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

What is the maximum allowable MTU size for a default GRE tunnel without IPv4 traffic fragmentation?
A . 1496 bytes
B . 1480 bytes
C . 1500 bytes
D . 1476 bytes

View Answer

Answer: D

Explanation:

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.