Azure Load Balancer
What it is
Azure Load Balancer is a Layer 4 (transport) load balancer that distributes TCP/UDP traffic across a set of backend virtual machines/instances (e.g. a VM scale set). It operates at the network (L3/L4) layer — no HTTP awareness.
Why it exists
A single VM is a single point of failure and a capacity ceiling. Load Balancer spreads traffic and handles failover/health so a collection of backends appears as one reliable service, without adding latency (it’s a pass-through forwarder).
Key ideas
- Versions: Public LB (receives internet traffic, maps public IP to backends) vs Internal LB (private, inside a VNet, no internet).
- SKUs: Basic (cost-effective, single front-end/regions, no SLA, health probe limited) vs Standard (multi-frontend, across zones/regions, outbound rules, SLA, required for VMSS) — & Gateway (for L3 VXLAN/gateway load balancing).
- Components: frontend (public/private IP), backend pool (VMs/VMSS instances by NIC/IP), health probes (HTTP/TCP/HTTPS), load-balancing rules (port mappings + session stickiness session affinity).
- Layer 4 only — no SSL offload, no path-based routing (that’s application-gateway).
- Outbound NAT — Standard LB provides outbound connectivity for VMs with no public IP (outbound rules).
How it fits
[Internet] ──► [LB public frontend 20.x.x.x]
│ health probe + rule
┌───────┼────────┐
[VM1] [VM2] [VM3] (backend pool, TCP 443)
Diagrams courtesy of Microsoft Learn / Azure docs: load-balancer/load-balancer-overview
Exam notes
- L4 (TCP/UDP) vs application-gateway L7 (HTTP/HTTPS) — the #1 distinction.
- Standard vs Basic: Standard = zones, cross-region, outbound rules, SLA; Basic = single region/zone, no SLA.
- Health probe determines backend availability; session affinity pins a client to a backend.
- Public = internet-facing; Internal = within VNet.
Related
Home · application-gateway · traffic-manager · azure-front-door · vnet · subnet
📘 Source: Microsoft Learn — Load Balancer