LTM vs GTM

===Global Traffic Manager===
 GTM load balances data centers.  It returns a IP   for client to connect to which is usually the LTM (VIP) . The LTM than load balances between servers .  For GTM to work properly, customers delegate DNS resolution to GTM. Clients send a DNS Lookup

-The GTM is an intelligent name resolver, intelligently resolving names to IP addresses.
-Traffic does not “route” through the GTM
-Tells best IP to route to based on metrics for the URL (WIP - Wide IP ) in question . WIP Common URL 

===Local Traffic Manager ===
LTM load balances servers and also does caching, compression, persistence, etc. 

- LTM doesn’t do any name resolution
-When traffic is directed to the LTM traffic flows directly through its’ full proxy architecture to the servers it’s load balancing.

Example
In a typical deployment, a customer will have 2 or more data centers, each with GTM and LTM devices. For GTM to work properly, customers delegate DNS resolution to GTM. 

So, a user could type into their browser "www.example.com." GTM would see the DNS request and see which data center was performing best. If it was data center 1, GTM would return the IP address from the LTM in data center 1. Then, the user would connect to that IP and LTM would load balance the user's request to a server.


1. Users Requests sent to common URL www.example.com (WIP)
2. Request goes to GTM acting as the DNS resolution (authoritative DNS) 
3.  GTM comes back with IP (or VIP) of  LTMs in one of the Datacenters 
4. The Traffic is forwarded to the LTM which load balances between the servers 


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.