MTU - Interface vs Tunnel Transport vs IP

[vv0bbLeS]

Hello all,

Trying to make sure I understand the different MTU settings on a Tunnel interface. Say I have the following output from a 

show interface Tunnel1

MTU 17916 bytes

Tunnel transport MTU 1476 bytes

And say I have the following config set in 

show run int Tunnel1:

ip mtu 1360

My understanding of these 3 MTU values is:

• The 17916 bytes is the physical interface's theoretical max MTU size that it *could* send (maybe this refers to the buffer size of the physical interface, or maybe it is calculated in some other way?)
• The 1476 is the Tunnel Interface's max MTU size that it *could* send (the 1500 - 24 byte GRE header in this case).
• The 1360 is the *actual* max MTU size that the Tunnel interface will send on the wire.

Is this a correct understanding? And if so, I'm guessing the 

ip mtu 1360

command would only work on a routed interface, i.e. it would not work on an interface that supports Layer 2?

[Joseph W. Doherty · Accepted solution]

"The 17916 bytes is the physical interface's theoretical max MTU size that it *could* send (maybe this refers to the buffer size of the physical interface, or maybe it is calculated in some other way?)"

Tunnels aren't physical interfaces.  It's possible that value was chosen as a maximum to support 16 Mbps token ring.

"The 1476 is the Tunnel Interface's max MTU size that it *could* send (the 1500 - 24 byte GRE header in this case)."

Correct.

"The 1360 is the *actual* max MTU size that the Tunnel interface will send on the wire."

Correct, but it's actually a logical L3 cap, where MTU is a physical L2 cap.  That's why it's labeled

IP MTU

and not just "MTU".

"And if so, I'm guessing the 

ip mtu 1360

command would only work on a routed interface, i.e. it would not work on an interface that supports Layer 2?"

Believe that's the case.  A L2 interface has a MTU.  Too large frames are just dropped, no fragmentation option, no notification to sender.  (In theory, physical interfaces on a shared medium should all support the same MTU.)

[vv0bbLeS]

Joseph,

Excellent, thanks again for the information! And yes I was curious where that 17916 came from - the 16Mbps token ring sounds very reasonable.