ENI's registered to Elastic Load Balancer - AWS

Question

When launching an Elastic Load Balancer in AWS, I happened to notice two ENI's get created that reference the ELB. The private IP's (assigned through the VPC subnet) for both of these ENI's appears in the httpd access log on my load balanced back-end instance during periodic health checks.

My questions are:

• Do these ENI's belong to the ELB (or abstracted ELB instances)?
• Are they solely for health check purposes?
• Also, under what circumstance would more than two ENI's be created or do only two ENI's appear no matter the number of instances being monitored (I experimented with a single and dual instances and in both cases, only two ENI's were generated)?

Answer 1

• Do these ENI's belong to the ELB (or abstracted ELB instances)?

ELB is a logical entity and the multiple ENI's belong to multiple concrete instances of the ELB.

• Are they solely for health check purposes?

No

• Also, under what circumstance would more than two ENI's be created or do only two ENI's appear no matter the number of instances being monitored (I experimented with a single and dual instances and in both cases, only two ENI's were generated)?

There can be multiple instances serving the ELB based on load. I think by default ELB's start off with 2 instances for redundancy. If the traffic to the ELB increases, more instances are added to handle the load. When the traffic decreases the instances are removed. This is the Elastic part of ELB! The actual IP address or ENIs of the ELB are not guaranteed at any time. So always use the DNS name of the ELB.