F5CAB3 BIG-IP Administration Data Plane Configuration (F5CAB3) exam Questions and Answers

The issue of "lost shopping carts" in an ecommerce environment is a classic symptom of a missing or improperly configured Persistence Profile. In modern web applications, session data—such as items added to a cart—is often stored locally on the specific web server that initially handled the user's request. If the BIG-IP system load balances a user's subsequent request (like clicking "Checkout") to a different server among the 50 webservers, the new server will not have the session data, and the user will appear to have an empty cart.

While Least Connections (node) is an efficient load balancing algorithm, it makes a new decision for every connection unless persistence is enabled. To resolve this, the administrator must implement a persistence mechanism. HTTP Cookie Persistence (Option A) is the industry standard for web applications. By assigning a cookie persistence profile to the Virtual Server, the BIG-IP inserts a unique cookie into the HTTP response. When the user's browser returns that cookie in future requests, the BIG-IP identifies the specific server that handled the first request and consistently directs the user back to that same server for the duration of their session.

Sip_info (Option B) is a persistence method for VOIP traffic and is not applicable to web traffic. Ratio load balancing (Options C and D) merely changes the distribution frequency but still does not guarantee that a specific user will stay on the same server across multiple requests. Therefore, adding a cookie persistence profile is the direct procedural fix to maintain session state and ensure ecommerce functionality.

For HTTP-based applications, cookie persistence is the most appropriate and commonly recommended persistence method.

According to the BIG-IP Administration: Data Plane Configuration documentation:

Cookie persistence inserts or uses an HTTP cookie to maintain session affinity.

It operates at Layer 7 (HTTP) and is application-aware.

It allows persistence to be maintained even when multiple clients are behind a NAT device.

Why the other options are incorrect:

A. Destination addressDestination address persistence is generally used for inbound traffic patterns such as firewall or proxy scenarios.

B. Hash persistenceHash persistence is less granular and not HTTP-specific.

C. SSL persistenceSSL persistence is typically used when SSL session IDs are reused and is less reliable than cookies for HTTP applications.

Correct Resolution:

Using cookie persistence ensures that HTTP web requests are consistently directed to the same pool member.

In BIG-IP systems, iRules influence traffic only when they are attached to a Virtual Server. If application traffic is being sent to nodes or pool members that are not defined in the pool, this typically indicates that an iRule is overriding the default load-balancing behavior by explicitly selecting a pool or node.

According to BIG-IP Administration: Data Plane Configuration and official F5 guidance:

iRules are associated with Virtual Servers, not directly with pools or nodes.

To determine whether an iRule is actively affecting traffic, the administrator must inspect the Virtual Server configuration.

Explanation of the correct answers:

B. Via the GUI at the Resources tab for the virtual serverThe Resources tab in the Configuration Utility displays all traffic-handling objects applied to the Virtual Server, including assigned iRules. This is the primary GUI location to verify whether an iRule is influencing data plane traffic.

C. Via TMSH with the list /ltm virtual commandThis TMSH command displays the full Virtual Server configuration, including any iRules listed under the rules section. It is the authoritative CLI method to confirm iRule usage.

Why the other options are incorrect:

A. Via TMSH with the list /ltm rule commandThis command only shows the contents of an iRule and does not indicate whether the iRule is attached to or used by any Virtual Server.

D. Via the GUI at the iRule tab for the virtual serverBIG-IP does not provide a dedicated “iRule” tab on Virtual Servers. iRules are viewed and managed under the Resources tab.

Correct Conclusion:

To verify whether an iRule is responsible for unexpected node selection, the BIG-IP Administrator must examine the Virtual Server configuration, either through the Resources tab in the GUI or by using TMSH to list the Virtual Server configuration.

WAN-optimized TCP profiles improve performance for high-latency Internet clients, while LAN profiles are optimal for backend servers.

This scenario describes a classic case of asymmetric routing in a "one-arm" or non-gateway deployment. When a BIG-IP system is configured for SSL offloading, the following traffic flow occurs:

Client-Side: The client establishes a successful SSL/TLS handshake with the Virtual Server. This explains why the user can "successfully establish an SSL connection."

Server-Side: The BIG-IP decrypts the traffic and forwards it as plain HTTP to the backend server. The packet trace confirms the server receives the HTTP GET request and responds with the content.

The Routing Failure: By default, the BIG-IP system preserves the client's original source IP address. If the backend server's default gateway is not the BIG-IP system (or if the server is on the same subnet as the client), the server will attempt to send the response directly back to the client's IP address, bypassing the BIG-IP.

