F5CAB5 BIG-IP Administration Support and Troubleshooting (F5CAB5) exam Questions and Answers

The failure of the Forwarding (IP) virtual server is caused by an incorrect Network Mask configuration for a wildcard destination.

Wildcard Destination: The administrator intends to create a "Wildcard" Virtual Server that listens for any destination IP address (0.0.0.0).

The Mask Conflict: A mask of 255.255.255.255 (or /32) tells the BIG-IP to look for a specific, single host address. When combined with 0.0.0.0, the system is literally looking for traffic destined for the IP 0.0.0.0, which is not a valid routable destination for standard traffic.

Correct Configuration: To allow the virtual server to catch traffic for any IP address, the mask must be changed to 0.0.0.0 (or /0). This signifies that the system should ignore all bits of the destination address and match everything.

Forwarding Logic: The rest of the configuration—including ip-forward (Forwarding IP type), translate-address disabled, and translate-port disabled—is correct for a BIG-IP acting as a router/gateway.

When analyzing HTTPS traffic using tools like tcpdump without access to the SSL private keys for decryption, only the Layer 2 through Layer 4 information remains visible.

Visible Information: You can see the Source and Destination IP addresses, TCP ports, and the TLS handshake headers (such as the Server Name Indication/SNI in the Client Hello).

Encrypted Information: Once the encrypted tunnel is established, all Layer 7 data is masked. This includes HTTP Request/Response Headers (Option A and D) and Cookies (Option C).

Troubleshooting Note: To see the headers or cookies, an administrator must either perform the packet capture on the "server-side" of the BIG-IP (if it is performing SSL Offload) or use a tool like Wireshark with the appropriate SSL keys loaded.

When a physical network link (like Interface 1.1) reaches its maximum capacity, it creates a bottleneck that negatively impacts network-level performance. To overcome the physical limits of a single interface, BIG-IP administrators use "Trunking," which is the F5 term for Link Aggregation (often implemented via LACP). A trunk object bundles multiple physical interfaces into a single logical link. By creating a trunk with two or more interfaces, the BIG-IP can spread the traffic load across all members of the trunk, effectively doubling or tripling the available bandwidth for the associated VLANs. Beyond performance, troubleshooting redundancy often leads to the use of trunks; if one cable in a trunk fails, the others continue to carry traffic, preventing a complete outage. This is a superior solution to simply increasing MTU (which requires end-to-end support) or manually setting media speeds. In a high-availability environment, configuring trunks is a foundational troubleshooting and optimization step to ensure that traffic spikes do not result in packet loss due to link saturation.

According toofficial F5 documentation (K52015891 – Troubleshooting BIG-IP power supply issues),hardware-related alerts for power supplies, fans, and chassis components are logged in /var/log/ltm.

When a BIG-IP device experiences a power supply issue—such as failure, intermittent outages, or fan-related faults—the system generates alerts through internal platform monitoring services. These alerts are written to the/var/log/ltmfile and often appear with messages similar to:

Chassis power supply 2 has experienced an issue. Status is as follows: FAN=bad; STATUS=bad.

This makes /var/log/ltm theauthoritative log filefor identifying and verifying power supply and chassis-related problems on BIG-IP systems.

The other log files are not appropriate for this purpose:

/var/log/daemon.logcontains general daemon messages but isnot the primary sourcefor chassis hardware alerts.

/var/log/kern.loglogs kernel-level events, not platform power status.

/var/log/auditrecords administrative actions and configuration changes.

Conclusion:

Per F5-supported guidance, when suspecting power supply outages or chassis hardware issues, the BIG-IP Administrator shouldalways check /var/log/ltm first.

The key clue in the exhibit is the pool member’s availability showing“Offline (Enabled) – Parent down”. In BIG-IP terminology, a pool member inherits the status of itsparent node. If thenodeis marked down (for example, by a node-level monitor or a default “node is down” condition), thenall pool members using that node IPwill also be marked down and will not receive any traffic, even if the application service on the member port might be healthy.

While the HTTPS monitor configuration (send/receive strings) is displayed, the statusspecificallyindicates anode (parent) failure, not a service-level failure. If the problem were the application not matching the receive string, you would typically see the member down due to themember’s monitorfailing (and the status would reflect monitor failure details), rather than “parent down.”

Option D is too broad; BIG-IP can generally reach the subnet (other servers work), and this symptom points to a specific node condition. Option C is incorrect because HTTP/1.1 is commonly used for monitoring and is valid when properly formatted (especially with a Host header). Therefore, the most likely cause is that thenode health monitor is not responding, causing the node—and consequently the member—to be marked down.

