FortiGate
FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
mribbans_FTNT
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).

 

 ------------------------------------- 

# 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

   next

  -------------------------------------

 

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):

 

 -------------------------------------

# fnsysctl ifconfig wan2

 

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

UP BROADCAST RUNNING ALLMULTI MULTICAST MTU:1500 Metric:1

 -------------------------------------

 

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).

 

Related articles:

 

PPPoE addressing mode on an interface:

https://docs.fortinet.com/document/fortigate/6.0.0/handbook/495735/pppoe-addressing-mode-on-an-inter...

 

How to adjust the MTU value on a FortiGate interface:

https://community.fortinet.com/t5/FortiGate/Technical-Note-How-to-adjust-the-Maximum-Transmission-Un...

 

MTU size and Jumbo frames support of Fortigate devices: https://community.fortinet.com/t5/FortiGate/Technical-Note-MTU-size-and-Jumbo-frames-support-on-Fort...