The data centers in a GSLB setup exchange metrics with each other through the metrics exchange protocol (MEP), which is a proprietary protocol for Citrix NetScaler appliance. The exchange of the metric information begins when you create a GSLB site. These metrics comprise load, network, and persistence information.
MEP is required for health checking of data centers to ensure their availability. A connection for exchanging network metric (round-trip time) can be initiated by either of the data centers involved in the exchange, but a connection for exchanging site metrics is always initiated by the data center with the lower IP address. By default, the data center uses a subnet IP address (SNIP) or a mapped IP address (MIP) to establish a connection to the IP address of a different data center. However, you can configure a specific SNIP, MIP, or virtual IP (VIP) address, or the NetScaler IP (NSIP) address, as the source IP address for metrics exchange. The communication process between GSLB sites uses TCP port 3011 or 3009, so this port must be open on firewalls that are between the NetScaler appliances.
Note: You cannot configure a GSLB site IP address as the source IP address for site metrics exchange.
If the source and target sites (the site that initiates a MEP connection and the site that receives the connection request, respectively) have both private and public IP addresses configured, the sites exchange MEP information by using the public IP addresses.
You can also bind monitors to check the health of remote services as described in "Monitoring GSLB Services." When monitors are bound, metric exchange does not control the state of the remote service. If a monitor is bound to a remote service and metric exchange is enabled, the monitor controls the health status. Binding the monitors to the remote service enables the NetScaler appilance to interact with a non-NetScaler load balancing device. The NetScaler can monitor non-NetScaler devices but cannot perform load balancing on them unless monitors are bound to all GSLB services and only static load balancing methods (such as the round robin, static proximity, or hash-based methods) are used.
With NetScaler release 11.1.51.x or later, to avoid unnecessary disruption of services, you can set a time delay for marking GSLB services as DOWN when a MEP connection goes DOWN.
This document includes the following information: