You add a GRE tunnel to transport packets between two routers. After implementing this, you
notice an increase in the number of dropped packets. While looking at packet captures, you notice
that the do-not-fragment (DF) bit is set in the IP header of all the dropped packets.
What would cause this changed behavior?
A.
The GRE tunnel has a lower MTU than the physical interface.
B.
GRE tunnels do not support fragmentation.
C.
GRE tunnels do not support the DF bit.
D.
The GRE tunnel has a higher MTU than the physical interface.
I would say that the answer is D, but after some thinking… A is correct and there will be drops if we compare the Incoming Physical interface (LAN) over the Outgoing GRE Tunnel, not considering the Outgoing Physical interface over which the Tunnel is built. In that case, as the GRE Tunnel has lower MTU and the packets are coming somehow with DF bit set, there will be drops.
Now, if the Outgoing Physical interface over which the tunnel is built has Higher MTU that the GRE itself, that would be actually ok (it will make room for the extra 24 bytes of GRE encapsulation)
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