While designing Data Center and Disaster recovery setups, network administrators and designers come across terms – GTMand LTM. Let’s understand what these terms are, their function and how they differ.

LTM load balances servers and also does caching, compression, persistence, etc. LTM is a full reverse proxy, handling connections from clients. The F5 LTM uses Virtual Services (VSs) and Virtual IPs (VIPs) to configure a load balancing setup for a service. LTMs can handle load balancing in two ways, first way is an nPathconfiguration, and second is a Secure Network Address Translation (SNAT) method. LTM load balances servers and also does caching, compression, persistence, etc.

GTM load balances traffic for application servers across Data Centers .GTM is used as an “Intelligent DNS” server, handling DNS resolutions based on intelligent monitors and F5’s own iQuery protocol used to communicate with other BIGIP F5 devices. It is used in multiple data center infrastructures, deciding where to resolve requesting traffic to.


Communication between GTM and LTM:
The gtmd agent on BIG-IP Global Traffic Manager (GTM) uses the iQuery protocol to communicate with the local big3d agent, and the big3d agents installed on other BIG-IP systems. The gtmd agent monitors both the availability of the BIG-IP systems, and the integrity of the network paths between the systems that host a domain and the local DNS servers that attempt to connect to that domain.


The GTM and LTM can work together or they can be totally independent. If your organization owns both modules it’s usually using them together, and that’s where the real power comes in. They do this via a proprietary protocol called iQuery. iQuery, functioning on TCP port 4353, reports VIP availability / performance back to the GTMs. The GTMs can then dynamically resolve VIPs that live on an LTM(s).

When a GTM has LTMs as servers in its’ configuration there is no need to monitor the actual VIP(s) with application monitors, as the LTM is doing that & iQuery reports the information back to the GTM.

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