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Question 381:

Which CoS component determines how traffic is assigned to a forwarding class based on packet markings?

1. Scheduler
2. Classifier
3. Shaper
4. Rewrite rule

Correct Answer: 2

Explanation:

A CoS classifier determines how incoming traffic is mapped to forwarding classes and, where applicable, loss-priority values. The classifier can examine packet characteristics such as DSCP or other configured fields and assign traffic to the appropriate forwarding class. Once classification occurs, other CoS components can provide differentiated treatment. A scheduler controls how queued traffic receives service, while a shaper regulates transmission rates. A rewrite rule modifies packet markings before transmission. Classification is therefore an important first step in implementing a CoS strategy because it determines how the device identifies and categorizes different types of traffic for subsequent processing.

Question 382:

What is the primary function of a CoS drop profile?

1. To define packet-loss behavior as congestion increases
2. To establish BGP sessions
3. To assign OSPF router IDs
4. To create VXLAN tunnels

Correct Answer: 1

Explanation:

A CoS drop profile defines how packet dropping can occur as queue utilization or congestion increases. It can be associated with queue behavior so that lower-priority traffic may be discarded more aggressively when resources become limited. This helps protect important traffic while managing congestion. Drop profiles work together with concepts such as Packet Loss Priority and scheduler configuration to provide differentiated congestion handling. They do not establish routing protocol sessions or create VXLAN tunnels. Properly designed drop behavior can help prevent a congested queue from treating all traffic equally, allowing critical applications to receive better service during periods when available buffer or bandwidth resources are insufficient.

Question 383:

Which CoS mechanism is primarily used to enforce a traffic-rate limit by taking action when traffic exceeds the configured rate?

1. Policer
2. Scheduler
3. Classifier
4. Routing policy

Correct Answer: 1

Explanation:

A CoS policer enforces a configured traffic-rate limit by monitoring traffic against specified thresholds and taking an appropriate action when traffic exceeds those limits. Depending on the configuration, excess traffic may be dropped or otherwise handled according to the policing behavior. Policing differs from shaping because a shaper generally buffers traffic and controls its transmission rate, while a policer typically acts immediately when traffic exceeds the permitted rate. Classifiers determine traffic categories, and schedulers control how queued traffic is serviced. Policing is useful when administrators need to enforce bandwidth limits for customers, applications, interfaces, or specific traffic classes.

Question 384:

Which statement best describes the difference between traffic shaping and policing?

1. Shaping usually buffers and delays excess traffic, while policing can drop or otherwise act on excess traffic
2. Policing always increases available bandwidth
3. Shaping is used only for OSPF traffic
4. Policing creates additional forwarding classes

Correct Answer: 1

Explanation:

Traffic shaping and policing both control traffic rates, but they generally behave differently when traffic exceeds the configured rate. A shaper can buffer packets and transmit them later at a controlled rate, smoothing traffic bursts. A policer typically measures traffic against a configured rate and takes an immediate action when traffic exceeds the allowed level, which may include dropping packets or changing their treatment. Shaping therefore trades delay and buffering for smoother transmission, while policing is commonly used for enforcement. Neither mechanism creates forwarding classes or is limited to routing protocols. Understanding this difference is important when designing bandwidth-control policies.

Question 385:

Which field in an IPv4 packet is commonly used for DSCP-based CoS classification?

1. IP header DSCP value
2. TCP acknowledgment number
3. Ethernet FCS
4. ARP hardware type

Correct Answer: 1

Explanation:

The DSCP value is carried in the IP header and is commonly used by network devices for differentiated services classification. A Junos CoS classifier can examine DSCP markings and map packets into configured forwarding classes and loss priorities. This allows traffic that has already been marked upstream to receive consistent treatment across the network. The TCP acknowledgment number, Ethernet FCS, and ARP hardware type are not the standard DSCP marking field. DSCP-based classification is particularly useful for applications such as voice and video because traffic can be marked at the edge and then handled according to its class throughout the network.

Question 386:

Which technology allows an Ethernet switch to supply electrical power to a connected compatible device?

1. LLDP
2. PoE
3. LACP
4. RSTP

Correct Answer: 2

Explanation:

Power over Ethernet (PoE) allows a compatible Ethernet switch or other power-sourcing device to deliver electrical power over Ethernet cabling to supported powered devices. Common powered devices include IP phones, wireless access points, and certain network cameras. PoE can simplify deployment because a separate electrical power connection may not be required at the endpoint location. LLDP is primarily used for device discovery and information exchange, while LACP provides link aggregation and RSTP provides loop prevention. PoE is particularly useful in enterprise networks where many powered devices are distributed throughout offices, buildings, or other locations.

Question 387:

What is the primary purpose of LLDP-MED in an enterprise network?

