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Manual Chapter: Configuration Guide for the BIG-IP® Link Controller: Synchronizing Link Controllers
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14

Synchronizing Link Controllers


Introducing synchronization

The primary goal of the Link Controller is to ensure that network traffic flows as efficiently and as cost-effectively as possible. Often, Link Controllers are installed in pairs on a given network segment, ensuring that if one Link Controller should fail, another is available to prevent network downtime. In environments where two Link Controllers are on the same subnet, you can configure these Link Controllers so that a change made to one Link Controller automatically transfers to the other. This process is called synchronization.

In network configurations that contain more than one Link Controller, synchronization means that each Link Controller regularly compares the timestamps of its configuration files with each other. If, at any time, a system discovers that its configuration files are too old, it will automatically transfer the newest configuration files to itself. With synchronization, you can change settings on one system and have that change distributed to all other systems.

You can separate the Link Controllers on your network into separate groups, called synchronization groups. A synchronization group is a collection of multiple Link Controllers that share and synchronize configuration settings. These groups are identified by a synchronization group name, and only systems that share this name will share configuration settings. These synchronization groups allow you to customize the synchronization behavior. For example, the Link Controllers residing in data centers in Europe might belong to one synchronization group, while the systems in North America belong to another group.

You interact with the Link Controller's synchronization settings in a variety of ways. You can:

  • Define NTP servers
  • Activate synchronization
  • Control file synchronization
  • Synchronize DNS zone files
  • Create synchronization groups
Note

If your network includes both Link Controllers and Global Traffic Managers, we highly recommend you review the section, Synchronizing Link Controllers and Global Traffic Managers , to ensure that you understand how these two products handle synchronization when they share the same network.

Defining NTP servers

Before you can synchronize Link Controllers, you must define the Network Time Protocol (NTP) servers that the Link Controller references. These servers ensure that the each Link Controller is referencing the same time when verifying timestamps for configuration files.

To define an NTP server

  1. On the Main tab of the navigation pane, expand System and then click General Properties.
    The General Properties screen appears.
  2. From the Device menu, choose NTP.
    The NTP screen appears.
  3. In the Address box, type either the IP address or fully-qualified domain name for the time server.
  4. Click the Add button to add the NTP server to your configuration.
    The time server appears as an entry in the Time Server List.
  5. Click the Update to save your changes.

Repeat this process for any additional time servers.

Activating synchronization

Activating synchronization for the Link Controller has an immediate affect on its configurations, provided that another Link Controller is already available on the network. We recommend that you activate synchronization after you have finished configuring one of the systems.

To activate synchronization

  1. On the Main tab of the navigation pane, expand System and then click General Properties.
    The general properties screen opens.
  2. On the menu bar, choose General from the Global Traffic menu.
    The general global properties screen opens.
  3. Check the Synchronization check box.
  4. Click the Update button to save your changes.

Controlling file synchronization

When you opt to synchronize multiple Link Controllers, you are instructing each system to share its configuration files with the other systems on the network. These files are synchronized based on their timestamp: if a Link Controller determines that its configuration files are older than those at another system, it acquires the newer files and begins using them to load balance name resolution requests.

You can control the synchronization by defining the maximum age difference between two sets of configuration files. This value is referred to as synchronization time tolerance.

By default, the value for the synchronization time tolerance is set to 10 seconds. The minimum value you can set for this value is 5 seconds, while the maximum you can set is 600 seconds.

To control file synchronization

  1. On the Main tab of the navigation pane, expand System and then click General Properties.
    The general properties screen opens.
  2. On the menu bar, choose General from the Global Traffic menu.
    The general global properties screen opens.
  3. In the Synchronization Time Tolerance box, type the maximum age difference, in seconds, between two sets of configuration files.
  4. Click the Update button to save your changes.

Deactivating file synchronization

In the event that you need to deactivate file synchronization, you can do so at any time. Situations in which you want to disable synchronization include updating the data center in which the Link Controller resides, or when you are testing a new configuration change.

To deactivate file synchronization

  1. On the Main tab of the navigation pane, expand System and then click General Properties.
    The general properties screen opens.
  2. On the menu bar, choose General from the Global Traffic menu.
    The general global properties screen opens.
  3. Clear the Synchronization check box.
  4. Click the Update button to save your changes.

Creating synchronization groups

Each Link Controller that you synchronize must belong to a specific group of systems, called a synchronization group. A synchronization group is a collection of multiple Link Controllers that share and synchronize configuration settings. Initially, when you enable synchronization for a Link Controller, the system belongs to a synchronization group called Default. However, you can create new groups at any time. This process allows you to customize the synchronization process, ensuring that only certain sets of Link Controllers share configuration values.

To illustrate how synchronization groups work, consider the fictional company, SiteRequest. SiteRequest has decided to add a new data center in Los Angeles. As part of bringing this data center online, SiteRequest has decided that it wants the Link Controllers installed in New York and in Los Angeles to share configurations, and the Paris and Tokyo data centers to share configurations. This setup exists because SiteRequest's network optimization processes require slightly different settings within the United States than the rest of the world. To accommodate this new network configuration, SiteRequest enables synchronization for the New York and Los Angeles data centers, and assigns them a synchronization group name of United States. The remaining data centers are also synchronized, but with a group name of Rest Of World. As a result, a configuration change at the Paris Link Controller immediately modifies the Tokyo system, but does not affect the systems in the United States.

Note

When you change the synchronization group name for a group, you must manually change it for each system within the synchronization group, as this value does not synchronize. In addition, synchronization stops for any systems with names that do not match.

To create a synchronization group

  1. On the Main tab of the navigation pane, expand System and then click General Properties.
    The general properties screen opens.
  2. On the menu bar, choose General from the Global Traffic menu.
    The general global properties screen opens.
  3. In the Synchronization Group Name box, type a name of either an existing synchronization group, or a new group.
  4. Note: If your network includes Global Traffic Managers as well as Link Controllers, they cannot share the same synchronization name. Doing so could cause unintended results.
  5. Click the Update button to save your changes.

Synchronizing Link Controllers and Global Traffic Managers

It is possible for a network to contain both Link Controllers and Global Traffic Managers. You must take care when implementing synchronization in such network configurations, because the synchronization feature treats both Link Controllers and Global Traffic Manager as the same. However, the Global Traffic Manager has a larger set of responsibilities than the Link Controller. As a result, if you do not use caution when implementing synchronization, you could configure a Global Traffic Manager in a way that is invalid for a Link Controller. This issue is especially relevant when configuring Wide IPs, as the Global Traffic Manager works with wide IPs at a more detailed level than the Link Controller.

When implementing synchronization in a network that has both Link Controllers and Global Traffic Managers, we recommend that you remember the following:

  • The synchronization group name for your Link Controllers should be different than the synchronization group name for your Global Traffic Managers. This setup ensures that the Link Controllers are synchronized separately from changes made to any Global Traffic Managers on the network.
  • If, through synchronization, a Link Controller receives a wide IP configuration that it cannot resolve, that wide IP appears in the Link Controller with a gray status code. This code indicates that the Link Controller knows of the wide IP, but does not have the ability to modify it.



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