Stateful Drop: Because the BIG-IP is a Full Proxy, it expects the response to return through its own internal state table to be encrypted and sent back to the client. Since the response bypasses the BIG-IP, the BIG-IP connection eventually times out, and the client receives no data despite the server having sent it.

Solution (SNAT): Enabling Secure Network Address Translation (SNAT), specifically SNAT Auto Map, ensures that the BIG-IP replaces the client's source IP with its own internal self-IP before sending the request to the server. This forces the server to send the response back to the BIG-IP, allowing the BIG-IP to complete the transaction and deliver the content to the user.

To provide encrypted communication between users and a virtual server, the BIG-IP system acts as a transparent SSL/TLS proxy. The administrative configuration required for this is a Client SSL profile.

When multiple hostnames (FQDNs) are associated with a single IP address, the system must determine which SSL certificate to present to the client during the initial TLS handshake. This is achieved using an extension of the TLS protocol called Server Name Indication (SNI).

The configuration logic is as follows:

Profile Type: The Client SSL profile is responsible for terminating the SSL connection from the client (the user) to the BIG-IP system.

The Setting: Within the Client SSL profile (under the "Advanced" view), there is a field specifically called Server Name. By entering the specific hostname (e.g., www.example.com) in this field, the BIG-IP system can match the hostname requested by the client in the ClientHello message to the correct profile.

Implementation: The administrator typically creates multiple Client SSL profiles—one for each hostname—and assigns them all to the same virtual server. One of these profiles must be designated as the Default SSL Profile for SNI to handle requests where the client does not provide a hostname or provides one that does not match any specific profile.

By using the Client SSL profile and the Server Name setting, the BIG-IP system ensures that each user receives the correct certificate for the specific site they are trying to reach, even though all sites share a single virtual server IP.

F5 provides several pre-configured (built-in) TCP profiles designed to optimize traffic for different network conditions. When an administrator is looking to improve general performance but does not have a specific, narrow use case (like strictly mobile or strictly long-haul WAN), the f5-tcp-progressive profile is the recommended starting point.

The f5-tcp-progressive profile is designed as a modern, high-performance replacement for the older default TCP settings. It incorporates several advanced congestion control algorithms and buffer management techniques that allow it to adapt dynamically to varying network latencies and packet loss scenarios. Unlike f5-tcp-legacy (Option A), which uses older, less efficient stacks, the progressive profile leverages modern TMM (Traffic Management Microkernel) optimizations.

While f5-tcp-mobile (Option B) is highly effective for high-loss cellular networks and f5-tcp-wan (Option D) is optimized for high-latency long-distance links, they can sometimes be too aggressive or poorly suited for standard campus or data center environments. The f5-tcp-progressive profile acts as a "best-of-all-worlds" template that typically provides an immediate performance boost for most web applications by improving window scaling and fast-retransmit behavior. Therefore, it is the procedural "first step" in TCP performance tuning before moving to more specialized, niche profiles.

This scenario describes a classic routing issue often encountered during SSL offload deployments. The fact that an SSL connection is established indicates the Client SSL profile is working correctly. The packet trace showing the server "receives and responds" to the request is the most critical diagnostic clue.

When a BIG-IP receives traffic, it typically passes the client's original source IP address to the backend server. If the backend server's default gateway is not the BIG-IP (a common "one-arm" network topology), the server will attempt to send its response directly back to the client's IP via its own default router. The client's browser will reject this response because it expects traffic to come from the Virtual Server's IP, not the backend server's IP.

To resolve this, the administrator must enable SNAT (Source Address Translation), typically using SNAT Automap. When SNAT is enabled, the BIG-IP replaces the client's original source IP with one of its own Self IPs before forwarding the request to the server. Because the source of the packet is now the BIG-IP, the backend server is forced to send its response back to the BIG-IP. The BIG-IP then receives the response, translates it back, and delivers the content to the user. Option A is unnecessary if the servers are expecting plain-text traffic after the BIG-IP performs offload. Option D would only worsen the existing routing discrepancy.

When a failover occurs in a standard BIG-IP High Availability (HA) pair, the newly active device takes over the floating IP addresses (Virtual Servers, Self IPs). By default, the new active device sends Gratuitous ARP (GARP) messages to the local network switch to inform it that these IP addresses are now associated with its own physical MAC addresses. However, network switches and intermediate routers often have ARP aging timers or security features that may delay the updating of their ARP tables, leading to "black-holed" traffic or dropped packets for several seconds or minutes until the network infrastructure correctly relearns the new path.

