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Manual Chapter: Configuration Guide for the BIG-IP® Link Controller: 3 - Defining Links
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3

Defining Links


Introducing links

Before you can load balance inbound and outbound traffic on the Link Controller, you must add at least one link. A link is a network device that connects your internal network with the Internet.

To add a link to the Link Controller, you must configure certain basic properties. These properties include settings such as the router address of the link, as well as the limit thresholds for inbound and outbound traffic. After you establish these basic properties, you can further customize the link through a variety of advanced configuration options.

One of the most critical aspects of adding links to the Link Controller is adding each link to a specialized group within the Link Controller, called the default gateway pool. The Link Controller uses this pool to load balance network traffic that is moving between your private network and the Internet at large.

In general, the tasks you accomplish with links are:

  • Adding links to the Link Controller
  • Adding health monitors to links to track their availability
  • Setting bandwidth restrictions
  • Configuring weighting and billing properties
Note

If your environment requires that the Link Controller operate in a fail-safe or high availability mode, see the section titled Configuring fail-safe in Chapter 13, Setting Up a Redundant System, in the BIG-IP Network and System Management Guide.

Adding links

A link is a network device that connects your internal network with the Internet. Links form the backbone of the Link Controller configuration, as they determine how traffic enters and leaves your network. The load balancing features of the Link Controller require at least two links; however, you can add as many links as you need to manage your inbound and outbound traffic.

To add a link

  1. On the Main tab of the navigation pane, expand Link Controller, and then click Links.
    The main link screen opens.
  2. Click the Create button.
    The New Link screen opens.
  3. In the Name box, type a name for the link.
  4. In the Router Address box, type the IP address that is defined as a member of the default gateway pool. This IP address is also considered the IP address assigned to the router that belongs to the local area network.
  5. In the Uplink Address box, type the IP address that identifies the router on the wide-area network. The Link Controller uses this address to obtain SNMP statistics from the routers WAN interface.
  6. In the Service Provider box, type the name of the Internet Service Provider.
  7. Select the health monitors that the Link Controller uses to determine the availability of the link:
    1. Click the name of the monitor from the Available list within the Health Monitors area.
    2. Then, use the Move [<<] button to add the monitor to the Active list.
      We recommend that you add at least the bigip_link and snmp_link monitors.
  8. Click the Finish button to save your changes.

Adding health monitors

A health monitor is a software utility that tracks the performance and availability of a given link. The Link Controller uses health monitors to ensure that is it selecting the best link when load balancing network traffic. You can add a health monitor when you first configure a link, or you can add it at a later time. For more detailed information on monitors, including how to configure the properties of existing monitors or create a custom monitor for your configuration, see Chapter 8, Configuring Monitors .

To add a health monitor to a link

  1. On the Main tab of the navigation pane, expand Link Controller, and then click Monitors.
    The main link monitor screen opens.
  2. Click the name of the link that you want to modify.
    The properties screen for the link opens.
  3. In the Health Monitors setting, select the appropriate monitors from the Available list, then use the Move [<<] button to move these monitors to the Active list.
    If you are unsure as to which monitors you should add to the link, we recommend that you at least assign the bigip_link and snmp_link monitors.
  4. Click the Update button to save your changes.

Monitoring the Link Controller

One of the first monitors you should add to any link is the bigip_link monitor. This monitor is responsible for gathering metrics and statistical data that the Link Controller gathers from other monitors that are verifying the availability of other network resources and services. We recommend that you at least add the bigip_link monitor to any link that you add to your Link Controller configuration.

For example, the IT department at the fictional company, SiteRequest, is using an FTP and HTTP monitor to track the health of one of a web servers. In addition to these monitors, the IT department also assigns the bigip_link monitor, which accumulates the data the FTP and HTTP monitors gather, allowing the IT department to view statistical and health information on the web server through the Link Controller's Configuration utility.

