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Manual Chapter: Configuration Guide for the BIG-IP® Link Controller: 1 - Introducing the Link Controller
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Introducing the Link Controller

Introducing the BIG-IP system

F5 Networks' BIG-IP® system is a port-based, multilayer switch that supports virtual local area network (VLAN) technology. Because hosts within a VLAN can communicate at the data-link layer (Layer 2), a BIG-IP system reduces the need for routers and IP routing on the network. This in turn reduces equipment costs and boosts overall network performance. At the same time, the BIG-IP system's multilayer capabilities enable the system to process traffic at other OSI layers. The BIG-IP system can perform IP routing at Layer 3, as well as manage and secure TCP, UDP, and other application traffic at Layer 4 through 7. The following software modules provide comprehensive traffic management and security for all traffic types. The modules are fully integrated to provide efficient solutions to meet any network, traffic management, and security needs.

  • BIG-IP® Local Traffic Manager
    The BIG-IP system includes local traffic management features that help you make the most of network resources such as web servers. Using the powerful Configuration utility, you can customize the way that the BIG-IP system processes specific types of protocol and application traffic. By using features such as virtual servers, server pools, profiles, and iRulesTM, you ensure that traffic passing through the BIG-IP system is processed quickly and efficiently, while meeting all of your security needs. For more information, see the Configuration Guide for BIG-IP® Local Traffic Management.
  • BIG-IP® Global Traffic Manager
    The Global Traffic Manager provides intelligent traffic management to your globally available network resources. Through the Global Traffic Manager, you can select from an array of load balancing modes, ensuring that your clients access the most responsive and robust resources at any given time. In addition, the Global Traffic Manager provides extensive monitoring capabilities so the health of any given resource is always available. For more information, see the Configuration Guide for BIG-IP® Global Traffic Management.
  • BIG-IP® Link Controller
    The Link Controller seamlessly monitors availability and performance of multiple WAN connections to intelligently manage bi-directional traffic flows to a site - providing fault tolerant, optimized Internet access regardless of connection type or provider. The Link Controller ensures that traffic is always sent over the best available link to maximize user performance and minimize bandwidth cost to a data center.
  • BIG-IP®Application Security Manager
    The Application Security Manager provides web application protection from application-layer attacks. The Application Security Manager protects Web applications from both generalized and targeted application layer attacks including buffer overflow, SQL injection, cross-site scripting, and parameter tampering. For more information, see the Configuration Guide for the BIG-IP® Application Security Manager.

Introducing the Link Controller

The Link Controller is a dedicated IP application switch that manages traffic to and from a site across multiple links, regardless of connection type or provider. The Link Controller provides granular traffic control for Internet gateways, allowing you to define how traffic is distributed across links in a way that meets your business priorities. The Link Controller also transparently monitors the availability and health of links to optimally direct traffic across the best available link.

The Link Controller includes the following features:

  • Dynamic load balancing, based on the following link attributes:
    • Total available bandwidth of the link
    • The costs of purchased incremental bandwidth segments
    • Inbound link capacity and resource limits
    • Outbound link capacity and resource limits
  • Router monitoring, to ensure high availability and continuous uptime

Understanding link load balancing

Link load balancing is defined as managing traffic across multiple Internet or wide-area network (WAN) gateways. Link load balancing ensures high availability in the network, and improves the performance of a web site or data center. Link load balancing provides a method by which an organization can reliably manage a multi-homed network. A multi-homed network is composed of one or more data centers that have more than one link to the Internet.

As enterprises increase their reliance on the Internet for delivering mission-critical applications and services, using only one link and ISP provider to access the public network represents a single point of failure. The Link Controller removes the risk of this single point of failure by enabling enterprises to control and monitor multiple links for their Internet connectivity.

The Link Controller:

  • Guarantees reliable network connections and eliminates downtime by detecting any type of connection outage, and transparently directing traffic away from malfunctioning or unavailable links.
  • Distributes traffic to optimize the capacity of each connection by monitoring line throughput so that links do not become over-saturated.
  • Increases site performance. The Link Controller measures and directs users over the best performing link to increase site response times.
  • Directs traffic over the least expensive link. Administrators can define the price of links and tiered pricing schemes. The BIG-IP Link Controller can direct traffic to the least expensive connection, lowering bandwidth costs.
  • Control traffic to match business priorities. Organizations can define traffic policies to direct traffic over specified connections.

Link management

With the Link Controller, you can manage both inbound and outbound traffic over multiple links. You can distribute traffic based on performance, bandwidth cost, and bandwidth availability. The metrics you can specify are the limits on bandwidth usage, and the pricing structure of your purchased bandwidth. When you specify the limits and pricing metrics for your links, the Link Controller then load balances the links based on those metrics.