In F5 TMOS administration, the traffic flow is processed through specific event huddles w3ithin iRules. To troubleshoot or m4odify traffic based on a URI (Uniform Resource Identifier), the BIG-IP system must first parse the application-layer data. The HTTP_REQUEST event is triggered when the system has fully received and parsed the HTTP request headers from the client5. This is the correct point to implement logic that selects a pool or pool member based on the path or file requested (e.g., /images or /api). Using CLIENT_ACCEPTED would be too early in the troubleshooting process because that event triggers at the L4 (TCP) connection establishment phase, before any URI information is available6. Conversely, HTTP_RESPONSE occurs during the return traffic from the server, which is too late to make a load balancing decision7. For troubleshooting virtual server behavior where URIs are involved, ensuring the iRule is attached to a Virtual Server with an HTTP profile and using the HTTP_REQUEST event is essential for proper traffic steering and inspection.

To gather granular Layer 7 data such as specific HTTP methods (GET, POST, etc.) and HTTP status codes (200, 404, 500), the BIG-IP system utilizes the Analytics (AVR) profile.

Analytics Profile (Application Visibility and Reporting): While a standard HTTP profile tracks basic byte counts and requests, the Analytics profile is designed specifically to capture, analyze, and display detailed application performance metrics.

Metrics Captured: When attached to a virtual server, it records URL-level statistics, response codes, page load times, and client-side metrics.

Why not others? * HTTP Profile (Option A): This profile handles the parsing of HTTP traffic but does not provide a built-in reporting dashboard for status code distribution.

Statistics Profile (Option C): This is a legacy profile used for custom user-defined counters and does not automatically categorize HTTP methods or status codes.

Request Adapt (Option D): This is used for integrating with ICAP servers (like virus scanners) and does not perform traffic reporting.

Comprehensive and Detailed Explanation From BIG-IP A41dministration Support and Troubleshooting documents:To confirm functionality across a complex environment, the "Network Map" is the most efficient troubleshooting tool in the Configuration Utility43. It provides a hierarchical, visual representation of the traffic management objects44. A single glance allows the administrator to see the status of a Virtual Server (Green/Red/Yellow), the status of its associated pool, the health of individual pool members, and which iRules are currently attached45. This view is superior to the standard "Virtual Server List" for troubleshooting because it maps the dependencies between objects46. For example, if a Virtual Server is "Red," the Network Map will show if that status is inherited from a failed pool or a specific monitor failing on a pool member. Reviewing these basic stats in the Network Map helps the administrator quickly isolate whether a failure is at the service level (Virtual Server), the logic level (iRule), or the hardware level (Pool Member).

To understand which device becomes active, we must look at how the BIG-IP system handles HA Order and Auto Fallback within a traffic group.

HA Order Mechanism: When a traffic group is configured with an "HA Order" list, the system prefers to host the traffic group on the highest-ranking available device in that list (1 being the highest).

The Impact of "Forced to Standby": BIGIP-D is the first choice in the order, but it has been "Forced to Standby." This state is persistent and manual; until an administrator releases the "Force to Standby" state, the device is ineligible to host the traffic group, effectively removing it from the top of the preference list.

Auto Fallback: When "Auto Fallback" is enabled, the traffic group will automatically migrate back to a higher-priority device in the HA order as soon as that device becomes available and is in a healthy "Standby" state.

The Scenario Logic: 1. BIGIP-D is ineligible (Forced Offline/Standby).

2. BIGIP-B is the next highest device in the HA Order (Rank 2).

3. While BIGIP-B was rebooting, the traffic group would have failed over to BIGIP-C (Rank 3).

4. Once BIGIP-B finishes booting and joins the cluster in a "Standby" state, the Auto Fallback setting triggers.

5. Because BIGIP-B is higher in the HA Order than the current active device (BIGIP-C) and the only device above it (BIGIP-D) is ineligible, the traffic group fails back to BIGIP-B.

The failure to connect is caused by a restrictive Source Address filter configured on the Virtual Server.

Source Address Filtering: In the BIG-IP system, the Source Address field on a Virtual Server acts as an implicit Access Control List (ACL). Only traffic originating from a client IP address that matches the specified network range will be accepted and processed by the Virtual Server.

Analyzing the Exhibit: The provided configuration for vs_http shows the Source Address is set to 10.128.10.0/24. This means the Virtual Server will only accept connections from the subnet ranging from 10.128.10.1 to 10.128.10.254.

Identifying the Conflict: The user trying to connect has the IP address 192.168.162.70. Since 192.168.162.70 does not fall within the allowed 10.128.10.0/24 range, the BIG-IP system will not match this traffic to the Virtual Server, effectively blocking the connection attempt.

Evaluation of Other Options:

All Ports (Option A): Configuring a Virtual Server for "All Ports" (port 0) allows it to handle traffic for any destination port, which would not block a standard HTTP application.

Destination Address (Option B): The destination address 192.168.162.80 is the Virtual IP (VIP) users should be connecting to; this is a standard configuration and not the cause of the failure for a user reaching out to it.

