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?
The default MTU size for Ethernet interfaces is 1500 bytes. GRE tunneling adds an overhead of 24 bytes (4 bytes for the GRE header and 20 bytes for the outer IP header). Therefore, the maximum allowable MTU size for a default GRE tunnel without IPv4 traffic fragmentation is 1476 bytes (1500 bytes - 24 bytes).
The GRE header is 4 bytes + 20 bytes IP header
D is correct.
D is correct - GRE tunneling adds 24 bytes (4 bytes for GRE header and 20 bytes for outer IP header). Simple math 1500B of standard MTU - 24 B = 1476 B
Whenever we create tunnel interfaces, the GRE IP MTU is automatically configured 24 bytes less than the outbound physical interface MTU. Ethernet interfaces have an MTU value of 1500 bytes. Tunnel interfaces by default will have 1476 bytes MTU. 24 bytes less the physical. Why do we need tunnel MTU to be 24 bytes lower (or more) than interface MTU? Because GRE will add 4 bytes GRE header and another 20 bytes IP header. If your outbound physical interface is configured as ethernet, the frame size that will cross the wire is expected be 14 bytes more, 18 bytes if link is configured with 802.1q encapsulation. If the traffic source sends packet with 1476 bytes, GRE tunnel interface will add another 24 bytes as overhead before handing it down to the physical interface for transmission. Physical interface would see a total of 1500 bytes ready for transmission and will add L2 header (14 or 18 bytes for ethernet and 802.q respectively). This scenario would not lead to fragmentation. Life is good.