Default Ethernet MTU = 1500 bytes (excluding the 18 bytes for the Ethernet header and Frame Check Sequence, FCS) (so MTU=1500 results in 1518 bytes ethernet frame)

802.1q (dot1q) tag is 4 bytes =⇒ MTU=1504 bytes (1522 bytes frame)

• IEEE_802.1Q
• Cisco ISL

Note:

Difference MTU for layer-2 switching process, and MTU for layer-3 routing!

Internet Protocol subnetworks require that all hosts in a subnet have an identical MTU.

Super jumbo frames (SJFs) are generally considered to be Internet packets which have a payload in excess of the tacitly accepted jumbo frame size of 9000 bytes.

Baby giants: The baby giants feature allows a switch to pass or forward packets that are slightly larger than the IEEE Ethernet MTU. Otherwise, the switch declares big frames as oversize and discards them.

• Wikipedia MTU and Jumbo_Frame
• Cisco info on MTU (also MPLS):
  Jumbo/Giant Frame Support on Catalyst Switches Configuration Example
• Mailing List: Cisco: NSP mtu issues on 3750
• Gigabit Ethernet Jumbo Frames - and why you should care
• Raising the Internet MTU
• presentation on MTU: http://staff.psc.edu/mathis/papers/MTU200302/index.html