AWS EC2 Load Balancer - Specific is instance simply “not available”?

Question

I've set up a VPC, with a private and public subnet. There are a handful of instances in the VPC that have very little difference between them other than the instance type, and security groups.

Now I want to add a Network Load Balancer, to route some TCP traffic to some of those instances. However, for some unclear reason one of the instances I need to route traffic to simply is not available for Load Balancer targets.

If I go via the Load Balancing -> Load Balancers -> Create Load Balancer -option, choose the correct VPC, the public subnet (and an elastic IP), choose to create a new target group and get to the "Register Targets" -page it simply does not list the instance at all. Searching for the instance by it's instance ID does not help.

If I go via Load Balancing -> Target Groups -> Create Target Group -option, I CAN register the instance to it fine. Then I go to Create Load Balancer and choose "Existing Target Group" the dropdown to choose the target group simply lists the Target Group I created as "not available". It is also listed as "not available" if I remove the only registered instance from it.

I've found no explanation on what this means, I've looked at all the instance settings I can think of, subnets, etc. and I just cannot find out what could possibly be wrong with this and why the registration lists literally every other instance except for this one.

Answer 1

From the network load balancer documentation :

• You cannot register instances by instance ID if they have the following instance types: C1, CC1, CC2, CG1, CG2, CR1, G1, G2, HI1, HS1, M1, M2, M3, and T1. You can register instances of these types by IP address.

• You cannot register targets in a peered VPC or linked through an AWS hardware VPN.