If MTU SIZE IS SET TO 1500, IS THERE ANY CHANCE THE FRAME WILL CROSS THE LIMIT? IF YES THEN HOW.

[Giuseppe Larosa · Accepted solution]

Hello,

on Cisco devices running IOS or IOS XE mtu value is actually the size of the L3 payload carried within the frame.

the 14 bytes ethernet header and 4 final bytes of FCS are not taken in account

So actually max frame size is 1518 bytes

When using 802.1Q tagging additional 4 bytes are added

802.1Q tagged frames of total size 1522 size should be allowed to pass.

Different for IOS XR where MTU is L2 and it skips only  the 4 bytes FCS. In this case default mtu size is 1514 bytes and increases to 1518 bytes for 802.1Q tagged frames or subinterfaces.

Hope to help

Giuseppe

[Joseph W. Doherty]

In addition to the information Giuseppe provides, I understand some/many Cisco switches will accept frames larger than their defined MTU but will not transmit a frame larger than an interface supports.