The routing table and forwarding table play distinct roles in a Junos OS device. The correct answers are A and D. The routing table (A) is used by the Routing Engine (RE) to select the best route among all the learned routes, while the routing table (D) stores all routes and prefixes learned from all routing protocols. The forwarding table, in contrast, contains only the active routes chosen by the RE and is used by the Packet Forwarding Engine for actual packet forwarding.

At the operational mode prompt on a Junos device, you can use various commands to view the device's status and request system operations. The show interface ge-0/0/0 command is used to display information about a specific interface, while the request system shutdown command is used to properly shut down the device. The set command is used in configuration mode, not operational mode, and the run command is used to execute operational mode commands from configuration mode.

Firewall filters in Junos OS are stateless, meaning they process each packet individually without regard to the state of a connection or sequence of packets. These filters can match various packet attributes, including those at Layer 4, such as TCP and UDP port numbers. This allows for granular control over traffic based on the type of service or application. Unlike stateless filters, stateful firewalls keep track of the state of active connections and make decisions based on the context of the traffic flow, which is not a capability of Junos firewall filters. Additionally, Junos firewall filters primarily operate up to Layer 4 and do not natively inspect Layer 7 parameters, which involve application-level data.

The load-balance per-flow statement in the Junos OS policy-options configuration enables flow-based load balancing in the forwarding table. This means that the traffic is distributed across multiple paths based on flows, where a flow is typically identified by attributes such as source and destination IP addresses, and possibly layer 4 information like TCP/UDP ports. This allows for more granular and efficient utilization of available paths, avoiding overloading a single path. The policy does not enable per-packet load balancing, which would send individual packets of the same flow over different paths, potentially causing out-of-order delivery issues. The policy's placement in the forwarding-table export suggests it's intended to influence forwarding behavior, not just routing protocol decisions, and does not necessarily have to be applied under the protocols hierarchy.

The TCP three-way handshake establishes a reliable connection between two hosts. During this process:

SYN: The client sends a TCP segment with the SYN flag set, including an initial sequence number (ISN) and port information.

SYN-ACK: The server responds with a SYN-ACK, which contains its own sequence number and acknowledgment of the client’s ISN.

ACK: The client replies with an ACK, confirming receipt of the server’s ISN.

Thus, port numbers (to identify the application endpoints) and sequence numbers (to ensure ordered and reliable data transfer) are exchanged.

Interfaces and application names are not exchanged during this process.

From Junos OS Fundamentals / TCP Concepts:

“The TCP three-way handshake establishes a reliable session by exchanging port numbers and sequence numbers between peers. This process synchronizes both sides before data transmission begins.”

The Packet Forwarding Engine (PFE) in Juniper Networks devices is the heart of the data plane, handling the actual forwarding of packets based on pre-computed forwarding tables. It provides several critical services to manage and control traffic flow, including policing (to enforce bandwidth limits for certain traffic types), stateless firewall filtering (to permit or deny traffic based on predefined criteria), and Class of Service (CoS) (to prioritize traffic to ensure quality of service for critical applications). The PFE utilizes both Layer 2 (MAC addresses) and Layer 3 (IP addresses) forwarding tables to make intelligent forwarding decisions, ensuring that packets are efficiently routed toward their final destination.

In Junos OS, when creating a new user account, the minimum required fields are the username and the login class. The username is the identifier for the account, while the login class specifies the level of access or permissions the user has on the device. Login classes allow for the differentiation between various roles, such as read-only access or full administrative rights. Other information, such as full name or user ID, is optional and not strictly necessary for the creation of a functional user account.

Referring to the firewall filter configuration in the exhibit, when a packet matches the specified term limit-icmp, two actions are defined in the then statement: count count-icmp and discard. The count count-icmp action means that each time a packet matches this term, a counter named count-icmp will be incremented, providing a tally of how many packets have matched the term. The discard action means that the packet will be dropped and not forwarded through the device. This effectively prevents the packet from reaching its intended destination. There is no action specified that would cause an ICMP destination unreachable message to be returned, nor is there any action that would allow the packet to be forwarded.

