- GSLB service down or manually shutdown all servers that are being used in GSLB Service.
- Could see GSLB answering queries for Vserver that are marked down after above step. 

EDR should be configured to control this behavior.

Note: You must configure this setting for each virtual server to which you want it to apply.

To configure a GSLB virtual server for empty down responses by using the command line interface:

At the command prompt, type:

Example

> set gslb vserver vserver-GSLB-1 -EDR ENABLED 
 Done 

To set a GSLB virtual server for empty down responses by using the configuration utility

1. Navigate to Traffic Management > GSLB > Virtual Servers.
2. In the GSLB Virtual Servers pane, select the GSLB virtual server for which you want to configure a backup virtual server (for example, vserver-GSLB-1).
3. Click Open.
4. On the Advanced tab, under When this VServer is “Down,” select the Do not send any service’s IP address in response (EDR) check box.
5. Click OK.

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Problem Cause

This is by design/expected behavior if EDR is not configured. 

When the GSLB vserver is down, with all the corresponding gslb services in the down state, the DNS query response can have the IP addresses of the down GSLB services. This is by design/expected behavior.

However, you can configure the GSLB virtual server to send an empty down response (enable EDR on GSLB Vserver). When this option is set, a DNS response from a GSLB virtual server that is in a DOWN state does not contain IP address records, and this prevents clients from attempting to connect to GSLB sites that are down.


https://docs.citrix.com/en-us/netscaler/10-1/ns-tmg-wrapper-10-con/netscaler-gslb-gen-wrapper-10-con/ns-gslb-protct-setup-against-fail-con.html 

Configuring a GSLB Virtual Server to Respond with an Empty Address Record When DOWN

A DNS response can contain either the IP address of the requested domain or an answer stating that the IP address for the domain is not known by the DNS server, in which case the query is forwarded to another name server. These are the only possible responses to a DNS query.

When a GSLB virtual server is disabled or in a DOWN state, the response to a DNS query for the GSLB domain bound to that virtual server contains the IP addresses of all the services bound to the virtual server. However, you can configure the GSLB virtual server to in this case send an empty down response (EDR). When this option is set, a DNS response from a GSLB virtual server that is in a DOWN state does not contain IP address records, but the response code is successful. This prevents clients from attempting to connect to GSLB sites that are down.