Link monitoring

You can monitor several aspects of your managed links using the following tools:

  • Link Statistics
    The Link Statistics screen, in the Configuration utility, displays information about the status, bandwidth usage, bandwidth limits, and bandwidth costs for each of the links managed by the Link Controller.
    For more information on link statistics, review Chapter 12, Viewing Statistics .
  • Transparent monitoring
    Transparent monitoring provides the health status of the routers for the managed links.

Using the Configuration Guide

The Configuration Guide for BIG-IP® Link Controller is designed to help you understand how you can use the features of the Link Controller to accomplish the tasks associated with managing name resolution request on a global level. These tasks include tracking the performance of different servers and services and identifying the load balancing methods that best suit the needs of your company.

The configuration guide contains the following chapters:

  • Introducing the Link Controller
    This chapter provides an overview of the Link Controller and this guide.
  • Essential Configuration Tasks
    This chapter describes the steps you need to follow to have a functional Link Controller on the network. This chapter is ideal if you want to get the Link Controller up and running quickly in order to explore and learn about its functionality.
  • Defining Links
    This chapter describes how to configure the links that determine how traffic flows in and out of your network.
  • Working with Listeners
    This chapter describes how to configure listeners for the Link Controller. A listener instructs the Link Controller to listen for network traffic destined for a specific IP address.
  • Configuring Pools
    This chapter describes how to configure collections of resources, called pools. The Link Controller can load balance incoming traffic to these pools using a variety of different methodologies.
  • Configuring Virtual Servers
    This chapter describes how to configure the virtual servers that are available to the Link Controller. The Link Controller uses these virtual servers when responding to incoming network traffic.
  • Defining Wide IPs
    This chapter describes how to define the inbound wide IPs for your network. An inbound wide IP is typically a web URL that local DNS servers use to request data from your network.
  • Configuring Monitors
    This chapter describes how to use monitors to track the components of your network. Monitors are components of the Link Controller that perform specific tests to see if a given component is available for load balancing.
  • Inbound Load Balancing
    This chapter describes the load balancing modes that the Link Controller supports, and how to apply those modes to your pools and wide IPs.
  • Working with Topologies
    This chapter describes topologies, which allow you to define load balancing modes and resolution controls based on the origin or destination of a given name resolution request.
  • Synchronizing Link Controllers
    This chapter describes how to synchronize the configuration settings between Link Controllers. You typically use synchronization when you have a redundant system consisting of two Link Controllers.
  • Viewing Statistics
    This chapter describes how to use the Link Controller to view statistics on the different physical and logical network components.
  • Understanding Profiles
    This chapter describes how to use the Link Controller to create profiles, which you can use to better mange your network resources.
  • Managing HTTP and FTP Traffic
    This chapter describes how to use the Link Controller to manage your HTTP and FTP traffic.
  • Enabling Session Persistence
    This chapter describes how to use the Link Controller to enable persistent connections between a client LDNS and an internal resource.
  • Managing Protocol Profiles
    This chapter describes the protocol profiles the Link Controller supports, such as TCP and Fast L4 profiles.
  • Using the Statistics Profile
    This chapter describes how to use the Statistics profile with the Link Controller.
  • Writing iRules
    This chapter describes how to write iRules; scripts that allow you to fully customize the load balancing capabilities of the Link Controller.
  • Configuring SNATs and NATs
    This chapter describes how to configure the address translation capabilities of the Link Controller.
  • Configuring Nodes
    This chapter describes how to configure the nodes that belong to different virtual servers.
  • Configuring Rate Shaping
    This chapter describes how to use rate shaping to customize how the Link Controller manages your network traffic.

In addition the preceding list of chapters, this guide contains the following appendix:

  • Additional Monitor Considerations
    This appendix describes some of the additional configuration options you should understand when working with Dynamic Ratio load balancing or the MSSQL monitor.

Additional information

In addition to this guide, there are other sources of the documentation you can use in order to work with other aspects of the Link Controller. The information is organized into the guides and documents described below. The following printed documentation is included with the BIG-IP system.

  • Configuration Worksheet
    This worksheet provides you with a place to plan the basic configuration for the BIG-IP system.
  • BIG-IP Quick Start Instructions
    This pamphlet provides you with the basic configuration steps required to get the BIG-IP system up and running in the network.

The following guides are available in PDF format from the AskF5SM web site, These guides are also available from the first web page you see when you log in to the administrative web server on the BIG-IP system.

