Azure load balancer - n-tier application

Hi,

I have a n-tier application, so the usual roles: web, processing, data. The would all go into a single vnet, but each tier will be deployed as active/passive, in this case using availability zones. 

Subnet 1

Web-1 (active in AZ 1)

Web-2 (passive in AZ2)

Subnet 2

Processing-1 (active in AZ1)

Processing-2 (passive in AZ2)

Subnet 3

Data-1 (active in AZ1)

Data-2 (passive AZ2)

I am planning to use a single Azure Standard LB, which would be in subnet 4 and have 3 front-ends (FE) and 3 backends (BE). There would be a LB rule using HA to map each FE to a BE. Additionally each subnet would be controller with NGS. The question I have is weather using a single Azure LB vs using one for each tier as well as the consideration for availability zones. To explain each app tier has a VM instance in zone 1 (active) and zone 2 (passive), however the LB also has zone redundancy options and I want establish what may be the best option to meet the best HA?