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L2 MTU

With L2 MTU we have to remember from the name this is on the ethernet frame itself before IP is involved and unlike IP or L3 MTU which can fragment, L2 MTU does not and instead the packet/frame is dropped if the size is over the MTU limit. MTU check happens on egress and the default payload L2 MTU is 1500.

This L2 MTU is usually a system wide configuration and depending on the Cisco Catalyst Family you are working the commands are different. So in general if you configure a system wide MTU setting like for example system mtu command that applies to all physical ports.

Things to keep in mind: - If the system mtu value is set to 9000, all ports use that MTU - If an interface is configured with a MTU value of 5000 and the system MTU is set at 9000, the interface will use 5000 while other ports will use 9000.

Generic L2 Header

  • Destination MAC (6 bytes)
  • Source MAC (6 bytes)
  • EtherType (2 bytes)
  • Payload (1500 - 9216 bytes)
  • CRC (4 bytes)

Total 18 bytes (excluding payload)

L2 Trunk

If this is trunk port we have to add 4 more bytes for the dot1q vlan tag. - Destination MAC (6 bytes) - Source MAC (6 bytes) - 802.1Q Header (4 bytes) - EtherType (2 bytes) - Payload (1500 - 9216 bytes) - CRC (4 bytes)

Total 22 bytes (excluding payload)