To eliminate this delay and ensure a seamless transition, a BIG-IP Administrator should Configure MAC Masquerade. MAC Masquerade allows the administrator to assign a unique, "virtual" MAC address to a specific traffic group. Instead of using the hardware-burned MAC address of the individual appliance, the active device uses this shared virtual MAC address for all communication involving floating IPs. When a failover occurs, the standby device assumes control of the traffic group and begins using the exact same virtual MAC address. Because the MAC address associated with the VIPs never changes from the switch's perspective, there is no need for the switch to update its MAC address table or for the surrounding infrastructure to update its ARP caches. This effectively eliminates the "stabilization period" reported by users, as the data plane transition happens almost instantaneously at Layer 2, maintaining continuous traffic flow without being hindered by external network re-convergence times.

Fallback persistence is used when the primary persistence method fails. Source Address Affinity is a Layer 4 persistence method and is fully supported as a fallback option for most virtual server types.

The issue described is a classic symptom of asymmetric routing, which frequently occurs when the BIG-IP system and the back-end servers reside on the same subnet (often referred to as a "one-arm" deployment).

The Routing Problem: By default, the BIG-IP system preserves the original client source IP address when forwarding traffic to a pool member. If the server is in the same subnet as the client or if the server's default gateway is not the BIG-IP, the server will attempt to send its response directly back to the client’s IP address, bypassing the BIG-IP.

Stateful Failure: Since the BIG-IP is a Full Proxy, it maintains a state table. Because the response packet never returns through the BIG-IP, the system cannot complete the three-way handshake or manage the application session, resulting in a connection failure for the user.

The Solution (SNAT): Enabling Source Network Address Translation (SNAT) solves this by changing the source IP address of the request to an IP address owned by the BIG-IP (typically a self-IP).

Requirement for Subnet Alignment: To ensure the server sends the response back to the BIG-IP, the translation address must be reachable. By using a self-IP configured in the same subnet as the servers, the BIG-IP ensures that the server sees the request coming from a local "neighbor." The server will then naturally send the response back to that self-IP, allowing the BIG-IP to translate the packet back and forward it to the client.

Why other options are incorrect:

A: Disabling address translation would ensure the server-side traffic uses the client IP, making asymmetric routing inevitable in this scenario.

B: This is technically contradictory; "Auto Map" specifically uses existing self-IPs and does not require or use a "SNAT pool" configuration.

C: While using a specific translation address can work, it does not inherently guarantee the Layer 2/Layer 3 reachability mentioned in the scenario as effectively as ensuring the self-IP is correctly placed in the server's subnet.

This question addresses the critical architectural distinction between "member-based" and "node-based" load balancing in environments where servers are multi-homed or host multiple virtualized services. On a BIG-IP, a node represents the underlying IP address of a physical or virtual server, while a pool member represents a specific combination of that IP and a service port (e.g., 10.1.1.10:80).

When multiple pools reference the same IP address across different ports or Virtual Servers, using Least Connections (member) (Option D) only balances connections relative to that specific pool. For example, if Pool_A and Pool_B both use Server_X, and Pool_A has 100 connections while Pool_B has only 5, a member-based algorithm for Pool_B only sees the 5 connections and may continue to send traffic there even if Server_X is CPU-saturated by Pool_A.

The Least Connections (node) algorithm (Option B) solves this by tracking the aggregate total of all active connections directed to that specific node (IP address) across every pool on the system. By selecting the node with the absolute lowest total connection count, the BIG-IP ensures a more equitable distribution of work at the hardware resource level. Predictive methods (Options A and C) use a ranking system based on the trend of connection counts over time, but for the specific requirement of maintaining an equal number of current connections on multi-service servers, the direct Least Connections (node) calculation is the standard and most effective procedural choice.

By default, BIG-IP load balancing algorithms (such as Round Robin) distribute connections evenly across all available pool members. However, persistence profiles override normal load balancing decisions by forcing subsequent connections from a client to be sent to the same pool member.

According to the BIG-IP Administration: Data Plane Configuration documentation:

Persistence creates a client-to-server mapping that is honored before load balancing algorithms are applied.

When persistence is enabled, BIG-IP may repeatedly select the same pool member even if others are available.

This directly alters load balancing behavior.

