Once a particular pool has been selected, a GSLB algorithm (as indicated in the GslbPool.algorithm parameter) balances load across the pool’s member services.

Round robin

Traffic is distributed evenly across all members and the traffic can optionally be skewed by weight. These are implied by the value of the member’s GslbPoolMember.ratio parameter, which defaults to 1, and may range from 1 to 20. For example, if virtual services A, B, and C have ratios of 1, 2, and 3 respectively, virtual service A will receive one-sixth, B will get one-third, and C will get one-half the load.

Consistent hash

Load is distributed based on the client’s source-IP address (likely a DNS resolver address). If Extension Mechanisms for DNS (EDNS) processing is enabled, the source IP address will be found in the ECS option. An integer mask ranging from 1 to 31 can be applied to the client IP address, if there are multiple local DNS in a given network, in one site. This algorithm can provide persistence. GSLB Site Cookie Persistence is the other.

Geolocation-based

Client requests are directed to the optimal site based on the longitude and latitude of the client relative to the GSLB members. Refer to Geolocation-based Load Balancing Algorithm for GSLB Members for details.

If Extension Mechanisms for DNS (EDNS) processing is enabled, the source IP address found in the ECS option will be used.

Topology-based algorithm

For more information, refer to Topology-Based GSLB Algorithm.

NSX Advanced Load Balancer supports the selection of n records from m GSLB pools. Refer to the Selecting n Number of Pool Members from m Number of Pools for more details.

Refer to Load Balancing Algorithm for more details.

Fallback Algorithm for GSLB in case of Geolocation Algorithm

GSLB service pools have a configuration option to change the fallback behavior, when the geolocation algorithm fails.

If fall back algorithm is not configured, the round-robin method is used as a default fallback algorithm.

Consistent hash method can be selected as the fallback algorithm for GSLB pool selection in addition to the default round robin algorithm.

Note:
  • It applies only to GSLB service pools having the following configuration:

    • Geolocation-based Load Balancing Algorithm for GSLB Members is configured as the main GSLB algorithm.

      In the geolocation-based load balancing method, a GSLB pool is selected based on its geolocation.

    • Consistent hash is configured as the fallback algorithm for the GSLB method. If the fallback algorithm is not selected, then the fallback continues to be round robin.

      In the consistent hash load balancing method, traffic is distributed based on the client’s source IP address (a DNS resolver address), unless EDNS processing is ON. In case EDNS processing is enabled, the source IP address is found in the ECS option. An integer mask ranging from 1 to 31 can be applied on the client IP address if there are multiple local DNSs in a given network on one site.

Refer to NSX Advanced Load Balancer GSLB Architecture and Terminologies for more details on GSLB algorithm.