1. To provide enhanced discovery and network policy information for endpoint devices such as IP phones
2. To calculate BGP paths
3. To create OSPF areas
4. To perform NAT translation

Correct Answer: 1

Explanation:

LLDP-MED extends the capabilities of the basic Link Layer Discovery Protocol to support additional information useful for endpoint devices such as IP phones. It can communicate information including network policy details and other device-related capabilities. In voice deployments, LLDP-MED can help an IP phone learn information needed to operate appropriately on the network, depending on the configured environment. LLDP-MED does not calculate BGP paths, create OSPF areas, or perform NAT. It is especially useful in enterprise networks where network infrastructure needs to provide standardized information to phones and other supported endpoint devices.

Question 388:

What is the primary purpose of a voice VLAN?

1. To provide a logical VLAN for voice traffic
2. To replace the IP routing table
3. To create a BGP autonomous system
4. To disable Ethernet switching

Correct Answer: 1

Explanation:

A voice VLAN provides a logical Layer 2 network dedicated to voice traffic, commonly from IP phones. Separating voice traffic from ordinary data traffic can simplify network design and make it easier to apply appropriate security and quality-of-service policies. Voice VLAN information can be provided through mechanisms such as switch configuration and, in suitable environments, LLDP-MED network policy information. A voice VLAN does not replace the routing table, create a BGP autonomous system, or disable switching. Proper voice VLAN configuration is important because IP phones need appropriate VLAN connectivity before voice services can operate reliably across the enterprise network.

Question 389:

Which authentication method uses a device’s MAC address as an identity when performing network access control?

1. MAC-based authentication
2. OSPF authentication
3. BGP authentication
4. DHCP relay authentication

Correct Answer: 1

Explanation:

MAC-based authentication uses the endpoint’s MAC address as part of the authentication or access-control process. It can be useful for devices that do not support traditional 802.1X authentication, allowing network access to be controlled based on known device identifiers. The switch can use the MAC address to query an authentication server or apply configured access-control behavior. This method is different from 802.1X, where the endpoint generally participates in an authentication exchange using an authentication supplicant. MAC-based authentication can therefore provide a practical alternative for devices such as certain printers, phones, or specialized equipment that cannot perform standard 802.1X authentication.

Question 390:

What is the primary role of RADIUS in an 802.1X deployment?

1. To provide centralized authentication and authorization services
2. To forward Ethernet frames between switches
3. To calculate OSPF shortest paths
4. To assign VXLAN VNIs

Correct Answer: 1

Explanation:

RADIUS can provide centralized authentication and authorization services in an 802.1X network-access-control deployment. The switch acts as an authenticator and communicates with the RADIUS server to validate credentials or authentication information supplied by an endpoint. Based on the server’s response, the switch can permit or deny network access and may apply additional authorization information. Centralizing authentication makes it easier for administrators to manage access policies across many switches. RADIUS does not perform Ethernet switching, OSPF path calculation, or VXLAN VNI assignment. It is an important component of enterprise access-control architectures where identity-based network access is required.

Question 391:

What is a common purpose of a guest VLAN in an access-control environment?

1. To provide limited network access to unauthenticated or guest devices
2. To carry only OSPF control traffic
3. To replace the management routing table
4. To provide BGP route reflection

Correct Answer: 1

Explanation:

A guest VLAN can provide limited network connectivity to devices that have not successfully completed the required authentication process or that are classified as guests. The exact behavior depends on the access-control configuration, but the purpose is generally to place such devices into a controlled network segment rather than granting them normal authenticated access. This can be useful for providing restricted Internet access or other limited services while protecting internal resources. A guest VLAN is not specifically designed for OSPF control traffic, BGP route reflection, or replacing a routing table. Proper security policies should still be applied to control what guest devices can reach.

Question 392:

Which Junos command is useful for viewing learned Ethernet switching MAC addresses?

1. show ethernet-switching table
2. show ospf database
3. show bgp neighbor
4. show system uptime

Correct Answer: 1

Explanation:

The Junos show ethernet-switching table command displays Ethernet switching information, including learned MAC addresses and their associated interfaces or VLANs. This command is useful when troubleshooting Layer 2 connectivity problems because it helps determine whether the switch has learned the expected MAC address. If a MAC address is missing, administrators can investigate interface status, VLAN configuration, traffic flow, or other Layer 2 issues. The other commands serve different purposes: show ospf database examines OSPF LSAs, show bgp neighbor examines BGP peers, and show system uptime provides system runtime information. MAC-table visibility is therefore fundamental to Ethernet-switching troubleshooting.

Question 393:

What happens when a switch receives a frame destined for an unknown unicast MAC address?

