Understanding Metric Exchange Protocol and Monitors for Global Server Load Balancing

Summary

This document provides an overview of Global Server Load Balancing (GSLB), Metric Exchange Protocol (MEP), monitors, and their relationship in the load balancing decision.

Prerequisites

• An understanding of the Domain Name System (DNS) process is assumed.

• Before configuring GSLB you must have a properly working load balancing virtual server/service configuration.

MEP

MEP is a proprietary protocol used by the NetScaler systems to exchange site metrics, network metrics, and persistence information to other sites participating in GSLB. The communication process is accomplished between each GSLB site on TCP port 3011 and therefore must be open on firewalls that are between the NetScalers.

The metrics that are exchanged are the load balancing metrics that are configured on the load balancing virtual server. For example, if you have three GSLB sites and each geographic site has a load balancing virtual server and this load balancing virtual server is configured for a method of Least Connections, each geographic site will have its own Least Connection value that is based upon that site’s current connection. When a request is made, the GSLB site compares these metrics for that particular virtual server receiving the request for resolution and then resolves that request to the site that has the least connections.

Configuring

By default MEP that has a UP status exchanges the necessary data for GSLB. To use the defined load balancing method as defined on the load balancing virtual servers as a means to resolution, it is necessary to ENABLE MEP on all GSLB sites.

To enable the MEP use the following command:

set gslb site <sitename> [-metricExchange (ENABLED | DISABLED)]

When this protocol is enabled, load balancing metrics are exchanged between all GSLB sites. If at any point the MEP status becomes DOWN on any one site, the entire load balancing method as defined by the virtual server is disabled and falls back to a Round Robin load balancing method. Once all the MEP statuses are UP, it starts to use the defined load balancing metrics. When configuring explicit monitors for a remote site, MEP no longer exchanges this data.

Monitors

Monitors used in GLSB are typically the same as any other monitoring configured for a load balancing virtual server. A ping monitor can be setup to a virtual server of a remote site to determine if it is reachable or an HTTP monitor can be used to monitor a specific page on a remote load balancing virtual server.

Note: No round trip times or metrics from the monitors are used to determine GSLB resolutions.

Configuring

Typically, there is no need to use monitors for LOCAL site load balancing virtual servers. GSLB is notified of a load balancing virtual server DOWN state for its own virtual servers and services.

To bind a monitor to a GSLB service use the following command:

bind monitor http <gslbservicename> -state ENABLED

It is also mandatory that a service type of HTTP be monitored by an HTTP monitor and a service type of SSL be monitored by an HTTPS monitor.

The following is a table that represents that interaction between MEP and monitors:

More Information

For additional background or reference information, consult the NetScaler Administrator’s Guide.

Reference Articles:

CTX110348 - Configuring Global Server Load Balancing