaces.
3. There are multiple sub-items under Interfaces, and each sub-item corresponds to one Nic. Select the NIC as follows:
A. Determine the NIC or IP address used by the local machine to connect to the Internet, for example,;
B. Click the subitem on Interfaces to view the IPAddress item in the key value list;
C. If the key value of IPAddress is the same as that of IP address A, that is,, this subitem is the network adapter to be found.

4. Enter this subitem, right-click in the window on the right, select "new"-> "DWORD Value", enter the name "MTU", and press Enter. Double-click "MTU" and the modification window will pop up. Fill in the MTU value (generally 1480 in decimal format ).
Set the base number to decimal. After the configuration, You need to restart the machine to make it take effect.
Windows 7 (XP, Vista not tested)
1. Run cmd with the administrator privilege
2. Run the netsh interface ipv4 show subinterfaces command to check the MTU and local connection name.
3. use netsh interface ipv4 set subinterface "connection name" mtu = 300 store = persistent
(Note: The connection name is the connection name corresponding to the MTU value you see in the above command. It is displayed on the right .)

Appendix: 1. This method does not need to be restarted. 2. If ipv6 is used, change ipv4 to ipv6.
In Linux, you can use the following command to modify the root permission.
Ifconfig Nic MTU Value
Ifconfig eth0 mtu 1460
MaxMTU is the largest unit for transmitting TCP/IP data. In TCP/IP, the data to be transmitted is divided into smaller groups for transmission. Each group is 576 bytes in size. In Windows, the default byte is 1500, which is the Ethernet grouping standard. The PPPoE used by ADSL is slightly smaller than this value, generally 1492. Some websites use MaxMTU greater than 1492. Therefore, some webpages may be inaccessible. Modifying the default MaxMTU in Windows can solve this problem. This problem may be encountered in the use of PPPoE software installed on PCs or PPPoE software built in Modem. If you use a vro, change MTU to manual in firewall configuration => Basic settings and set it to 1492.

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