## TL;DR

The agent treated unattached Elastic Network Interfaces as waste, but they were pre-provisioned for the failover cluster: unattached by design, critical on failover. Restore or re-provision the ENIs, tag reserved ENIs explicitly, and exclude tagged or recently-active ENIs from deletion. Unattached does not mean unused.

## Error

```text
agent deleted unattached ENIs  -  they were reserved for the failover cluster
```

## Steps

1. Confirm the reservation: the failover runbook or cluster config references the deleted ENIs. Expected: the ENIs' purpose is documented.
2. Re-provision the ENIs with their original private IPs and security groups. Expected: the failover cluster's network layout is restored.
3. Tag all reserved ENIs with a protection tag and document their purpose. Expected: the reservation is explicit.
4. Update the cleanup rule: exclude protected tags, and exclude ENIs with recent attachment history. Expected: the rule skips reserved interfaces.
5. Test the failover path to confirm it works with the restored ENIs. Expected: failover is proven functional.

## When to use

- ENI cleanup deletes failover-reserved interfaces.
- Unattached ENIs have a documented purpose.
- Scoping automated ENI deletion.

## When not to use

- The ENIs are genuinely abandoned with no references.
- The failover design no longer needs them.
- The ENIs were never attached to anything meaningful.

## Tool compatibility

AWS EC2 Elastic Network Interfaces (current 2026 releases).

### Variant phrasings

### "unattached" enis were failover spares

Tag reserved interfaces and exclude them.

### eni cleanup broke the dr plan

Re-provision and protect by tag.

### pre-provisioned network interfaces deleted

Unattached is a state, not a verdict.

## Why it happens

Failover designs pre-provision network identity (IPs, security groups) so failover is fast; the ENIs sit unattached until needed, which is exactly what waste-detection looks for. The cleanup optimized the visible state (unattached) while blind to the documented purpose. Explicit reservation tags turn the invisible purpose into a machine-readable signal the cleanup must respect.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_CYg5D0A1f8Tn80AVSh2pGw