The forwarding table in a network device requires specific information to efficiently forward traffic toward its destination. This includes the next hop IP address, which indicates the next router or device in the path to the destination. The outgoing interface name identifies the physical or logical interface through which the packet should be sent to reach the next hop. Lastly, the next hop MAC address is crucial for Layer 2 forwarding decisions, allowing the device to encapsulate the IP packet in a frame that can be understood by Ethernet or other Layer 2 protocols. OSPF metric values and BGP local preference values are used in the routing decision process to select the best path and populate the forwarding table but are not directly used by the forwarding table to forward traffic.

Junos OS supports several external authentication methods for administrative access, with TACACS+ (Terminal Access Controller Access-Control System Plus) and RADIUS (Remote Authentication Dial-In User Service) being among the most commonly used. Both TACACS+ and RADIUS are protocols that allow network devices to communicate with a central authentication server, enabling centralized control over user authentication and authorization. This centralization simplifies the management of user credentials and access policies, especially in larger networks with multiple devices.

When an interface on a router is configured with an IPv4 address and is in an up state, two types of routes are automatically created in the routing table: a local route and a direct route, making B and D the correct answers. The local route represents the interface's IP address itself, indicating that the router can directly receive packets addressed to this IP. The direct route represents the subnet or network segment to which the interface is connected, indicating that the router can directly forward packets to destinations within this subnet.

In Junos OS, to navigate from operational mode to configuration mode, you can use either the edit or configure command. Both commands move the CLI from operational mode, where you can view the state of the device, to configuration mode, where you can make changes to the device's configuration.

Transit traffic that traverses the forwarding plane of a Layer 3 router includes both unicast and multicast traffic types. Unicast traffic is directed from a single source to a single destination, while multicast traffic is sent from one source to multiple destinations that are part of a multicast group. These types of traffic are efficiently routed through the network by leveraging the router's forwarding plane capabilities. Exception traffic, which requires special handling by the control plane, and broadcast traffic, which is typically limited to a single broadcast domain and not usually forwarded by Layer 3 routers, are not considered standard types of transit traffic for the forwarding plane of a router.

In Junos OS, as with many network operating systems, the control plane is responsible for processes that determine how to route traffic. This includes maintaining routing tables, which store information about network paths and protocols. Therefore, routing tables are indeed stored in the control plane.

Exception traffic refers to packets that cannot be processed by the normal fast-path processing of the Packet Forwarding Engine (PFE) in the forwarding plane, and thus are sent to the control plane for further processing. This might include packets destined for the router itself, packets that need to be fragmented, or packets that match certain firewall filter criteria, among other reasons.

Routing tables are not stored in the forwarding plane. However, the forwarding plane contains the forwarding table (sometimes referred to as the forwarding information base or FIB), which is a distilled version of the routing table optimized for fast packet forwarding. The forwarding plane uses this information to perform the actual transfer of packets across the network device interfaces.

Junos OS supports multiple methods for automatically archiving configurations upon commit. Two of the supported methods are SCP (Secure Copy Protocol) and RCP (Remote Copy Protocol). These methods can be configured to save the configuration files to a remote server automatically whenever a commit is made.

The candidate configuration in Junos OS is a temporary configuration that allows network administrators to make and stage multiple configuration changes before applying them to the device. This approach enables the deployment of multiple changes in a single operation, ensuring that all configurations work together as intended before making them active. Additionally, the candidate configuration can be discarded if the administrator decides not to apply the staged changes, allowing for a "trial and error" approach without affecting the currently active configuration. This feature provides flexibility and reduces the risk of disruptive changes to the network.

Exception traffic includes traffic that is not simply forwarded by the router but requires special handling, such as routing updates (B) and log messages (C). These types of traffic are processed by the router's control plane rather than just being forwarded through the data plane.