Technical Tip: MTU size on a Physical interface is displayed differently to the explicit MTU in PPPoE mode

markdr_FTNT · ‎06-28-2022

Description

This article describes why an Interface set in PPPoE mode will display a different MTU size to the explicitly set MTU.

Scope

FortiGate.

Solution

An MTU can be explicitly set on an interface (as shown below), however the displayed MTU size may be different to what was actually configured.

Using the following configuration example, the MTU (Maximum Transmission Unit) on this interface is set to 1492, but notice also that the mode is set to PPPoE(Point-to-Point Protocol over Ethernet).

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# config system interface

edit "wan2"

set vdom "root

set mode pppoe

set type physical

set role wan

set snmp-index 2

set mtu-override enable

set mtu 1492

Despite the MTU being set to 1492, when viewing the interface using ifconfig, the MTU size displays as 1500 bytes (8 bytes more than was explicitly set):

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# fnsysctl ifconfig wan2

wan2 Link encap:Ethernet HWaddr E8:1C:BA:76:60:AE

UP BROADCAST RUNNING ALLMULTI MULTICAST MTU:1500 Metric:1

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The reason for the discrepancy is due PPPoE’s encapsulation requirements.

When configured for PPPoE, the interface will use an extra 2 bytes for the PPP protocol plus an extra 6 bytes for PPPoE giving a total overhead of 8 additional bytes.

This effectively restricts the maximum size to 1492 bytes (1500 as the maximum frame size, less 8 bytes overhead for PPPoE).