In Google Cloud’s Zonal Network Endpoint Groups (NEGs), GCE_VM_IP serves as an endpoint identifier. It specifically points to a Compute Engine VM’s network interface, utilizing the primary internal IPv4 address of that interface to identify the interface itself, rather than merely an IP address.
Traditional Instance Group: When you target a VM by its primary IP, you are targeting the entire VM’s network stack, which implicitly includes its primary network interface.
GCE_VM_IP Endpoint’s “Network Interface Itself”: The distinction here becomes critical when a VM might have multiple network interfaces. While less common for basic setups, a VM can have several network interfaces, each with its own IP address, connected to different VPC networks or subnets.
Example:
If your VM has nic0 (IP 10.0.0.5/24) and nic1 (IP 10.1.1.5/24), and you want to direct traffic to services specifically on nic1 because it belongs to a particular network segment or has unique routing, then using nic1 directly as a GCE_VM_IP endpoint ensures the load balancer targets only that interface. This allows for more granular targeting within a VM.
Why use a GCE_VM_IP endpoint when an instance group seems sufficient?
Granular Control over Multi-NIC VMs: This is the primary reason. If you’ve got complex virtual machines with lots of network interfaces connected to different networks or subnets, GCE_VM_IP lets you set up load balancing pools that point to specific VM interfaces. Usually, an instance group will go for the primary network interface (nic0) by default.
Example: You have a security appliance VM that acts as a firewall, and it has nic0 for management and nic1 for processing user traffic. You can use a GCE_VM_IP NEG to load balance traffic specifically to nic1.
Diversified Backends: Instance groups typically consist of homogeneous instances, meaning they are all configured similarly. However, Network Endpoint Groups (NEGs), particularly with GCE_VM_IP endpoints, offer significant flexibility. They allow you to include the network interface of any Compute Engine VM, regardless of whether it’s part of an instance group. This capability proves highly advantageous for:
Custom Deployments: VMs managed by other orchestration tools, or individual VMs that aren’t part of a uniform scaling group.
Hybrid Setups: While not directly for hybrid (that’s usually Hybrid Connectivity NEGs), the principle of individually specifying endpoints offers similar flexibility to connect disparate components.
Specific Load Balancer Types: As the documentation states, GCE_VM_IP endpoints are intended for Passthrough Network Load Balancers (both Internal and External). These Layer 4 (TCP/UDP) load balancers are not proxying load balancers; they simply forward the client’s original IP and port to the backend.