Junos devices support various types of tunnels for different purposes 1 2 .

Option B is correct.  Generic Routing Encapsulation (GRE) is a tunneling protocol that can encapsulate a wide variety of network layer protocols inside virtual point-to-point links over an Internet Protocol network 1 .  Junos devices support GRE tunnels 1 .

Option D is correct.  IPsec (Internet Protocol Security) is a protocol suite for securing Internet Protocol (IP) communications by authenticating and encrypting each IP packet of a communication session 1 .  Junos devices support IPsec tunnels 1 .

Option A is incorrect. Spanning Tree Protocol (STP) is not a type of tunnel.  It’s a network protocol designed to prevent loops in a bridged Ethernet local area network 2 .

Option C is incorrect.  While Junos devices do support IP-IP (also known as IP tunneling), it’s not supported on all Junos devices 1 .

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

Default BGP keepalive timer in Junos is 30 seconds

The default keepalive time for BGP is 30 seconds, which means that BGP peers send keepalive messages to each other every 30 seconds to verify the liveness of the BGP session. If no keepalive message is received from a peer within the hold time, which is 90 seconds by default, the peer is considered to be unavailable and the BGP session is closed. The keepalive time and the hold time can be configured manually for each BGP peer or peer group, but they must match on both ends of the BGP session. The keepalive time should be one third of the hold time for optimal performance. Therefore, option C is correct and the other options are incorrect. References:  BGP keepalive messages and hold time - Juniper Networks