XDCR secure replication and external networking guide

There has been some confusion in recent days regarding XDCR when it comes to securing replication, and also the external networking situation. A few CBSEs have been raised where the lack of information on our doc pages lead to misconfiguration, and ultimately repeated efforts to help customers resolve their set ups.

Some customers have referred to the lack of the information on the doc pages as a reason to not follow Support or Engineering help. They stated that the lack of such information (or concepts) mean whatever resolution advice was offered was not necessary since they do not constitute "a pre-requisite requirement" of sorts.

There is an internal Google Doc that XDCR team has come up with to act as a concept guide and/or troubleshooting guide. It has been compiled with concepts that need to be clarified on our doc pages for customers, a few repeated occurrences that took place, as well as some ideas for resolution.

It would be great to translate these into official pages so that it can strengthen our product offering.