Standard Type (Option C): A "Standard" Virtual Server is the most common type used for HTTP applications as it allows for Layer 7 profiles and full proxy capabilities.

Troubleshooting failed SSL handshakes involves interpreting the resource limits defined by the system's license8888. The log message SSL transaction (TPS) rate limit reached indicates the BIG-IP is dropping SSL connections because it has exceeded its licensed "Transactions Per Second" capacity. When analyzing stats to determine the correct license level, the administrator must focus on "Client-side" SSL TPS. This represents the initial encrypted handshakes between users and the BIG-IP virtual servers91. In this scenario, the peak client-side demand is 1200 TPS. While the 800 server-side transactions represent re-encryption toward the backend, F5's primary SSL TPS license limits typically apply to the client-facing side of the traffic flow. Therefore, to resolve the intermittent connectivity issues and ensure the virtual server works reliably during peaks, the license must be upgraded to at least 1200 TPS949596969696.9798Confirming this peak via statistics andcomparing it to the current license is a standard troubleshooting step for SSL performance issues.

The exhibit shows the virtual serverapplication_vswith a status indicating it isoffline but enabled. In BIG-IP terminology, this status means the virtual server itself is administratively enabled, but it is unable to pass traffic becauseno usable pool members are available.

Two common and documented causes for this condition are:

Pool member(s) administratively disabled (Option A):When all pool members are administratively disabled, BIG-IP removes them from load-balancing decisions. Even though the virtual server remains enabled, it has no available pool members to send traffic to, resulting in an offline status.

Node(s) administratively disabled (Option C):Pool members inherit the status of their parent nodes. If a node is administratively disabled, all associated pool members are also marked unavailable. This condition causes the virtual server to show as offline, even though the virtual server configuration itself is correct.

The other options are incorrect:

Forced offline pool members (Option B)result in a different operational intent and are explicitly set for maintenance scenarios.

Virtual server administratively disabled (Option D)would show the virtual server as disabled, not enabled/offline.

This behavior is consistent with BIG-IP traffic management logic and is commonly verified by reviewingpool and node availability stateswhen diagnosing virtual server availability issues.

Uneven load balancing on a BIG-IP system typically occurs when traffic is not distributed evenly across all available pool members. One common reason is thatmonitors have marked down multiple pool members (Option B). When health monitors fail for specific pool members, BIG-IP automatically removes those members from load-balancing decisions. As a result, traffic is sent only to the remaining healthy member, creating the appearance that load balancing is not functioning correctly. This behavior is expected and aligns with BIG-IP’s design to ensure traffic is sent only to healthy resources.

Another frequent cause isthe presence of a persistence profile on the pool or virtual server (Option C). Persistence (such as source address or cookie persistence) forces subsequent client connections to be sent to the same pool member for session continuity. While persistence is critical for certain applications, it can override the load-balancing algorithm and cause most or all traffic to be directed to a single pool member, especially during low traffic volumes or testing scenarios.

The other options are incorrect because avirtual server marked down (Option A)would not pass traffic at all, andall pool members marked down (Option D)would result in no connections rather than uneven distribution. This analysis follows standard BIG-IP troubleshooting methodology using pool status, monitor results, and persistence configuration review.

When load balancing is not working as expected across hardware hosting multiple services, the administrator must distinguish between "member" and "node" level algorithms102102102102. A "member" is a specific IP and port (e.g., 10.1.1.1:80), while a "node" is the physical server's IP (10.1.1.1) regardless of the port103. If servers host both FTP and HTTP services in separate pools, using "Least Connections (Member)" only balances connections within each individual pool. This can lead to a skewed distribution where one server is selected for a new HTTP connection because it has the fewest HTTP connections, even if it is currently overloaded with hundreds of FTP connections. By applying "Least Connections (Node)," the BIG-IP tracks the total number of connections to the physical hardware across all ports and pools106106106106. This ensures that the administrator can maintain an even distribution of the total workload across the server fleet, resolving the reports of uneven traffic distribution reported by the server support team

These log messages indicate that the BIG-IP system is under significant resource pressure and has activated its Adaptive Connection Management.

Global Eviction Policy: The message Aggressive mode ... activated (global memory) confirms that the system has reached a memory utilization threshold that triggers the eviction policy. This happens because TMM (the data plane) is running low on available memory pages.

Sweeper/Reaping: Once in "Aggressive mode," the BIG-IP "sweeper" starts reaping (terminating) connections to free up memory. The log Connections killed confirms this is occurring.

Impact: The system does not drop all connections; it targets connections based on the eviction policy (e.g., oldest connections or those exceeding limits) to bring memory usage back to safe levels. Thus, some connections will be dropped (Option A), and the cause is TMM memory exhaustion (Option C).