MTU vs MSS

[martinbuffleo]

I have been reading up on DMVPN and noticed the tunnel configuration had the following:

iinterface Tunnel0

 ip mtu 1408

 ip tcp adjust-mss 574

 
 Would someone be able to explain to me why the mss is so much lower than the MTU.

I thought the MSS was 28 less than the MTU.
 
 

[Marcin Latosiewicz]

MSS is 40 bytes (IP and TCP header) less than IP MTU.

Makes little sense to lower the MSS so low, PMTUD would _typically_ be a better mechanism.

[martinbuffleo]

https://www.cisco.com/en/US/docs/solutions/Enterprise/WAN_and_MAN/VPNLoad/ch8_Dual.pdf

The MSS is calculated in the V3PN QoS Service Policy -> DMVPN (GRE Transport Mode)

But I still dont get it.

[Marcin Latosiewicz]

From same doc, I think this is valid

"The goal is to select an optimum value for ip tcp adjust-mss that minimizes both the IPSec padding and

ATM adaption layer (AAL) 5 padding."

Is that your objective in live network?

For the rest it's pretty self explanatory.

IP MTU of transport network > IP MTU overlay network > TCP MSS set on overlay