To add a bigip_link monitor

  1. On the Main tab of the navigation pane, expand Link Controller, and then click Links.
    The main link screen opens.
  2. Click the name of the link that you want to modify.
    The properties screen for the link opens.
  3. In the Health Monitors setting, select the appropriate monitors from the Available list, then use the Move [<<] button to move these monitors to the Active list.
  4. Click the Update button to save your changes.

Monitoring with SNMP

Another option when determining how to monitor the health of network resources, such as a web server, is to use the Simple Network Management Protocol, or SNMP. Tracking the health of a resource through SNMP requires that you:

  • Enable the given resource to send SNMP traps
  • Assign the snmp_link monitor to the given resource in the Configuration utility

The snmp_link monitor is a specialized health monitor that acquires SNMP data that a given resource sends, and makes that information available to you through the Link Controller Configuration utility.

To add an snmp_link monitor

  1. On the Main tab of the navigation pane, expand Link Controller, and then click Links.
    The main link screen opens.
  2. Click the name of the link that you want to modify.
    The properties screen for the link opens.
  3. In the Health Monitors setting, select snmp_link from the Available list, then use the Move [<<] button to move the monitor to the Active list.
  4. Click the Update button to save your changes.

Determining availability requirements

By default, the Link Controller requires that all health monitors assigned to a resource verify that the resource is available for load balancing traffic. However, you can configure the Link Controller so that only a certain number of monitors must verify that the link is available.

To determine availability requirements

  1. On the Main tab of the navigation pane, expand Link Controller, and then click Monitors.
    The main link monitor screen opens.
  2. Click the name of the link that you want to modify.
    The properties screen for the link opens.
  3. From the Configuration list, select Advanced.
    The advanced properties for the link open.
  4. From the Availability Requirements list, select the number of monitors that must identify the link as available.
    You can select All from this list if you want to require that all monitors return that the link is available. Alternatively, you can select At Least to specify a minimum number of monitors that must return that the link is available.
  5. Click the Update button to save your changes.

Setting bandwidth restrictions

Often, the links that you want to manage with the Link Controller have bandwidth restrictions that you want to maintain to ensure that your network is operating in a cost-effective manner. Through the Link Controller, you can establish bandwidth restrictions for each link. When the traffic on a link exceeds a specified value, the Link Controller marks the link as unavailable, and attempts to use another link.

To set bandwidth restrictions for a link

  1. On the Main tab of the navigation pane, expand Link Controller, and then click Links.
    The main link screen opens.
  2. Click the name of the link that you want to modify.
    The properties screen for the link opens.
  3. From the Configuration list, select Advanced.
    The advanced properties for the link open.
  4. In the Traffic Limit section, specify the inbound, outbound, and total traffic limits, using the corresponding options.
    When you specify these limits, you have two choices:
    • None, which instructs the Link Controller to ignore this statistic when determining load balancing options.
    • Up to, which allows you to specify a value, in megabits per second, that denotes the maximum amount of traffic of that type for the link.
  5. Click the Update button to save your changes.

Configuring link weighting and billing properties

With the link weighting and billing properties, you can refine the link load balancing and statistics reporting for the links in your configuration. On the Link Weighting screen, you determine how the Link Controller manages and distributes traffic across the links, using the following settings.

  • Duplex Billing
    If your ISP provider uses duplex billing, you can set the Duplex Billing option so that the statistics and billing report screens on the Link Controller accurately reflect the bandwidth usage for the link. Note that by default, the duplex billing setting is on.
  • Price Weighting
    If you want the Link Controller to load balance traffic based on the cost of the bandwidth, then select the Use Price Weighting option. You can use this weighting option to avoid the costs associated with exceeding your prepaid bandwidth. You can also use this weighting option to direct traffic over the least expensive link first.
  • Ratio Weighting
    If you want the Link Controller to load balance the total traffic to the controller based on a ratio, then select the Use Ratio Weighting option. When you have links of varying bandwidth sizes, you can use this weighting option to avoid oversaturating a smaller link with too much traffic.
Important

You can use either the price weighting option or the ratio weighting option to load balance your link traffic for all of your links. You cannot use both options. Regardless of which weighting option you use, you must use the same weighting option for all links.

