The Link Controller handles name resolution requests at an international level. Consequently, one of the methods you can employ to load balance requests is through the use of topologies. A topology is a set of characteristics that identifies the origin of a given name resolution request. In the Link Controller, topologies belong to one of several categories, including:
In addition to these topology types, the Link Controller also supports regions. A region is a customized collection of topologies. For example, you could create a topology for Denmark, Iceland, Finland, Norway, and Sweden. These topologies could then compose a custom region called Scandinavia.
Through topologies, you can instruct the Link Controller to select a data center or resource based on its physical proximity to the client making the name resolution request. This process helps ensure that name resolution requests are answered and managed in the fastest possible time.
You can instruct the Link Controller to use topologies to load balance name resolution requests across pools at the wide IP level, and across virtual servers at the pool level.
A fictional company, SiteRequest, allows its customers to download applications from its web site. SiteRequest has three data centers: New York, Paris, and Tokyo. To ensure that customers can download their purchased application as quickly as possible, the IT department has decided to create topologies with which to load balance name resolution requests.
The New York data center is chosen as the designated data center for any name resolution requests originating in the western hemisphere. To ensure that these requests go only to the New York data center, the IT department first creates a custom region, called Western Hemisphere, that contains the continents North America and South America. With this custom region created, the next step is to create a topology record for the Link Controller. A topology record is a statement that tells the Link Controller how to handle name resolution requests based on topologies. In this case, the IT department creates the record as follows:
The final step to implement this topology is to configure the corresponding wide IP, www.siterequest.com, to use topology load balancing. See Using topology load balancing in a wide IP for more information.
When you want to load balance connection requests using one or more topologies, you must complete two tasks:
To configure a wide IP or pool to use topology as a load balancing method, see Configuring inbound load balancing, in Chapter 9 .
See Setting up and removing topology records , for more information.
A topology record has several elements: a request source statement, a destination statement, an operator, and a weight.
A request source statement defines the origin of a name resolution request. You can define the origin of a request as one of the following:
A destination statement defines the resource to which the Link Controller directs the name resolution request. The types of resources available for a destination statement are as follows:
You can select one of two operators for both a request source and a destination statement. The is operator indicates that the name resolution request matches the statement. The is not operator indicates that the name resolution request does not match the statement.
The last element of a topology record, called the topology score or weight, allows the Link Controller to evaluate the best resolution option for a DNS request. In the event that a name resolution request matches more than one topology record, the Link Controller uses the record with the highest weight attribute to determine which statement it uses to load balance the request.
As your network changes, you might find that you need to refine your existing topology records, or remove outdated topology records.
For example, the fictional company SiteRequest has an existing topology statement that routes all traffic originating from the United States to the New York data center. Last week, a new data center in Los Angeles came online. One of the results of this new data center is that the topology record that the Link Controller used to direct traffic was obsolete, and needed to be removed.
You can use the Topology load balancing mode to distribute traffic among the pools in a wide IP. To do this, you must have at least two pools configured in the wide IP. With topology load balancing, you send name resolution requests to specific data centers or other resources based on the origin of the request.
Repeat this process for each wide IP as needed.
To further refine the topology load balancing capabilities of the Link Controller, you can create custom topology regions. A region is a customized collection of topologies. For example, you could create a topology for Denmark, Iceland, Finland, Norway, and Sweden. These topologies could than compose a custom region for Scandinavia. Regions allow you to extend the functionality of your topologies by allowing you to define specific geographical regions that have meaning for your network.
You create a custom region by adding one or more region member types to the region member list. The available region member types are as follows:
Once you select a region member type, you then fill in the details about that region member and add it to the region member list. The region member options change based on the region member type that you select. When you have finished adding region members to your new region, the new region becomes an option in the Create Topology screen.
The Link Controller supports additional options that affect how the system load balances name resolution requests. These options are:
The ACL Threshold creates an exclusion list based on the topology record score of a given name resolution request. If the topology record score is lower than the value entered into this option, the name resolution request does not have access to the listed virtual servers. This option is set to 0 by default, which disables it.
The Longest Match option instructs the Link Controller to use the topology statement that most completely matches the source IP address of the name resolution request. For example, two topology statements exist: one that matches a source IP address of 10.0.0.0 and one that matches 10.15.0.0. A name resolution request arrives with a source IP address of 10.15.65.8. With the Longest Match setting enabled, the Link Controller will use the topology statement with 10.15.0.0 because it has the longest, and therefore most complete, match. If this option was disabled, the Link Controller could use either topology statement, depending on factors such as the weight of the statement or the order in which the statements are listed. This option is enabled by default.