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F5 LTM Top 20+ Interview Questions and Answers

F5 lTM Interview Question and Answers

F5 LTM has been placed in the Leader’s Quadrant for 4-5 years straight by Gartner, Inc. This indicates that you will be marking a successful career by choosing F5 LTM Networks. 

Becoming an F5 LTM Engineer is not easy, yet there are some things that need your proper attention. In this blog, I have combined all the top most-asked F5 LTM interview questions and answers.  

It will help you to boost up your interview preparation. You can keep this F5 LTM questions cheat sheet with you for future reference too. However, I do not recommend you to keep this blog as the only source of your interview preparation. 

Without any further ado, we are going to start off with the list of interview questions. These F5 LTM interview questions will help you to make your concepts stronger. 


Let’s begin! 

F5 LTM Interview Questions and Answers

1. What are iRules in F5 LTM? 

An iRule is a powerful and adaptable feature with BIG-IP Local Traffic Manager. It can be used to manage your network traffic.  

Using syntax, the iRules feature does not only let you select pools based on header data but it allows you to direct traffic. It does so by searching for any type of defined content data. 

Therefore, iRules allows you to customize your content according to your own needs. 

In other words, iRules lets you specify the direction of your traffic to a particular destination. 

With the help of iRules, you can send your traffic to the following: 

  • Pools 
  • Individual pool members 
  • Ports 
  • URIs 


   if {[IP::addr [IP::client_addr] equals]} { 

   pool my_pool 



The iRule is activated when a client-side connection is accepted. It then lets the Local Traffic Manager to send the packet to the pool my_pool , if the client’s address matches

2. What is Load Balancing on Servers?  

Server Load Balancing (SLB) helps in increasing network speed and content delivery. It does so by using a variety of techniques and prioritization to react to network requests.  

SLB exposes the users to a set of servers and guarantees that users are not routed to any failing server. 

3. What is iControl in F5 LTM? 

In F5 LTM, iControl is a web services-enabled open API. It provides granular control over the configuration and management of BIG-IP which is F5’s application delivery platform. 

iControl can be integrated with the help of SOAP (Simple Object Access Protocol). It can then be used virtually on any platform. 

iControl can be used for the following functions: 

  • To build custom management. 
  • To monitor applications. 
  • To integrate with Business Process Management (BPM) and other workflow applications. 
  • It can be integrated directly into applications for better control over delivery of an application. 
  • Integrating with virtual computing platform management tools like VMware vCenter and Microsoft System Center Virtual Machine Manager (SCVMM). 

4. Is F5 Load Balancer a software app or a hardware device? 

There are two types of load balancers: 

  • Software-based 
  • Hardware-based 

Vendors of hardware such as F5 Networks and Citrix include the software-based load balancer within the products. 

5. What are the advantages of using SLB? 

The three main benefits of using SLB are as follows: 

  • Increased scalability. 
  • It provides redundancy. 
  • Upkeep and performance.

6. What is GSLB?

  • Global Server Load Balancing (GSLB) is very similar to SBL. Yet, it is different from SBL in many aspects. 
  • It enables us to load balance VIPs from various geographical locations as a single entity. 
  • The GSBL also enables geographic site expansion as well as low latency. 

7. How to enable GSLB?

  • One of the easiest ways to enable GSLB is with the use of CDN (Content Delivery Network). 
  • Global CDN collects data from their client’s origin servers. 
  • It then caches it on geographically distributed servers. 
  • It offers dependable internet content to consumers all across the globe.

8. What is Health Check in reference to load balancer? 

Health Check is a feature present in the load balancer. It allows you to set parameters to perform diagnostic observations on the performance of web servers and web server farms. These farms are associated with each appliance. 

Health checking lets you determine if a particular server or a service has failed or it stopped working. When a health check fails, the SLB algorithm stops sending clients to that server until the service passes health checks again. 

9. Which server will be accessed first when load balancing to a real server? 

It completely depends on the load balancing method you choose. A few examples include: 

  • Least connections method:  

The first connection is received by the real server with the lowest number of concurrent connections

  • Round robin method: 

The real server with the lowest entry index will receive the first connection. 

  • Shortest response: 

The load balancer looks for the real server with the lowest response time. It does so by establishing connections with each server and then calculating the round-trip time.

10. What is Reverse Proxy Cache? 

Reverse Proxy Cache is a cache present in front of the origin servers, hence the use of the term ‘reverse’ in the name. 

If a client requests a cached object, the proxy will service the request from the cache.  

It then does not use the origin server.

11. How does the cache decide what to cache? 

The cacheability of an object is measured by the HTTP Cache-Control header. It can also determine how long the object should be cached. The cache can be configured to cancel the cacheability of an object by specifying the host-name and a regular expression. It looks for the object whose URL matches these metrics.

12. What is Overflow? 

  • When the server cannot handle the queries anymore due to excessive queries, it is called overflow.  
  • Overflow is a technique in array GSBL that addresses this issue. 
  • It does so by redirecting queries to another server when the local site reaches 80% of its capacity. 

13. What is a pool in F5? 

  • A pool is a collection of equipment. These include web servers that work together to collect and handle the traffic. 
  • BIG-IP does not route client information to the specified destination IP address in the user requests. 
  • Instead of that, it transmits the request to the subscribers of that group.

14. What is the difference between dynamic and static content? Is it possible for the Array appliance to cache dynamic content? 

A web server responds to a client’s request for a web page by returning the required content. Static content is the content which is obtained from disk/memory and provided to the user without further intervention. 

Dynamic content is determined by measuring the amount that is created on the fly by the host. 

15. What is meant by True Clustering? 

  • True clustering allows the synchronization of all configurations across all machines in the cluster. 
  • Only universal parameters are synced, such as the SLB configuration. Local characteristics, such as the IP addresses of the interface are not synced. 

16. What is Virtual Clustering? 

  • Virtual clustering allows the availability of VIPs between cluster members.  
  • For example, if one appliance responsible for traffic processing fails, another device in the cluster assumes the responsibility for traffic processing. 

17. What parameters do we need to provide while configuring the cluster? 

The following parameters must be defined in the cluster configuration: 

  • Address of the VIP 
  • Cluster-ID 
  • Method of Authorization 
  • User interface 
  • Priorities 

18. What exactly is SNAT? 

SNAT translates the source client’s IP address included inside the query to the BIG-IP device’s transcription address. 

19. What exactly is an iRule? 

It is a type of script that was developed to access features that are not available through GUI or CLI. This iRule allows you to engage directly with the traffic flowing by that device.

20. What are the components of iRule?

  • There are three most important components of iRule:
  • Declarations of Events 
  • Commands of iRule

21. What algorithms are used for cache content replacement? 

The Array (Hardware load balancer) Cache uses various algorithms for recognizing cache replacement behavior, depending on the context. The Array will use LRU (Least Recently Used) to remove older content when the space is needed. The LRU is used in most cases. 

22. What is recursion depth in tree structure?

Recursion depth refers to the number of levels that can be cross the main or parent page. This is often used to specify how much of a website or how many deep levels are needed to preload into the cache. 


You no longer need to worry. You can go through these F5 LTM interview questions and answers. These questions are technical F5 LTM questions as well as basic questions with answers.  

Make your concepts stronger with these questions. Good luck! 

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