Configuring MTU on a DrayTek Vigor Router

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The MTU (Maximum Transmit Unit) setting on a DrayTek Vigor router controls the maximum packet size that can be sent through an interface and because DrayTek Vigor routers typically support multiple Internet connections, this can be specified per WAN interface.

Packets that are sent through the router that exceed the MTU value are fragmented into smaller packets so that they can be sent through the Internet / network that the router is connected to. It is beneficial to have larger packets where possible because this can result in lower overheads, allowing more data to be sent in the same time and giving increased throughput.

If the MTU specified on the router is larger than the packet size that the Internet connection is able to send, or perhaps the destination service is able to receive, then this can result in strange connection issues, such as some websites loading but not others, or being unable to load webpages but able to send pings to addresses on the Internet.

The default MTU setting on a DrayTek Vigor router will typically be 1442 but may vary depending on the firmware version that the router was originally configured with.

Many PPPoE and PPPoA connections will be capable of a maximum MTU of 1492, because of additional data required for routing information, with some newer connections allowing up to 1500.

Most Ethernet / Cable Internet connections will use an MTU of 1500.

The MTU value is specified for each WAN interface on the router; to change it, go to [WAN] > [Internet Access] and click on the Details Page button for that WAN interface:

In the WAN settings, the MTU setting will show in the lower-left portion of the WAN settings:

If this is not present, update the router to current firmware.

Change the MTU value to what your ISP or Internet connection requires then click OK to apply the change.

The router will then prompt to restart and the new MTU will be utilised once the Internet connection has reconnected.

Path MTU Discovery

DrayTek Vigor routers are able to auto detect the MTU that is ideal for the connection using the Path MTU Discovery feature; this has the router send packets to a specified Internet IP address until they begin to fragment and recommends the highest size that does not fragment the packet.

To detect the MTU, the Internet connection must be online and able to send traffic through that connection. Go into the WAN configuration as shown above and click the Detect button for Path MTU Discovery:

This will pop-up a browser window with the Path MTU detection facility. Specify an IP address (not a hostname) that the router will be able to ping, such as an ISP gateway address or DNS server.

The option to reduce MTU size will find the highest result and deduct the specified amount to account for packet overheads. This can be left on its default value of 8.

Click the Detect button to begin the detection and the router will gray out the options while it does this:

Once that has completed, if it has been able to ping that target successfully, it will show Success as the status along with the recommended MTU size. Click Accept and the router will change MTU setting for the WAN. Then click OK to apply the WAN setting change and restart the router to fully apply the MTU setting change.

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