Maximum Transfer Unit - MTU

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

Cisco ISL


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:

Original source:

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