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.ratioparameter, which defaults to 1, and can 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 of 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. For more information on ECS option see Extension Mechanisms for DNS (EDNS) Client Subnet Option Insertion in the VMware NSX Advanced Load Balancer Configuration Guide.
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.
Client requests are directed to the optimal site based on the longitude and latitude of the client relative to the GSLB members. For more information, see Geolocation-based Load Balancing Algorithm for GSLB Members.
If Extension Mechanisms for DNS (EDNS) processing is enabled, the source IP address found in the ECS option will be used.
GSLB service pools have a configuration option to change the fallback behavior when the geolocation algorithm fails.
If fallback 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 instead of the default round robin algorithm.
For more information, see GSLB Site Persistence.
For more information, see Topology-Based GSLB Algorithm.
- Preference order
Preference order algorithm can be used to load balance the traffic among the GSLB pool members.
To use this algorithm, you must assign the preference order to each pool member. Once preference order is assigned, NSX Advanced Load Balancer will direct the traffic to the first available member in the ordered list specified by the user as per the preference order.
The lower the preference order, the higher the preference, which means that the DNS Service chooses the member with the lowest preference that is operationally up.
NSX Advanced Load Balancer supports the selection of n records from m GSLB pools. For more information, see Selecting n Number of Pool Members from m Number of Pools.
Under certain circumstances, it could be ideal to bypass the load-balancing algorithm for certain clients based on certain conditions being met. For more information, see GSLB Site Selection with Fallback and Preferred-Site Options.