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Manual Chapter: Synchronizing Link Controllers
Manual Chapter
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11 
The primary goal of the Link Controller system is to ensure that network traffic flows as efficiently and as cost-effectively as possible. Often, Link Controller systems are installed in pairs on a given network segment, ensuring that if one system should fail, another is available to prevent network downtime. In environments where two Link Controller systems are on the same subnet, you can configure these systems so that a change made to one Link Controller system automatically transfers to the other. This process is called synchronization.
In network configurations that contain more than one Link Controller system, synchronization means that each system regularly compares the timestamps of its configuration files with each other. If, at any time, a system discovers that its configuration files are older than the configuration files on another Link Controller system, it automatically transfers 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 Controller systems on your network into separate groups, called synchronization groups. A synchronization group is a collection of multiple Link Controller systems that share and synchronize configuration settings. These groups are identified by a synchronization group name, and only systems that share this name share configuration settings. These synchronization groups allow you to customize the synchronization behavior. For example, the Link Controller systems residing in data centers in Europe might belong to one synchronization group, while the systems in North America belong to another group.
One exception to this process occurs when you add an additional Link Controller system to the network. In this scenario, there is a chance that the timestamp of the configuration file on the system you are adding to the network is newer than the configuration files on the already-installed Link Controller systems. When you add the new Link Controller system to the network, if you enable synchronization on the new system, the configuration file on the new system is distributed to the existing Link Controller systems, effectively removing the existing configurations. Therefore, you should run the gtm_add script on the new Link Controller system before you enable synchronization on that system.
The gtm_add script acquires the configuration file from an existing Link Controller system and applies it to the new system. As a result, the new system has the current configuration for your network.
Control file synchronization, including running the gtm_add script when you add a new Link Controller system to your network
Note: If your network includes both Link Controller systems and Global Traffic Manager systems, we highly recommend you review the section, Synchronizing Link Controller and Global Traffic Manager systems, to ensure that you understand how these two products handle synchronization when they share the same network.
Before you can synchronize Link Controller systems, you must define the Network Time Protocol (NTP) servers that the systems reference. These servers ensure that the each Link Controller system is referencing the same time when verifying timestamps for configuration files.
1.
On the Main tab of the navigation pane, expand System and then click Configuration.
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 Update to save your changes.
Activating synchronization for the Link Controller system has an immediate effect on its configurations, provided that another Link Controller system is already available on the network. We recommend that you activate synchronization after you have finished configuring one of the systems.
1.
On the Main tab of the navigation pane, expand System and then click Configuration.
The general properties screen opens.
2.
From the Global Traffic menu, choose General.
The general global properties screen opens.
3.
Check the Synchronization check box.
4.
Click the Update button to save your changes.
When you opt to synchronize multiple Link Controller systems, 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 system 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.
1.
On the Main tab of the navigation pane, expand System and then click Configuration.
The general properties screen opens.
2.
From the Global Traffic menu, choose General.
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.
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 system resides, or when you are testing a new configuration change.
1.
On the Main tab of the navigation pane, expand System and then click Configuration.
The general properties screen opens.
2.
From the Global Traffic menu, choose General.
The general global properties screen opens.
3.
Clear the Synchronization check box.
4.
Click the Update button to save your changes.
Each Link Controller system that you synchronize must belong to a specific group of systems, called a synchronization group. A synchronization group is a collection of multiple Link Controller systems 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 Controller systems 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 Controller systems 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 SiteRequests 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 system 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.
1.
On the Main tab of the navigation pane, expand System and then click Configuration
The general properties screen opens.
2.
From the Global Traffic menu, choose General.
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.
Note: If your network includes Global Traffic Manager systems as well as Link Controller systems, they cannot share the same synchronization name. Doing so could cause unintended results.
4.
Click the Update button to save your changes.
When you add an additional Link Controller system to a network in which a Link Controller system already exists, you run the gtm_add script before you activate synchronization on the new system. You run this script because, if you do not you run the risk of having the new Link Controller system, which is unconfigured, replace the configuration of the existing Link Controller systems on the network.
2.
At the command prompt, type gtm_add.
A prompt appears, describing what the gtm_add script does and asking if you are sure you want to run the process.
3.
Press the y key to start the gtm_add script.
The script prompts you for the IP address of the system from which you want to acquire configuration settings.
5.
Press Enter.
At this point, both Link Controller systems share the same configuration. In addition, they also belong to the same synchronization group, because the gtm_add script copied the settings from the existing Link Controller system to the new Link Controller system.
It is possible for a network to contain both Link Controller systems and Global Traffic Manager systems. You must take care when implementing synchronization in such network configurations, because the synchronization feature treats both Link Controller systems and Global Traffic Manager systems as the same. However, the Global Traffic Manager system has a larger set of responsibilities than the Link Controller system. As a result, if you do not use caution when implementing synchronization, you could configure a Global Traffic Manager system in a way that is invalid for a Link Controller system. This issue is especially relevant when configuring wide IPs, as the Global Traffic Manager system works with wide IPs at a more detailed level than the Link Controller system does.
When implementing synchronization in a network that has both Link Controller systems and Global Traffic Manager systems, we recommend that you remember the following:
The synchronization group name for the Link Controller systems should be different than the synchronization group name for the Global Traffic Manager systems. This setup ensures that the Link Controller systems are synchronized separately from changes made to any Global Traffic Manager systems on the network.
If, through synchronization, a Link Controller system receives a wide IP configuration that it cannot resolve, that wide IP appears in the system with a gray status code. This code indicates that the system knows of the wide IP, but does not have the ability to modify it.
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