MAC limiting protects against flooding of the Ethernet switching table (also known as the MAC forwarding table or Layer 2 forwarding table). This feature is enabled on interfaces (ports). MAC limiting sets a limit on the number of MAC addresses that can be learned on a single Layer 2 access port. The MAC limit value in the EX-series switch's default configuration is five MAC addresses. You can configure the MAC limit on an interface, on an interface in a VLAN, or on all interfaces. You can also specify the action to be taken when the MAC limit is exceeded, such as dropping packets, logging messages, or shutting down the interface.
Additionally, you can configure specific "allowed" MAC addresses for the access port. Any MAC address that is not in the list of configured addresses is not learned. Allowed MAC binds MAC addresses to a VLAN so that the address does not get registered outside the VLAN. References:
[EX] How to control MAC address access through Junos OS for EX Series switches1 Configuring MAC Limiting2 Understanding MAC Limiting and MAC Move Limiting3