An Application Layer Gateway (ALG) is a software component that is designed to manage specific protocols such as Session Initiation Protocol (SIP) or FTP on Juniper Networks devices running Junos OS. The ALG module is responsible for Application-Layer aware packet processing on switches1. The ALG can perform various functions such as modifying the payload and header of packets, opening secondary connections, translating addresses and ports, and applying security policies1. The ALG does not use software processes for permitting or disallowing specific IP address ranges, as this is the function of firewall filters or security zones2. The ALG does not use software that is used by a single TCP session using the same port numbers as the application, as this is the definition of a stateful firewall3. The ALG does not contain protocols that use one application session for each TCP session, as this is the characteristic of some application protocols such as HTTP or SMTP4. References:

• 1: ALG Overview | Junos OS | Juniper Networks
• 2: Firewall Filters Overview | Junos OS | Juniper Networks
• 3: Stateful Firewall Overview | Junos OS | Juniper Networks
• 4: Application Layer Protocols | Junos OS | Juniper Networks

Policy Enforcer is a Junos Space Security Director component that allows updated security policies to be deployed across Juniper SRX Series firewalls, MX Series 5G Universal Routing Platforms, EX Series Ethernet Switches, QFX Series Switches, and third-party network devices1. Policy Enforcer can leverage the DDoS protection feature of Juniper devices to detect and mitigate DDoS attacks on the network. The DDoS protection feature is based on two main components: the classification of host-bound control plane traffic and a hierarchical set of individual- and aggregate-level policers that cap the volume of control plane traffic that each protocol type is able to send to the Routing Engine (RE) for processing2. The DDoS protection feature is supported on MX Series routers and QFX Series switches, among other devices3. Therefore, the correct devices to use for DDoS protection with Policy Enforcer are MX and QFX.

The other options are not correct for the following reasons:

• vQFX is a virtual switch that emulates the QFX Series switches for testing and development purposes. It does not support the DDoS protection feature4.
• vMX is a virtual router that emulates the MX Series routers for testing and development purposes. It does not support the DDoS protection feature.

References: Policy Enforcer DDoS Protection Case Study Protection against distributed denial of service (DDoS) attacks vQFX10000 Overview [vMX Overview]

• Cluster failovers occur when one node in a chassis cluster becomes inactive or unreachable, and the other node takes over the processing of traffic and services. To control when cluster failovers occur, you can configure two specific parameters on an SRX Series device: heartbeat-interval and heartbeat-threshold12.
• Heartbeat-interval is the time interval, in milliseconds, between heartbeat messages sent by each node to the other node. The default value is 1000 ms. You can configure the heartbeat-interval value from 100 through 3000 ms1.
• Heartbeat-threshold is the number of consecutive heartbeat messages that can be missed before a node is considered to be down. The default value is 3. You can configure the heartbeat-threshold value from 2 through 2551.
• The combination of heartbeat-interval and heartbeat-threshold determines the failover time, which is the maximum time it takes for a node to detect the failure of the other node and initiate a failover. The failover time is calculated as follows: failover time = heartbeat-interval x heartbeat-threshold1.
• For example, if the heartbeat-interval is 1000 ms and the heartbeat-threshold is 3, the failover time is 3000 ms. This means that if a node does not receive three consecutive heartbeat messages from the other node within 3000 ms, it will assume that the other node is down and initiate a failover1.
• You can configure the heartbeat-interval and heartbeat-threshold parameters using the set chassis cluster redundancy-group group-id node node-id priority priority heartbeat-interval interval heartbeat-threshold threshold command1.

References:

• 1: Configuring Chassis Cluster Redundancy Group Properties | Junos OS | Juniper Networks
• 2: Example: Configuring an Active/Passive Cluster Deployment | Junos OS | Juniper Networks