MTU size in Cisco IOS XR and IOS software

In the classical Cisco IOS software (not the Cisco IOS XR software), the interface mtu command configures the L2 payload size, but does not include the L2 header. This is different from the Cisco IOS XR software which includes both the L2 and L3 overhead in the interface mtu command.

The L3 MTU commands, as in the case of the ipv4 mtu command, configure the maximum packet size of that protocol which includes the L3 header. This is similar to the case of Cisco IOS XR software.

https://networklessons.com/ip-routing/pppoe-mtu-troubleshooting-cisco-ios

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