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I'm looking at changing the subnets that our RDS instances are currently in.

The subnet group currently has /20 for both a & c zones. I've created new /22 subnets for a & c and associated them for the subnet group, removing the old ones.

I've been running dig on the rds dns name and the IP that is being returned is still from one of the old /20 subnets.

I'm looking for suggestions on how to resolve this so I can remove the old eni's and subnets.

I've tried toggling the "multi-az" on/off as well as forcefully removing the old (and in use) eni (won't detach).

Short of snapshotting the database and restoring using a new subnet group (i.e. downtime) is there anything else I can do?

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You probably figured it out by now, but you have to reboot the RDS. It will force the update. However you still have a small downtime.

Alternatively, a more elegant approach is to create a read replica (in a new subnet group, with the new subnets added to it), add the new endpoint to your db driver's server list & promote it to master. Which, if your db driver supports it, can be without downtime from app standpoint(using for e.g. silent failover or other similar mechanisms).

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