title: Two Cassandra DCs become Unreachable in nodetool describecluster gradually
body: <p>I set up a Cassandra cluster of two DCs, one in East US and one West US. There is not VPN/gateway. Every time I restart the whole cluster, nodetool describecluster reports everything normal, and a long read with consistent level ALL works fine.</p>
  
  <p>However, after a few minutes, the "describeculster" on each node shows a UNREACHABLE: [xxx.xxx, ...], the unreachable set increase gradually until the two DCs are not reachable to each other. and the read request fail on ReadTimeoutException.</p>
  
  <p>But any run of "nodetool status" on each node reports all nodes Up &amp; normal. And ssh to the node and ping the other DC works fine, too.</p>
  
  <p>Increasing read_request_timeout_in_ms, tuning GC does not solve the problem.</p>
  
  <p>Any ideas why this can happen?</p>
  
  <p>casandra.yaml:
  listen_address: {{private_ip}}
  rpc_address: 0.0.0.0
  broadcast_rpc_address: {{public_ip}}
  broadcast_address: {{public_ip}}</p>