Issue with NetScaler GSLB service. Effective state is showing as down.

To resolve this issue add vServer to GSLB service.

Problem Cause

Configuration issue.

Effective State is based on MEP which checks the health of the vServer. When adding a vServer to a GSLB Service, there is an option to add an Existing Server.
Existing Server lists any remote vServers that may have previously been added to a GSLB Service. It also lists any other servers that are listed under 'Servers' as a user may want to add a non-NetScaler server. For MEP to work, the 'existing server' needs to be a vServer of some sort. Also, the vServer needs to be in TD0.