L2TP MTU
Overview
The payload size (MTU) of traffic going through the L2TP tunnel will be lower than that of the underlying ISP - this is due to the extra 'headers' that are applied to make the tunnel.
Some L2TP clients may auto discover the MTU, but you may want to specify it in your L2TP client configuration so as to obtain the optimum size.
You'd first want to find out what the maximum MTU is of your underlying ISP. you can do this with some pings, see our MTU Page. Once you know your MTU, you can calculate what the L2TP MTU should be.
- Generally, the L2TP MTU would be 38 bytes less than the MTU you have on the underlying internet connection.
Rough Guide
| ISP's Type | ISP MTU | L2TP MTU |
|---|---|---|
| Most broadband providers | 1492 | 1454 |
| Broadband using Ethernet or baby jumbo frames RFC4638 | 1500 | 1462 |
| Mobile or other | lower than 1492 | Lower than 1454 |
Notes
- To find out the MTU of your underlying internet connection, see our MTU Page
- In theory fragments will work to allow 1500 MTU on our service, but fragments are inefficient, and if everyone sends fragmented packets that could degrade the service.
Technical Breakdown:
Here's a breakdown of the headers and the bytes used. This is using a standard PPP broadband connection where traffic through the ISP is 1492
Ethernet 1500 1500 802.11Q* - 4* 1500 * Treated as part of the ethernet header and doesn't affect the MTU PPPoE - 6 1494 PPP - 2 1492 <-- The MTU of the underlying internet connection IPv4 - 20 1472 UDP - 8 1464 L2TP - 6 1458 (HDLC) - 2 1456 PPP - 2 1454 <-- The MTU of the L2TP service