max ethernet frame size

[sarahr202]

hi every body!

I read in cisco book that maximum size for ethernet frame is 1518 byte.

My question is why? it should be larger than 1518 because:

(assuming mtu=1500)

ethernet(dix)header=26 byte.

IEEE ethernet(802.3) header= upto 28 bytes

IEEE 802.3 with snap= upto 35 bytes.

If we use any ethernet standard with mtu=1500, it would be above 1518 bytes.

Any idea?

thanks alot!

have a nice day!

[padramas · Accepted solution]

Hello Sarah,

The Ethernet frame size will remain the same i.e. 1518 unless they are configured( ex vlan tagging) to understand frames gt > 1518

You cannot mix match different ethernet headers.Generally,when use different ethernet headers, MTU for the ethernet layer reduces below 1500.

Support for frame size gt>1518 are highly vendor/implementation specific.

Good info on different types of Ethernet frame formats

http://www.firewall.cx/ethernet-frames-intro.php

HTH

Padmanabhan