As your capture system does not have a GRE endpoint configured, the IP layer does not know what to do with these packets (but you see them anyway as they passed the point in the stack where the capture library is implemented). You can do three things:
- Make sure the capture system is a GRE endpoint so that the packets will be processed. But this is not needed for your purpose of just getting the packets to your capture system. It could be beneficial though to capture on the decapsulated side of the tunnel to see the traffic without the GRE tunneling.
- Create a filtering rule in the IP stack of your capturing system to not send out these ICMP packets back to the switch. If you are on Linux, an iptables rule can do this for you. This would be the cleanest solution.
- Do nothing and have the ICMP packets be sent, And to clean up your capture, you can then add a capture filter
ip proto 47 to only capture the GRE encapsulated packets.
