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:

  • 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.

Links

https://forum.networklessons.com/t/mtu-troubleshooting-on-cisco-ios/1078/228?u=lagapidis

https://forum.networklessons.com/t/mtu-troubleshooting-on-cisco-ios/1078/226?u=lagapidis

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750x_3560x/software/release/15-2_4_e/configurationguide/b_1524e_consolidated_3750x_3560x_cg/b_1524e_consolidated_3750x_3560x_cg_chapter_010000.pdf

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9400/software/release/17-1/configuration_guide/int_hw/b_171_int_and_hw_9400_cg/configuring_per_port_mtu.pdf