This section provides definitions for key terms related to F5 LTM Load Balancer and its Virtual Servers.

• F5 LTM (Local Traffic Manager): A core module of the F5 BIG-IP platform that manages and optimizes inbound and outbound network traffic to ensure application reliability, security, and high performance.
• Virtual Server (VS): A logical object that appears as a single endpoint for client connections. It processes incoming traffic based on configuration and forwards it to one or more backend servers (pool members).
• Pool: A group of backend servers (nodes) to which the virtual server can dispatch incoming client requests.
• Node: An individual physical or virtual backend server, identified by its IP address, that receives traffic from the pool.
• Profile: A set of predefined settings or features (such as TCP, HTTP, or SSL) applied to a virtual server to control how traffic is handled and optimized.
• Persistence Profile: Ensures that a user's session is consistently directed to the same backend server during their interaction, improving user experience for stateful applications.
• iRule: A flexible, event-driven script written in Tcl (Tool Command Language) for custom traffic handling and advanced logic on the F5 device.
• SNAT (Secure Network Address Translation): A feature that changes the source IP address of client traffic, allowing server responses to return properly and facilitating routing in complex network topologies.
• Health Monitor: Automated checks that verify the availability and responsiveness of backend pool members or nodes, ensuring that traffic is only sent to healthy servers.

Below are essential parameters you will encounter when configuring a Virtual Server in F5 LTM. Understanding these helps you properly design and deploy your load balancing solution:

• Name: A unique identifier for the virtual server object.
• Destination Address: The IP address where clients connect. This acts as the virtual endpoint exposed to the users.
• Service Port: The destination port that the virtual server will listen on (such as 80 for HTTP or 443 for HTTPS).
• Default Pool: The backend server pool where incoming client requests are forwarded.
• SNAT (Source Network Address Translation): Controls whether the source IP is translated, which can be required for routing responses correctly.
• Persistence Profile: Ensures user sessions consistently connect to the same backend server for the duration of the session if needed.
• Protocol Profiles: Attach protocol-specific behaviors such as TCP, HTTP, SSL for optimization and security.
• iRules or Policies: Logic and rules you can attach to inspect, manipulate, or reroute traffic based on advanced criteria.
• Connection Limit: The maximum number of simultaneous connections allowed to this virtual server (can be used to protect backend resources).
• Health Monitors: Defines criteria for checking backend pool member health before routing traffic to them.

These configuration settings ensure that your virtual server is correctly tailored to your application's requirements, both for functionality and resilience.

An iRule is a customizable script written in Tcl that runs on the F5 LTM to manage and manipulate network traffic based on specific criteria. iRules allow for advanced traffic manipulation by responding to events such as connections, requests, or responses.

Step-by-step example of a simple iRule:

1. Define the rule to inspect HTTP requests as they are received.
2. Evaluate the URI path of incoming requests.
3. Direct requests to different backend server pools based on the URL path.

Sample iRule code:

when HTTP_REQUEST {
  if {[HTTP::uri] starts_with "/admin"} {
    pool admin_pool
  } else {
    pool web_pool
  }
}

In this example, if the requested URI begins with /admin, traffic is sent to the admin_pool. All other requests are directed to the web_pool. iRules like this provide flexibility for traffic routing, security enforcement, and application-specific logic on your F5 LTM virtual servers.

This checklist outlines step-by-step actions for identifying and resolving common issues with F5 LTM Virtual Servers:

1. Verify Virtual Server State:
   • Check that the virtual server is enabled and in an online state through the GUI or CLI.
   • Use: tmsh show ltm virtual [virtual-server-name] to confirm status.
2. Validate Pool Member Health:
   • Ensure at least one pool member is up and available.
   • Use: tmsh show ltm pool [pool-name] to check member status.
3. Check SNAT and Routing Configuration:
   • Verify SNAT settings are correct for your environment.
   • Ensure backend servers route return traffic through the BIG-IP device.
4. Review Health Monitor Assignments:
   • Confirm health monitors are properly assigned and configured for each pool.
   • Investigate monitor failures in the event of pool member downtime.
5. Look for Layer 2/3 Issues:
   • Check for network connectivity, duplicate IPs, or ARP cache issues on the virtual server IP address.
6. Analyze iRules and Profiles:
   • Inspect attached iRules and protocol profiles for errors that could disrupt traffic.
7. Check Logs and Statistics:
   • Review logs (/var/log/ltm) and virtual server statistics for clues on dropped or rejected connections.
8. Use CLI Tools for Deep Troubleshooting:
   • Run packet captures with tcpdump to trace traffic flow and diagnose issues.
   • Example: tcpdump -ni 0.0 host [virtual-server-ip]
9. Test End-to-End Connectivity:
   • Ping pool members from the BIG-IP to ensure connectivity.
   • Test application health directly against backend servers to rule out server-side issues.
10. Escalate When Needed:
    • If all checks fail, contact F5 Support and provide relevant configuration, logs, and packet captures.

These steps cover the most frequent causes of virtual server outages and misconfigurations, ensuring a faster resolution for typical deployment and operational issues.

Below are some essential CLI commands for working with F5 LTM Virtual Servers. These commands help you display configurations, check health and status, and troubleshoot traffic management:

1. Enter TMSH Shell:
   • tmsh Launch the Traffic Management Shell interface, where you run most F5-specific commands.
2. List All Virtual Servers:
   • tmsh list ltm virtual Shows the configuration details of all virtual servers.
   • tmsh list ltm virtual virtual-server-name Displays configuration for a specific virtual server.
3. View Virtual Server Status:
   • tmsh show ltm virtual Shows the availability and status for all virtual servers.
   • tmsh show ltm virtual virtual-server-name Checks status of a specified virtual server.
4. List Pools and Pool Members:
   • tmsh list ltm pool Displays all pools and their member configuration.
   • tmsh show ltm pool pool-name Shows real-time status and statistics for a selected pool.
5. Find Virtual Server by Destination IP:
   • tmsh list ltm virtual one-line | grep IP-address Filters the list to locate the virtual server associated with a specific IP.
6. Monitor Logs and Capture Traffic:
   • tail -f /var/log/ltm Follows live LTM log updates for troubleshooting errors and events.
   • tcpdump -ni 0.0 host virtual-server-ip Captures and analyzes live traffic for in-depth diagnostics.

These CLI commands allow you to effectively manage, audit, and troubleshoot your F5 LTM Virtual Server environment directly from the command line, streamlining operational workflows and supporting rapid issue resolution.