this will be the description of the different modes

Topology Load Balancing Mode

Topology is a proximity based load balancing mode that allows you to direct traffic by defining topology records and selecting the Topology load-balancing mode for the wide IP or pool. The Topology mode bases the distribution of requests on the topology records and the weighted scores configured for each record. The topology records direct DNS queries to the closest virtual server, based on geographical information. The BIG-IP DNS system is designed to support an unlimited number of topology records. The only limit is the amount of memory available to the gtmd process.

When planning topology load balancing, you should consider the following factor:

• The BIG-IP DNS system looks up topology records in the order they appear in the Configuration utility and the configuration file. As a result, you should place more-specific topology records toward the top of the topology statement and less-specific records toward the end of the topology statement. You can change the order of existing topology records by clicking Change Order on the Topology Records page.

Topology record format

Topology records are stored in the bigip_gtm.conf file on the BIG-IP DNS system and are used to match a Local Domain Name System (LDNS) request with a specific destination object. A topology record has the following fields: an LDNS request source statement, a destination statement, an operator, and a weight.

LDNS Request Source.

• The LDNS request source statement defines the origin of a name resolution request. For example, the topology statement can be defined to direct an LDNS request from a particular source to a desired destination. The LDNS source can be any of the following elements:
  • Continent
  • Country
  • ISP
  • IP subnet (CIDR format)
  • Region (user-defined)
  • State
  • Geolocation ISP (12.0.0 and later)

Destination

• A destination statement defines the resource to which the BIG-IP DNS system directs the incoming DNS request. For example, the topology statement can be defined to direct an LDNS request that originates from a particular source to any of the following destination elements:
  • Continent
  • Country
  • Data center *
  • ISP
  • IP subnet (CDIR format)
  • Pool of virtual servers
  • Region (User-defined)
  • State
  • Geolocation ISP (12.0.0 and later)

Quote

Note: When using the topology load balancing method at a wide IP level with topology records that have a data center destination, the topology record has no effect. This is because load balancing at a wide IP level selects between BIG-IP DNS pools, and BIG-IP DNS pools do not have a data center associated with them. Such topology records only have an effect when using the topology load balancing method at the pool level.

Operator

An operator statement indicates whether the name resolution request matches the statement. You can select an operator for both a request source and a destination statement. For example, an operator statement can be one of the following:

• is
• is not

Weight (score)

The weight specifies the score that will be given to a destination object which matches the topology record. In the event that a name resolution request matches more than one topology record, the BIG-IP DNS system uses the destination object with the highest weight to determine which statement it uses to load balance the request.

Understanding topology records matching modes and the importance of order

The BIG-IP DNS system orders topology records based on how the records are created and whether Longest Match is enabled. Topology records are passed through in order looking for the first matching record for each possible destination. Each matching record is then evaluated to determine which destination has the highest weight. This means that if a pool contains four pool members there may be up to four matching records (one for each possible destination), at which point the weight of each is evaluated to determine which record to use.

Longest Match (enabled)

The Longest Match option, which is enabled by default, instructs the BIG-IP DNS system to use the topology statement that most completely matches the source IP address of the name resolution request.

The Longest Match option ensures that the BIG-IP DNS system processes topology records as fast as possible. The system achieves this by automatically sorting the topology records so that the records that more specifically match the LDNS IP appear first in the topology list. When the Longest Match option is enabled, the BIG-IP DNS system evaluates all records in the topology statement and uses the record that most completely matches the source IP address of the name resolution request. In the event that a name resolution request matches more than one topology record, the BIG-IP DNS system uses the destination object with the highest weight (score) to determine which statement it uses to load balance the request.

Longest Match (disabled)

If the Longest Match option is not selected, the topology records can be manually ordered, but the method used to evaluate the topology records when choosing a match does not change.