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Containers in Swarm (Classic) network cannot communicate

We’re running Swarm (Classic) 1.2.9 on Docker 18.06.3-ce. Several containers cannot ping to another container. And we have a second symptom: when I inspect the overlay network on each node, all containers are displayed, but if I modify the DOCKER_HOST to point to Swarm (i.e., docker -H tcp://<swarm_ip:swarm_port> network inspect `, the containers do not show up. More precisely, a random set of containers shows up. I don’t see anything in the containers or the network that would indicate a problem. It seems to lie in Swarm’s networking utilities (which I know nothing about; what does Swarm do re. requests across the network that Docker does not?).

(We’re in a production environment with customer containers running; any help would be greatly appreciated.)

This gist shows the output difference between docker inspect and swarm inspect (it also includes a copy of the swarm script): https://gist.github.com/briandant/a03253f583bf6a6fb317077581f4613e

just out of curriosity: what advantages does Swarm classic provide that the build in Swarm mode does not?