What is the difference between Load Balancer and GTM; which one should I use?

Short answer

Load Balancer - ADC works within a data center: after the connection is established, it distributes the request to one of the healthy servers. GTM, on the other hand, works between data centers: before the connection is established, at the moment of the DNS query, it decides which data center's address to give the client. For distribution between servers in a single data center, Load Balancer is sufficient; if more than one data center, geographic routing or Disaster Recovery is required, GTM is used. The two are not alternatives to each other but complementary.

How Does It Work?

• Load Balancer: The client connects to the address of the vService. TR7 receives the request and forwards it, according to the algorithm, to a healthy Backend Service in the data center. The traffic flows through TR7.
• GTM: TR7 becomes the authoritative DNS server for the domain name. When the client queries the domain name, GTM returns the suitable address according to the health of the targets, the location of the client and the data center metrics. The client connects to this address directly; the application traffic does not pass through GTM.
• Failover speed: In GTM, switching from one data center to another depends on DNS caches. The default TTL of the records is 3600 seconds; for fast failover, 60–300 seconds is recommended.

Comparison

	Load Balancer - ADC	GTM
Scope	Within a data center	Between data centers / geographic
Layer	Connection and application (L4–L7)	DNS
Decision moment	After the connection is established	Before the connection is established, at the DNS query
Target	Load distribution between servers	Routing between data centers and regions
Does the traffic pass through TR7?	Yes	No; only the DNS response is given

Which One Should I Use?

Your situation	Product
Single data center; I want to distribute the traffic between servers, terminate TLS and route by content	Load Balancer - ADC
I have two data centers; if one goes down, the users should go to the other	GTM, in Disaster Recovery mode (+ Load Balancer in each data center)
I want to send users to the nearest data center according to their location	GTM
I want to select data centers according to load and resource status	GTM, in Datacenter mode

In TR7

• Separate products: Load Balancer - ADC (TR7 ADC) and GTM (TR7 GTM) are licensed separately.
• GTM operating modes: One of the Standalone, Disaster Recovery and Datacenter modes is selected per domain name. In Datacenter mode, the status and metrics of the vServices in the data centers also become decision inputs.

Common Misconceptions

• "GTM distributes the traffic through itself." GTM only determines the DNS response; the client connects directly to the returned address.
• "In GTM, failover happens instantly." Clients and DNS servers keep the old response in the cache for the TTL duration; the failover time depends on the TTL.
• "GTM is also required in a single data center." For distribution between servers in a single data center, Load Balancer is sufficient.

How it is done in the documentation

• Load Balancer - ADC · Difference from GTM
• GTM · Its Difference From Local Load Balancing
• GTM · Operating Modes
• GTM · Use Cases

Related questions

• Can I use Load Balancer and GTM together?
• For redundancy across data centers, should I use Load Balancer or GTM?
• What is a load balancer and what does it do?
• What does the TR7 Load Balancer - ADC module cover, and what does it not cover?