Why the other options are incorrect:

A. Adding a OneConnect profileOneConnect optimizes server-side TCP connections but does not change which pool member is selected.

B. Enabling SNAT automapSNAT affects source address translation, not pool member selection.

C. Enabling a fallback host in the HTTP profileA fallback host is only used when no pool members are available.

Correct Resolution:

Adding a persistence profile alters load balancing behavior by maintaining client affinity to a specific pool member.

HTTP Compression on a BIG-IP system is an optimization feature that reduces the size of HTTP responses sent to clients, thereby saving bandwidth and improving page load times for end-users. To implement this functionality on a Virtual Server, the system requires two specific profiles to work in tandem: an HTTP profile and a Compression profile.

The HTTP profile (Option E) is a prerequisite because compression is an application-layer (Layer 7) operation. The BIG-IP must be "HTTP-aware" to parse the incoming requests and outgoing responses. The HTTP profile allows the BIG-IP to inspect the Accept-Encoding header from the client (to see if the client supports gzip or deflate) and to modify the Content-Encoding and Vary headers in the server's response. Without the HTTP profile, the Virtual Server operates at Layer 4 (TCP) and sees the traffic only as an opaque stream of bytes, making compression impossible.

The Compression profile (Option B) contains the actual settings and engine for the task. It defines which content types should be compressed (e.g., text/html, application/javascript), the compression level, and the minimum content length required before compression is applied. While a Stream profile (Option D) is used for data replacement (search and replace) and SSL profiles (Option C) are used for encryption, they do not provide compression logic. By applying both the HTTP and Compression profiles, the BIG-IP can successfully intercept server responses, compress the payload according to the configured policy, and deliver the optimized data to the user, effectively offloading the CPU-intensive compression work from the backend web servers.

A red diamond indicates that the Virtual Server is enabled but unavailable, typically due to all pool members being down.

SSH is a TCP-based service. A TCP monitor validates service availability without requiring application-layer inspection.

Slow Ramp Time prevents new pool members from receiving full traffic immediately, avoiding overload.

When multiple hostnames resolve to the same IP address and encrypted communication is required, the BIG-IP must be able to present the correct SSL certificate based on the hostname requested by the client. This is accomplished using Server Name Indication (SNI).

According to BIG-IP Administration: Data Plane Configuration documentation:

SNI is a client-side TLS extension, where the client includes the requested hostname during the SSL handshake.

BIG-IP evaluates this hostname using the Client SSL profile, not the Server SSL profile.

The “Server Name” setting in the Client SSL profile enables BIG-IP to select the appropriate SSL certificate for the requested hostname.

Why option C is correct:

Client SSL profile handles inbound (client-side) encryption.

Server Name enables SNI-based certificate selection when multiple DNS names share the same virtual server IP.

Why the other options are incorrect:

A. Client SSL, Client NameThere is no Client SSL setting called Client Name for SNI certificate selection.

B. Server SSL, Server NameServer SSL is used for encryption between BIG-IP and backend servers, not for client-side hostname identification.

D. Server SSL, Client NameServer SSL does not process client-requested hostnames during TLS negotiation.

Correct Resolution:

Configure a Client SSL profile and enable the Server Name (SNI) setting to support multiple encrypted hostnames on the same virtual server IP.

The configuration provided utilizes Priority Group Activation in conjunction with the min_active_members setting. Priority groups allow an administrator to define primary servers and "backup" servers within the same pool. The BIG-IP prioritizes traffic based on the assigned priority number, with the highest number receiving traffic first.

In this specific configuration, the priority 3 group is the primary group. Therefore, connections are first distributed to all pool members with priority 3 as long as they are available. The system will continue to use only the priority 3 group unless the number of available members in that group falls below the min_active_members value, which is set to 2.

If the priority 3 group has fewer than two active members, the BIG-IP "activates" the next available priority group (priority 2) and distributes traffic among the remaining members of priority 3 and all members of priority 2. This cascading logic continues down the list. Consequently, if both the priority 3 group and the priority 2 group have fewer than two members available, traffic is directed to the priority 1 group. This ensures that even in a multi-server failure scenario, the system has a last-resort group of servers to handle the traffic.

Option D is incorrect because if only one member of priority 3 goes down, there are still two members active (10.12.10.8 and 10.12.10.9). Since 2 is not less than the min_active_members threshold of 2, the priority 2 group will not yet be activated. Option B is incorrect because traffic flows from high priority to low priority, not the other way around.