MTU - Interface MTU configuration considerations

When configuring the MTU of a Layer 2 interface, there are several things that must be considered. These are considerations that have been determined from both documentation and from practical experience.

• The MTU size is calculated differently on Cisco IOS and IOS-XR network operating systems.

• On physical hardware, the interface MTU is typically configured in two ways:

  • Globally using the system mtu command
  • Some platforms allow you to configure it on a per-port basis using the mtu interface command
  • In both cases, the minimum MTU that can be configured is 1500 bytes.
  • The default interface MTU is 1500 bytes.

• On physical hardware, the MTU setting will allow for an additional 18 bytes for the frame size on the wire beyond the configured MTU:

  • 14 bytes for the typical Ethernet frame
  • 4 byes for the VLAN Tag

• On an emulator, and in particular using CML with Cisco IOS Software, vios_l2 Software (vios_l2-ADVENTERPRISEK9-M), Experimental Version 15.2(20200924:215240) switch, the MTU can range from 64 to 9600 bytes.

• When using CML, the MTU setting will allow for an additional 24 bytes for the frame size on the wire beyond the configured MTU:

  • 14 bytes for the typical Ethernet frame
  • 4 bytes for the VLAN Tag
  • 6 additional bytes - there is no clear indication of why this is added

For all of the above, the FCS field is not counted at all.