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.