The maximum allowable MTU size for a default GRE tunnel without IPv4 traffic fragmentation is 1476
bytes1. This is because GRE packets are formed by the addition of the original packets and the
required GRE headers1. 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
problems1. 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 use1. 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-Bytes1.
