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

Redundant Trunk Groups (RTGs) on EX Series switches provide a simple solution for network recovery when a trunk port on a switch goes down 1 .  They are configured on the access switch and contain two links: a primary or active link, and a secondary link 1 .  Therefore, option B is correct because if the active link fails, the secondary link automatically starts forwarding data traffic without waiting for normal spanning-tree protocol convergence 1 .

Option D is also correct.  In a typical enterprise network composed of distribution and access layers, RTGs are used where one Access switch is connected to two different uplink switches 2 .  This implies that RTGs must be connected to the same aggregation switch 2 .