What is the recommended MTU value for a GRE tunnel to accommodate most IP packets?

D

When packets traverse a GRE tunnel, they may need to be fragmented if their size exceeds the MTU (Maximum Transmission Unit) of the tunnel. This can cause performance degradation, increased latency, and packet loss. To avoid this problem, it is recommended to set the MTU of the tunnel to a value that can accommodate most IP packets without fragmentation.

The MTU of a GRE tunnel is calculated as follows:

MTU = Path MTU - (IP header + GRE header)

The Path MTU is the MTU of the underlying network path, which is the minimum of the MTUs of all links between the tunnel endpoints. The IP header is 20 bytes, and the GRE header is 24 bytes.

Therefore, the recommended MTU for a GRE tunnel is:

MTU = min(MTUs of all links) - 44

In practice, the MTU of most Ethernet networks is 1500 bytes. However, some links may have a lower MTU due to factors such as MPLS encapsulation or PPPoE overhead. As a result, the actual Path MTU may be less than 1500.

To accommodate most IP packets without fragmentation, we need to set the MTU of the GRE tunnel to a value that is lower than the Path MTU but high enough to avoid fragmentation as much as possible. The recommended value is 1476 bytes, which is calculated as follows:

MTU = min(1500, MTUs of all links) - 44 = 1500 - 44 = 1456

However, this value may not be sufficient for some scenarios, such as when the network path includes tunnels nested within tunnels or when the application generates large packets. In such cases, a lower MTU value may be necessary.

Option A (1476) is therefore the recommended value to prevent fragmentation issues across a GRE tunnel to accommodate most IP packets.