Load Balancing with Avi in VMware Cloud Director

VMware Cloud Director 10.2 is adding network load balancing (LB) functionality in NSX-T backed Organization VDCs. It is not using the native NSX-T load balancer capabilities but instead relies on Avi Networks technology that was acquired by VMware about a year ago and since then rebranded to VMware NSX Advanced Load Balancer. I will call it Avi for short in this article.

The way Avi works is quite different from the way load balancing worked in NSX-V or NSX-T. Understanding the differences and Avi architecture is essential to properly use it in multitenant VCD environments.

I will focus only on the comparison with NSX-V LB as this is relevant for VCD (NSX-T legacy LB was never viable option for VCD environments).

In VCD in an NSX-V backed Org VDC the LB is running on Org VDC Edge Gateway (VM) that can have four different sizes (compact, large, quad large and extra large) and be in standalone or active / standby configuration. That Edge VM also needs to perform routing, NATing, firewalling, VPN, DHCP and DNS relay. Load balancer on a stick is not an option with NSX-V in VCD. The LB VIP must be an IP assigned to one of external or internally attached network interfaces of the Org VDC Edge GW.

Enabling load balancing on an Org VDC Edge GW in such case is easy as the resource is already there. 

In the case of Avi LB the load balancing is performed by external (dedicated to load balancing) components which adds more flexibility, scalability and features but also means more complexity. Let’s dive into it.

You can look at Avi as another separate platform solution similar to vSphere or NSX – where vSphere is responsible for compute and storage, NSX for routing, switching and security, Avi is now responsible for load balancing.

Picture is worth thousand words, so let me put this diagram here first and then dig deeper (click for larger version).


Control Path

You start by deploying Avi controller cluster (highly available 3 nodes) which integrates with vSphere (to use for compute/storage) and NSX-T (for routing LB data and control plane traffic). The controllers would sit somewhere in your management infrastructure next to all other management solutions.

The integration is done by setting up so called NSX-T Cloud in Avi where you define vCenter Server (only one is supported per NSX-T Cloud) and NSX-T Manager endpoints, NSX-T overlay transport zone (with 1:1 relationship between TZ and NSX-T Cloud definition). Those would be your tenant/workload VC/NSX-T.

You must also point to pre-created management network segment that will be used to connect all load balancing engines (more on them later) so they can communicate with the controllers for management and control traffic. To do so, in NSX-T you would set up dedicated Tier-1 (Avi Management) GW with the Avi Management segment connected and DHCP enabled. The expectation is the Tier-1 GW would be able through Tier-0 to reach the Avi Controllers.

Data Path

Avi Service Engines (SE) are VM resources to perform the load balancing. They are similar to NSX-T Edge Nodes in a sense that the load balancing virtual services can be placed on any SE node based on capacity or reservations (as Tier-1 GW can be placed on any Edge Node). Per se there is no strict relationship between tenant’s LB and SE node. SE node can be shared across Org VDC Edge GWs or even tenants. SE node is a VM with up to 10 network interfaces (NICs). One NIC is always needed for the management/control traffic (blue network). The rest (9) are used to connect to the Org VDC Edge GW (Tier-1 GW) via a Service Network logical segment (yellow and orange). The service networks are created by VCD when you enable load balancing service on the Org VDC Edge GW together with DHCP service to provide IP addresses for the attached SEs. It will by default get subnet, but the system admin can change it, if it clashes with existing Org VDC networks. Service Engines run each service interface in a different VRF context so there is no worry about IP conflicts or even cross tenant communication.

When a load balancing pool and virtual service is configured by the tenant Avi will automatically pick a Service Engine to instantiate the LB service. It might even need to first deploy (automatically) an SE node if there is no existing capacity. When SE is assigned Avi will configure static route (/32) on the Org VDC Edge GW pointing the virtual service VIP (virtual IP) to the service engine IP address (from the tenant’s LB service network).

