TCP MSS & IP MTU considerations when using DMVPN if you click Ethernet, you will see VLAN and QinQ header option checkboxes. 1. PDU value will be your MTU, whatever you encapslate into GRE must not exceed that size. However the tunnel needs to add the GRE header, in this case an additional 28 bytes. This tool allows you to easily see what each protocol adds to your packet. IPSec & MSS - VPN: Site to Site and Remote Access - Sophos MTU, MSS, JUMBO FRAMES explained - LinkedIn The basics - MTU, MSS, GRE, and PMTU - Das Blinken Lichten MTU is maximum IP packet size of a given link. Most platforms will either calculate the effective MTU of the tunnel based on the egress interface and tunnel overhead and/or do PMTUD on the tunnel path. PDF Configuring MTU and TCP MSS clamping - Michael McNamara MTU Test in a VPN Environment experiencing throughput issues. Some protocols have additional options, e.g. Everything else is pure header size exclusing any outer or inner protocols, e.g. MTU is used for fragmentation i.e packet larger than MTU is fragmented.But in case of MSS, packet larger than MSS is discarded. IPsec Overhead Calculator - Cisco MTU is used for fragmentation i.e packet larger than MTU is fragmented.But in case of MSS, packet larger than MSS is discarded. 8 -l 1472 -f Pings google DNS with an ICMP packet that has an MTU of 1500 with the don't fragment bit set. By default, the MSS value is defined as 1380 bytes in a NetScaler SD-WAN appliance, assuming the WAN infrastructure allows a standard IP packet maximum transmission unit (MTU) size of 1,500 bytes. Great - so now we have:- 28 Bytes - GRE 20 Bytes - IP The MSS value advertised at the 3-way handshake is based on the interface MTU (Maximum Transmission Unit), whereas the MSS value used in the command show system connection extensive reflects the current MSS value used.