MTU in GRE Tunnels

I wanted to understand why is it recommended to set

ip mtu 1400
 ip tcp adjust-mss 1360

Hello Justine

Take a look at this lesson. It answers your question with an example as well. If you have further questions after you review the lesson, let us know!

Strictly speaking, you will see that you don’t really need 1400 but something slightly larger (like 1420 when using GRE and IPSec in tunnel mode for example). But 1400 and subsequently 1360 for the TCP adjust-mss, are used simply because they cover the most common GRE + IPSec mode combinations. As Cisco states:

“There is no discernable downside to allowing for an extra 20 or 40 bytes overhead. It is easier to remember and set one value and this value covers almost all scenarios.”

I hope this has been helpful!