F5 Load Balancer Interview Questions

1. Is an F5 load balancer software or hardware?

Well, the load balancer is available in two types: software-based and hardware-based. Hardware vendors like F5 networks and Citrix load the software-based load balancer(VIPRION of BIG-IP) into these hardware(machines).

2. What do you know about Server Load Balancing?

SLB(Server Load Balancing) provides content delivery and network services by using a sequence of load-balancing algorithms. One of its exceptional qualities is that it automatically prioritizes the responses that must be sent to a particular client request on the network.

SLB guarantees application reliability, high availability, scalability, and delivery.

3. How does SLB work?

There are two types of load balancing on which SLB works they are:

Application-level load balancing: traffic load is used to make the balancing decisions, including WSLB(windows server load balancing).

Transport-level load balancing: this is an approach based on DNS, which doesn’t need an application payload.

4. List three uses of SLB?

The three benefits of using SLB are:

1. Scalability is increased
2. Redundancy is provided
3. Maintenance and performance      

5. What data must be given to set up any appliance to perform cookie load balancing?

The following data must be given to set up any appliance to perform cookie load balancing:

1. Cookie name
2. Cookie values
3. VIP
4. Groups to balance

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6. What is the use of the Health Check feature in load balancing?

The health check feature is used to determine the Server status. For example, if there is a failed server in the network, the Health server identifies that failed server, and SLB will avoid sending the client requests to that failed server.

7. What do you know about iControl?

As iControl is a web service-enabled API, it gives granular control on the management and configuration application delivery platform of F5.

8. What is GSLB?

Global Server Load Balancing is very similar to SBL, but on a global level, it enables us to load balance the VIPs from various geographical places as one entity. GSBL also provides geographical site scalability and fault tolerance.

9. How can we enable GSLB?

The simplest way to enable GSLB is with the help of CDN(Content Delivery Network). Global CDN will obtain information from the origin servers of their customers and cache the data on the geographical servers and deliver reliable internet content to users across the world.

10. What are persistent cookies?

Persistent cookies are usually stored in the user’s device. These cookies help us remember our user credentials, it also allows us to remember information, preferences, settings. Basically, these cookies are designed to make a convenient website experience for the user.

11. Is turning on clustering important to access GSLB?

Yes, turning on clustering is very important to utilize GSLB. Every proxy inside the cluster should have a similar configuration. Therefore, every appliance can serve as a DNS if it becomes a site master.

12. What is Overflow?

When there are more requests than the site capacity, that specific situation is called Overflow. Overflow is one of the methods in array GSBL that solves this problem by sending the requests to another site when 80% of the local site is loaded.

13. What does the “least connections” method do?

The “least connections” method will send the clients to the sites that contain the least number of contemporary connections.

14. What is the use of a round-robin algorithm for load balancing?

Round-robin is one of the simplest methods to distribute the client request over a cluster of servers. As each request is forwarded to one of the servers in the group, this algorithm directs the load balancer to repeat the same process until there are no more requests.

15. List the different load balancing techniques?

Different load balancing techniques are:

• Round robin
• Least connection
• Weighted Round Robin
• Weighted Least Connection
• SDA adoptive Resource-Based
• Adoptive Resource-Based
• Weighted Response Time
• Fixed Weighting       

16. What is a pool in F5?

A pool is a group of devices like web servers grouped to get and process the traffic. BIG-IP doesn’t send the client traffic to the target IP address mentioned in the client request. Instead, it sends that request to the nodes that are members of that pool.

17. What is the use of nodes in your load balancer?

Nodes in your load balancer will help you to transfer the client requests to the registered destination. In simple words, the node acts as a transporting medium that carries the client request to your targets.

18. What do you know about RAM cache?

RAM cache provides us with the capacity to serve content to the clients directly from your F5 appliance memory. Both the server and client can be benefited because the RAM cache reduces the server load and response latency.

19. What is the non-cacheable content in RAM Cache?

Following is the non-cacheable content in RAM cache:

• PUT, DELETE, HEAD, CONNECT, and TRACE are non-cacheable by default.
• Private data, which is specified by the control headers of the cache, are also non-cacheable. 

20. What is the shortcut to clear the cache?

ctrl+F5 is the shortcut to refresh or to clear the cache.

21. What is pipelining?

Pipelining is a technique within which various requests are sent upon a unique TCP connection without expecting the responses. The server will send back the responses in the same order in which it received the requests.

22. What is an iRule?

CLI and GUI do not provide all the extended capabilities required, so the iRule came into existence. It is a kind of script that we write to use those capabilities that the GUI and CLI do not provide. This iRule will help us to directly interact with the traffic that is passing by that device.

23. List the different elements of iRule?

There are three essential elements in iRule they are:

1. Operators
2. Event declarations
3. iRule commands

24. What are the parameters we have to define within the cluster configuration?

These are the parameters we have to define within the cluster configuration:

• VIP address
• Cluster-ID
• Authorization method
• Interface
• Priorities 

25. What is SNAT?

SNAT(source network address translation) maps the IP address of a source client within the request to the translation address that is defined upon the BIG-IP device.

26. What is the full form of DNAT?

DNAT is abbreviated as Destination network address translation. This helps us to redirect the destined packets to a specific IP address. 

27. What do you know about recursion depth?

Recursion depth is a structure that refers to the levels that can be traversed from the main page. We use this to specify the number of levels of the website to preload inside the cache.

28. What are the different types of cookie policies in F5 load balancing? 

There are two different types of cookie policies in F5 load balancing they are:

1. QoS cookie policy 
2. Persistent cookie policy 

29. Differentiate between checked and unchecked exceptions?

We can check the checked exceptions at programs run time, whereas we can check the unchecked exceptions at compile time of that program. Both the checked and unchecked exceptions can be generated manually.

30. What are the different algorithms utilized for cache content replacement?

The hardware load balance cache utilizes several algorithms to define the behavior of the cache replacement based upon the context. In many cases, the hardware load balancer utilizes Least Recently User(LRU) to remove the older content when we need space.