I faced with problem due to connect two cassandra clusters placed in different datacenters. Each datacenter have public ip – lets assume there are 111.111.111.111 and 222.222.222.222. Cassandra nodes starts in docker contaners – 3 nodes in each cluster. The problem is clusters don’t see each other.

Here are docker-compose files for first cluster on host with ip 111.111.111.111

version: '3.3'

services:
  cass1:
    image: cassandra
    container_name: cass1
    hostname: cass1
    healthcheck:
        test: ["CMD", "cqlsh", "-e", "describe keyspaces" ]
        interval: 5s
        timeout: 5s
        retries: 60
    networks:
      - cassandra
    ports:
      - "9042:9042"
      - "7000:7000"
    environment: &environment
        HEAP_NEWSIZE: 128M
        MAX_HEAP_SIZE: 2048M
        CASSANDRA_SEEDS: "cass1, cass2, 222.222.222.222”
        CASSANDRA_CLUSTER_NAME: ks
        CASSANDRA_DC: DC1
        CASSANDRA_RACK: West
        CASSANDRA_ENDPOINT_SNITCH: GossipingPropertyFileSnitch
        CASSANDRA_NUM_TOKENS: 256

  cass2:
    image: cassandra
    container_name: cass2
    hostname: cass2
    healthcheck:
        test: ["CMD", "cqlsh", "-e", "describe keyspaces" ]
        interval: 5s
        timeout: 5s
        retries: 60
    networks:
      - cassandra
    ports:
      - "9043:9042"
      - "7001:7000"
    environment: *environment
    depends_on:
      - cass1

  cass3:
    image: cassandra
    container_name: cass3
    hostname: cass3
    healthcheck:
        test: ["CMD", "cqlsh", "-e", "describe keyspaces" ]
        interval: 5s
        timeout: 5s
        retries: 60
    networks:
      - cassandra
    ports:
      - "9044:9042"
      - "7002:7000"
    environment: *environment
    depends_on:
      - cass2

networks:
  cassandra:

And the second cluster on host with ip 222.222.222.222

version: '3.3'

services:
  cass1:
    image: cassandra
    container_name: cass1
    hostname: cass1
    healthcheck:
        test: ["CMD", "cqlsh", "-e", "describe keyspaces" ]
        interval: 5s
        timeout: 5s
        retries: 60
    networks:
      - cassandra
    ports:
      - "9042:9042"
      - "7000:7000"
    environment: &environment
        HEAP_NEWSIZE: 128M
        MAX_HEAP_SIZE: 2048M
        CASSANDRA_SEEDS: "cass1, cass2, 111.111.111.111”
        CASSANDRA_CLUSTER_NAME: ks
        CASSANDRA_DC: DC2
        CASSANDRA_RACK: East
        CASSANDRA_ENDPOINT_SNITCH: GossipingPropertyFileSnitch
        CASSANDRA_NUM_TOKENS: 256

  cass2:
    image: cassandra
    container_name: cass2
    hostname: cass2
    healthcheck:
        test: ["CMD", "cqlsh", "-e", "describe keyspaces" ]
        interval: 5s
        timeout: 5s
        retries: 60
    networks:
      - cassandra
    ports:
      - "9043:9042"
      - "7001:7000"
    environment: *environment
    depends_on:
      - cass1

  cass3:
    image: cassandra
    container_name: cass3
    hostname: cass3
    healthcheck:
        test: ["CMD", "cqlsh", "-e", "describe keyspaces" ]
        interval: 5s
        timeout: 5s
        retries: 60
    networks:
      - cassandra
    ports:
      - "9044:9042"
      - "7002:7000"
    environment: *environment
    depends_on:
      - cass2

networks:
  cassandra:

After startup each cluster is seeing only itself and doesn’t see other cluster. For example nodetool status output for 111.111.111.111

Datacenter: DC1
===============
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
--  Address     Load        Tokens  Owns (effective)  Host ID                               Rack
UN  172.18.0.4  108.81 KiB  256     60.1%             3ceecd38-0c30-4189-9a52-cb23465ca0ca  West
UN  172.18.0.3  113.77 KiB  256     70.4%             f1f6f63a-7cc5-4883-8b90-5159c6b6613c  West
UN  172.18.0.2  108.74 KiB  256     69.5%             94395942-8efa-447b-9961-2d80375bce35  West

netcat show that another host is available

nc -v 222.222.222.222 7000
Connection to 222.222.222.222 7000 port [tcp/*]
succeeded!

No errors in cassandra logs.
Maybe is some missconfiguration happened? I tried to run 2 cluster on same host and it worked properly with intercommunication between clusters.
TY for answers.