Unable to connect to subscription URL from Subscribed Content Library of vCenter server | Error : "The remote library is not reachable"

Products

• VMware vCenter Server

Issue/Introduction

• Subscriber vCenter server is unable to connect to the Publisher URL.
• Following error received while accessing subscription URL: "The remote library is not reachable"
• Same error seen when trying to synchronize the subscribed library in Publisher vCenter.

Environment

VMware vCenter Server 8.0.x
VMware vCenter Server 7.0.x

Cause

• Subscriber VCenter is unable to ping/connect to the Publisher VCenter
• OR Publisher VCenter is not reachable over HTTPS port 443 from Subscriber VCenter

Resolution

• Test the connectivity between the Subscriber and Publisher vCenters server using Ping / Curl. 
• Test the DNS name resolution on both Subscriber and Publisher vCenters.
• Use the openssl command to test connectivity and also to confirm the current SSL information
  # openssl s_client -connect destination-ip:ssl-port

• Based on the results, engage Internal Networking team to investigate the underlying network communication issues between Publisher and Subscriber vCenter servers.