  • Platform Guide
    This guide includes information about the BIG-IP system. It also contains important environmental warnings.
  • Installation, Licensing, and Upgrades for BIG-IP Systems
    This guide provides detailed information about installing upgrades to the BIG-IP system. It also provides information about licensing the BIG-IP system software and connecting the system to a management workstation or network.

Introducing the Configuration utility

The Configuration utility is a web-based application that you use to configure and monitor the Link Controller. Using the Configuration utility, you can define the load balancing configuration along with the network setup, including data centers, synchronization groups, and servers used for load balancing and path probing. In addition, you can configure advanced features such as topology settings and SNMP agents. The Configuration utility also monitors network traffic, current connections, load balancing statistics, performance metrics, and the operating system itself. The home screen of the Configuration utility provides convenient access to downloads such as the SNMP MIB, and documentation for third-party applications such as ZebOS.

Configuration utility components

The Configuration utility consists of three main components:

  • The navigation pane
    This component contains the following tabs: the Main tab, which allows you to select the area of your network (global, local, and so on); and the Help tab, which displays online help relevant to the main screen.
  • The menu bar
    The content of this component changes depending on what you select on the Main tab in the navigation section. Through the menu bar, you can access more detailed aspects of a given network component.
  • The active screen
    The active screen changes depending on what you select on the Main tab in the navigation section. Through the active screen you configure the different aspects of the Link Controller.

It is important to note that the Link Controller often co-exists with other BIG-IP system modules, such as a Local Traffic Manager. Consequently, you might see features in the Configuration utility that are not described in this guide. See Finding help and technical support resources for a list of other guides that will help you learn about the BIG-IP solution for your enterprise.

Browser support

The Configuration utility, which provides web-based access to the Global Traffic Manager configuration and features, supports the following browser versions:

  • Netscape® Navigator® 4.7X
  • Microsoft® Internet Explorer®, version 5.0, 5.5, or 6.0

Stylistic conventions in this document

To help you easily identify and understand certain types of information, this documentation uses the following stylistic conventions.

Using the solution examples

All examples in this documentation use only private IP addresses. When you set up the solutions we describe, you must use IP addresses suitable to your own network in place of our sample IP addresses.

Identifying new terms

When we first define a new term, the term is shown in bold italic text. For example, a wide IP is a mapping of a fully-qualified domain name to one or more pools of virtual servers that host the domain's content.

Identifying references to products

We refer to all products in the BIG-IP product family as BIG-IP systems. We refer to the software modules by their name; for example, we refer to the Link Controller module as simply the Link Controller. If configuration information relates to a specific hardware platform, we note the platform.

Identifying references to objects, names, and commands

We apply bold text to a variety of items to help you easily pick them out of a block of text. These items include web addresses, IP addresses, utility names, and portions of commands, such as variables and keywords. For example, the nslookup command requires that you include at least one <ip_address> variable.

Identifying references to other documents

We use italic text to denote a reference to another document. In references where we provide the name of a book as well as a specific chapter or section in the book, we show the book name in bold, italic text, and the chapter/section name in italic text to help quickly differentiate the two. For example, you can find information about the Local Traffic Manager in the Configuration Guide for BIG-IP® Local Traffic Management. For more information, see Introducing the BIG-IP system .

Identifying command syntax

We show actual, complete commands in bold Courier text. Note that we do not include the corresponding screen prompt, unless the command is shown in a figure that depicts an entire command line screen. For example, the following command sets the Link Controller load balancing mode to Round Robin:

lb_mode rr

Table 1.1 explains additional special conventions used in command line syntax.

Table 1.1 Command line conventions used in this manual
Item in text
Continue to the next line without typing a line break.
< >
You enter text for the enclosed item. For example, if the command has <your name>, type in your name.
Separates parts of a command.
[ ]
Syntax inside the brackets is optional.
Indicates that you can type a series of items.


Finding help and technical support resources

You can find additional technical documentation and product information using the following resources:

  • Online help for the Link Controller
    The Configuration utility has online help for each screen. The online help contains descriptions of each control and setting on the screen. Click the Help tab in the left navigation pane to view the online help for a screen.
  • Welcome screen in the Configuration utility
    The Welcome screen in the Configuration utility contains links to many useful web sites and resources, including:
    • The F5 Networks Technical Support web site
    • The F5 Solution Center
    • The F5 DevCentral web site
  • F5 Networks Technical Support web site
    The F5 Networks Technical Support web site,, provides the latest documentation for the product, including:
    • Release notes for the Link Controller, current and past
    • Updates for guides (in PDF form)
    • Technical notes
    • Answers to frequently asked questions
    • The AskF5 natural language question and answer engine.

To access this site, you need to register at

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