What is the maximum allowable MTU size for a default GRE tunnel without IPv4 traffic...

The maximum allowable MTU size for a default GRE tunnel without IPv4 traffic fragmentation is 1476 bytes 1 .  This is because GRE packets are formed by the addition of the original packets and the required GRE headers 1 .  These headers are 24-bytes in length and since these headers are added to the original frame, depending on the original size of the packet we may run into IP MTU problems 1 .  The most common IP MTU is 1500-bytes in length (Ethernet) 1 .  When the tunnel is created, it deducts the 24-bytes it needs to encapsulate the passenger protocols and that is the IP MTU it will use 1 .  For example, if we are forming a tunnel over FastEthernet (IP MTU 1500) the IOS calculates the IP MTU on the tunnel as: 1500-bytes from Ethernet - 24-bytes for the GRE encapsulation = 1476-Bytes 1 .