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

 
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Hall of Fame Expert

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

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

 

2 REPLIES 2
Hall of Fame Expert

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

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

 

VIP Expert

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

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.
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