This article outlines the recommended naming conventions for aggregated (AE) interfaces, particularly when integrating new device models into an existing network infrastructure. 

In this scenario, a network currently have a QFX switch (Core) connected to EX1 access switches using aggregated Ethernet interfaces ae0 and ae1. Now, if we plan to introduce EX2 switches on a new floor and connect them to the same QFX. Is it recommended to reuse ae0 and ae1 for the new EX2 links?

AE interfaces group specific physical ports. Reusing an AE ID can lead to conflicts in port-channel definitions. Hence, it is not recommended to reuse AE interface IDs for different devices connected to the same core switch. AE interfaces should be uniquely assigned for each set of links to avoid configuration overlaps and ensure separation of traffic.

Configuring unique AE IDs to avoid LACP or STP inconsistencies and make troubleshooting easier. A scalable naming scheme aids network growth and ensures seamless future upgrades.

As a best practice, assign new AE interface IDs for the new connections, such as ae3 and ae4 (or the next available numbers). For further sample LACP configuration, please refer: https://supportportal.juniper.net/s/article/LACP-and-AE-interface-configuration