Note: The VIP contrary to NSX-V LB can be almost any arbitrary IP address. It can be routable external IP address allocated to the Org VDC Edge GW or any non-externally routed address but it cannot clash with any existing Org VDC networks. or with the LB service network. If you use an external Org VDC Edge GW allocated IP address you cannot use the address for anything else (e.g. SNAT or DNAT). That’s the way NSX-T works (no NAT and static routing at the same time). So for example if you want to use public address for LB on port 80 but at the same time use it for SNAT, use an internal IP for the LB (e.g. and create DNAT port forwarding rule to it ( to

Service Engine Groups

With the basics out of the way let’s discuss how can service provider manage the load balancing quality of service – performance, capacity and availability. This is done via Service Engine Groups (SEG).

SEGs are (today) configured directly in Avi Controller and imported into VCD. They specify SE node sizing (CPU, RAM, storage), bandwidth restrictions, virtual services maximums per node and availability mode.

The availability mode needs more explanation. Avi supports four availability modes:
A/S … legacy (only two nodes are deployed), service is active only on one node at a time and stand by on the other, no scale out support (service across nodes), very fast failover

A/A … elastic, service is active on at least two SEs, session info is proactively replicated, very fast failover

N+M … elastic, N is number of SE nodes service is scaled over, M is a buffer in number of failures the group can sustain, slow failover (due to controller need to re-assign services), but efficient SE utilization

N+0 … same as N+M but no buffer, the controller will deploy new SE nodes when failure occurs. The most efficient use of resources but the slowest failover time.

The base Avi licensing supports only legacy A/S high availability mode. For best availability and performance usage of elastic A/A is recommended.

As mentioned Service Engine Groups are imported into VCD where the system administrator makes a decision whether SEG is going to be dedicated (SE nodes from that group will be attached to only one Org VDC Edge GW) or shared.

Then when load balancing is enabled on a particular Org VDC Edge GW, the service provider assigns one or more SEGs to it together with capacity reservation and maximum in terms of virtual services for the particular Org VDC Edge GW.

Use case examples:

  • A/S dedicated SEG for each tenant / Org VDC Edge GW. Avi will create two SE nodes for each LB enabled Org VDC Edge GW and will provide similar service as LB on NSX-V backed Org VDC Edge GW did. Does not require additional licensing but SEG must be pre-created for each tenant / Org VDC Edge GW.
  • A/A elastic shared across all tenants. Avi will create pool of SE nodes that are going to be shared. Only one SEG is created. Capacity allocation is managed in VCD, Avi elastically deploys and undeploys SE nodes based on actual usage (the usage is measured in number of virtual services, not actual throughput or request per seconds).

Service Engine Node Placement

The service engine nodes are deployed by Avi into the (single) vCenter Server associated with the NSX Cloud and they live outside of VMware Cloud Director management. The placement is defined in the service engine group definition (you must use Avi 20.1.2 or newer). You can select vCenter Server folder and limit the scope of deployment to list of ESXi hosts and datastores. Avi has no understanding of vSphere host, and datastore clusters or resource pools. Avi will also not configure any DRS anti-affinity for the deployed nodes (but you can do so post-deployment).


The whole Avi deployment process for the system admin is described in detail here. The guide in the link refers to general Avi deployment of NSX-T Cloud, however for VCD deployment you would just stop before the step Creating Virtual Service as that would be done from VCD by the tenant.

Avi licensing is basic or enterprise and is set at Avi Controller cluster level. So it is possible to mix both licenses for two tier LB service by deploying two Avi Controller cluster instances and associating each with a different NSX-T transport zone (two vSphere clusters or Provider VDCs).

The feature differences between basic and enterprise editions are quite extensive and complex. Besides Service Engine high availability modes the other important difference is access to metrics, amount of application types, health monitors and pool selection algorithms.

The Avi usage metering for licensing purposes is currently done via Python script that is ran at the Avi Controller to measure Service Engine total  high mark vCPU usage during a given period and must be reported manually. The basic license is included for free with VCPP NSX usage and is capped to 1 vCPU per 640 GB reported vRAM of NSX base usage.

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