Configuring duplex billing for each link

Certain ISPs employ duplex billing when managing the costs of an Internet connection. Duplex billing refers to a billing method in which the current traffic rate is considered to be the maximum of either the inbound or outbound traffic. By default, any links that you add in the Link Controller have the Duplex Billing option enabled.

To configure the duplex billing for a link

  1. On the Main menu of the navigation pane, expand Link Controller, and then click Links.
    The main screen for links opens.
  2. Click the name of one of the links in the link list.
    The properties screen for that link opens.
  3. From the Configuration list, select Advanced.
    The advanced options for the link open.
  4. Check Duplex Billing, located at the bottom of the screen.
  5. Click the Update button to save your changes.

Configuring price weighting for each link

You can set the price weighting that you want the Link Controller to use when load balancing link traffic. The price weighting is based on the billing structure that your ISP uses. The Link Controller load balances traffic to another link if the lowest cost link reaches a threshold that you have set. This helps you control traffic based on the cost the ISP is charging for the bandwidth.

To configure the price weighting for a link

  1. On the Main menu of the navigation pane, expand Link Controller, and then click Links.
    The main screen for links opens.
  2. Click the name of one of the links in the link list.
    The properties screen for that link opens.
  3. From the Configuration list, select Advanced.
    The advanced options for the link open.
  4. From the Weighting list, select Price (Dynamic Ratio).
    A set of options related to price weighting open.
  5. Configure the following cost elements associated with the link:
    • In the Prepaid Segment box, you can type the maximum bandwidth usage you pay for each month, regardless of how much you use. For example, if your ISP charges $1000 for bandwidth usage in the 0 to 1.54 Mbps range, you would type 1540 in the Prepaid Segment, Up to Kbps box. That means that you pay every month for up to 1.54 Mbps even if you do not use the link at all.
    • In the Incremental Segment box, type the bandwidth (in Kbps) and the associated cost of the next pricing tier. For example, if your ISP charges $2200 for bandwidth usage in the 1.54 to 3 Mbps range, you would type 3000 in the Up to Kbps box and 2200 in the Cost box. Click the Add button to add the new cost tier to the configuration. You can add additional cost tiers to the configuration if required.
  6. Click the Update button to save your changes.
Important

If you configure price weighting for one link on the Link Controller, you must configure price weighting for all of the remaining links in the configuration. If you do not, the Link Controller load balances only to the link for which price weighting is defined.

Configuring link ratio weighting for each link

In a configuration in which you have multiple links available to handle inbound and outbound traffic, it is not uncommon for the Link Controller to determine that one or more links are valid options for load balancing traffic. You can control how the Link Controller handles this situation by assigning a link ratio weight for each link. When multiple links are available to handle traffic, the Link Controller takes the value of the link ratio for each link and selects a link based on that value.

For example, the fictional company, SiteRequest, has two links available for handling network traffic during business hours. One link is a T1 line, while the other is a DSL link. To ensure their network traffic flows efficiently, the IT team at SiteRequest sets the link ratio value for the T1 link to 2 and the value for the DSL link to 1. This configuration instructs the Link Controller to send twice as much traffic through the T1 link as through the DSL link.

To configure the link ratio weighting for a link

  1. On the Main menu of the navigation pane, expand Link Controller, and then click Links.
    The main screen for links opens.
  2. Click the name of one of the links in the link list.
    The properties screen for that link opens.
  3. From the Configuration list, select Advanced.
    The advanced options for the link open.
  4. In the Link Ratio box, type a link ratio value.
  5. Click the Update button to save your changes.

Additional considerations for link ratio weighting

If you want to load balance link traffic using link ratio weighting, you need to configure the following settings in addition to specifying the ratio weight on the link.

  • Inbound load balancing
    The inbound load balancing mode, on the wide IP, is Quality of Service. All QOS coefficients should be set to 0, except for the Link Capacity coefficient, which should be any number other than 0.
  • Outbound load balancing
    The outbound load balancing mode, for the router pool (usually the default gateway pool), is Dynamic Ratio.



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