Switch between a target instance and a backend service

You can switch between internal protocol forwarding and Internal TCP/UDP Load Balancing by updating the forwarding rule with the set-target command as described on this page. When switching between internal protocol forwarding and Internal TCP/UDP Load Balancing, you don't need to delete and re-create the forwarding rule. The forwarding rule continues to use the same internal IPv4 address (whether static or ephemeral).

You can also switch between a target instance and backend service by deleting and re-creating the forwarding rule. However, this method requires a static IPv4 address in order to use the same address with the new forwarding rule. Deleting and re-creating the forwarding rule is the only way to switch between external protocol forwarding and backend service-based external TCP/UDP Network Load Balancing.

The following diagram shows an example using internal protocol forwarding.

Switching between a target instance and a backend service.

A target instance does not specify any protocol–it inherits the protocols and ports of the forwarding rule. However, when switching a forwarding rule to reference a backend service, you must ensure the IP protocol of the forwarding rule and the protocol of the backend service are compatible as shown in the following table:

Forwarding rule protocol	Backend service protocol
TCP	TCP or UNSPECIFIED
UDP	UDP or UNSPECIFIED
L3_DEFAULT For external protocol forwarding only	UNSPECIFIED

To change a forwarding rule from pointing to a target instance to pointing to a regional backend service:

gcloud compute forwarding-rules set-target FORWARDING_RULE \
    --backend-service=BACKEND_SERVICE

You can then transition back to a target instance:

gcloud compute forwarding-rules set-target FORWARDING_RULE \
    --target-instance=TARGET_INSTANCE \
    --target-instance-zone=ZONE