Why minumum MTU of 2500 for bridged iSCSI ??

Hello,

I am working my way through the ISM book for E20-001 exam.

At the end of Chapter 8 the is a question about iSCSI:

"Why should an MTU value of at least 2500 bytes be configured in a bridged iSCSI environment?"

This one has got me stumped. Can anybody think of a possible reason why this might be the case?