Maximum Transfer Unit - MTU
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)
Protocol MTU frame-size
Ethernet 1500 1518
IEEE 802.1q VLAN-trunk 1504 1522
Cisco ISL VLAN-trunk 1530 1548
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
Original source: http://john.de-graaff.net/wiki/doku.php/index/mtu
