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Why int tunnel MTU is 1514?

khj1
Level 1
Level 1

Hello!

When i simple create interface tunnel, i can see interface's mtu is 1514.

Why not 1500? Why not 1500-GRE overhead? Why not 1500+GRE overhead?

How Cisco calculate it?

4 Replies 4

a.manosca
Level 4
Level 4

Hi, this is a good question. I tried looking for some documentation but

unable to find one. I hope somebody out there (preferably from Cisco)

could explain or give the calculations. But I think the 1514 bytes of MTU

was based on the assumption that the maximum MTU size allowed

(without the GRE header) is 1490 bytes.

r.sneekes
Level 1
Level 1

I had the same problem when you do a show int it says mtu 1514 but when you do a sh ip int

it will show a diffrent mtu size in mine case standard for a tunnel interface = 1476 . 1500 - gre overhead.

I don't know why its different from the sh int tunnel command . its a bit confusing. but the value with the sh ip int command changes when you change the mtu size. while on the tunnel it will still stay at 1514. So het value displayed by the sh ip int command should be considerd al the real MTU size for that interface

Hi

sh ip int you can see no mtu, but IP MTU

This is two different parameters (mtu and ip mtu) and commands.

Refer to IOS Command reference to know difference between this commands.

BR

Your right my bad. this counts for ip only.