I have (2) RB750's tied back to back and I have a L2TP connection between the 2 and I also have a laptop hanging off each one. The laptops can ping every IP address in-between on each router, except each other. They should be able to ping each other but cant. . I suspect ARP related but cant be sure where to start. Wireshark on both laptops does show the ICMP data flowing so it doesn't seem to be a routing issue. Torch also shows the traffic passing back and forth.

Any ideas? I am tempted to try arp-proxy but not sure that will do it. I took both exports and compared them in notepad++ and there is no difference between the 2 that should prevent the end to end ping. So maybe there is something I am missing on both RB705's that is preventing the end to end pings.

I am working on a project that requires us to run L2TP through someone elses Layer 3 network . I have another set of RB750's that I do have working but its really not optimal or efficient and I cant access them remotely due to the tagging we are doing. Thats another subset of the project though, assigning bridges to each RB750 with a management IP in VLAN10. Here is the 1st one Here is the 2nd one

What about the laptops, do they like to get pinged from non-local subnets? If they have Windows, it's by default blocked in firewall.

I am tempted to try arp-proxy but not sure that will do it.

It won't - proxy-arp only helps if the remote end of a point-to-point tunnel has an IP address which fits into the own subnet of the LAN device, so the LAN device sends an ARP request to obtain its MAC address to send the packet directly rather than looking for a route. This is not the case here as the two laptops are in different subnets. So what @Sob suggests is the most likely reason. You can run /tool sniffer quick ip-protocol=icmp at both the 750s simultaneously and try pinging one laptop from the other to see how far the echo requests get through the network.

Wireshark on both laptops shows that the icmp packets are getting to them. So routing is ruled out. Torch on both RB's shows flow so the tunnel is working.

I may have to do a simple laptop to laptop test to rule out their internal firewalls. Even though they can ping remote addresses/interfaces....they themselves may not want to be pinged.