1. It normally floods the frame within the appropriate VLAN
2. It always sends the frame to the default gateway
3. It converts the frame into an OSPF packet
4. It automatically drops every unknown unicast frame

Correct Answer: 1

Explanation:

When a switch receives a frame for an unknown unicast destination MAC address, it normally floods the frame out appropriate ports within the same VLAN, except the port on which the frame was received. This behavior allows the switch to locate the destination device when its MAC address has not yet been learned. Once the destination MAC address is learned, subsequent frames can normally be forwarded only toward the associated interface. Unknown-unicast behavior can be controlled or restricted by specific switch features and configurations, but normal Ethernet switching relies on flooding to discover destinations. This mechanism is different from routing, where IP destinations are handled using Layer 3 forwarding information.

Question 394:

Which protocol is commonly used to prevent Layer 2 loops in Ethernet switched networks?

1. STP
2. BGP
3. OSPF
4. RADIUS

Correct Answer: 1

Explanation:

Spanning Tree Protocol (STP) is designed to prevent Layer 2 forwarding loops in Ethernet networks that contain redundant paths. Without loop-prevention mechanisms, redundant links can cause frames to circulate indefinitely, resulting in broadcast storms, MAC-table instability, and severe network disruption. STP logically blocks selected redundant paths while keeping them available as potential backup paths. If the active topology changes, a previously blocked path can become part of the forwarding topology. BGP and OSPF are routing protocols operating at Layer 3, while RADIUS provides authentication services. STP is therefore the appropriate protocol for preventing Layer 2 loops in traditional Ethernet switching environments.

Question 395:

What is a major improvement provided by Rapid Spanning Tree Protocol compared with traditional STP?

1. Faster convergence after topology changes
2. Automatic BGP route reflection
3. IPv6 address assignment
4. Centralized user authentication

Correct Answer: 1

Explanation:

Rapid Spanning Tree Protocol (RSTP) improves upon traditional STP primarily by providing faster convergence after topology changes. When a link or switch fails, RSTP can transition appropriate ports toward forwarding more quickly than traditional STP mechanisms. Faster convergence reduces the period during which network connectivity may be disrupted. RSTP still performs the fundamental Layer 2 loop-prevention role associated with spanning-tree protocols. It does not provide BGP route reflection, IPv6 address assignment, or user authentication. In enterprise networks with redundant Ethernet links, faster convergence can be especially valuable because it helps restore connectivity more rapidly after a topology change.

Question 396:

What is the primary purpose of LACP when used with aggregated Ethernet interfaces?

1. To negotiate and maintain link aggregation
2. To assign OSPF areas
3. To authenticate users
4. To perform DNS resolution

Correct Answer: 1

Explanation:

Link Aggregation Control Protocol (LACP) is used to dynamically negotiate and maintain a link aggregation group between compatible devices. Multiple physical Ethernet links can be combined into a logical aggregated interface, providing increased aggregate bandwidth and redundancy. LACP helps verify that member links can participate in the aggregation and manages the operational state of the group. This provides advantages over manually configuring an aggregation without protocol assistance. LACP is unrelated to OSPF area assignment, user authentication, or DNS resolution. In Junos, aggregated Ethernet interfaces such as ae0 can use LACP to manage their member links and provide resilient connectivity.

Question 397:

Which interface type commonly represents an aggregated Ethernet interface in Junos?

1. lo0
2. ae0
3. fxp0
4. irb.0

Correct Answer: 2

Explanation:

In Junos, an aggregated Ethernet interface is commonly represented using an ae interface, such as ae0. Multiple physical Ethernet interfaces can be configured as member links of the aggregated interface. LACP may be used to dynamically manage the aggregation when supported and configured on both sides. Other interface names serve different purposes. lo0 is commonly used as a loopback interface, fxp0 may be associated with management functions on supported platforms, and irb interfaces provide integrated routing and bridging functionality. Understanding Junos interface naming is useful when configuring and troubleshooting link aggregation and determining which logical interface represents the combined physical links.

Question 398:

Which Junos command provides detailed operational information about an interface, including extensive statistics?

1. show interfaces extensive
2. show route summary
3. show ospf database
4. show bgp summary

Correct Answer: 1

Explanation:

The Junos show interfaces extensive command provides detailed operational information about interfaces. Depending on the platform and interface, the output can include administrative and operational states, packet and byte counters, error statistics, physical information, and other interface details. This makes the command useful for troubleshooting connectivity, performance, and physical or logical interface problems. show route summary focuses on routing-table information, show ospf database displays OSPF LSDB information, and show bgp summary provides BGP session information. When an interface appears to be experiencing errors or unexpected traffic behavior, detailed interface statistics can provide valuable diagnostic information.

Question 399:

Which Junos command provides a concise summary of interface operational and administrative status?

1. show interfaces terse
2. show configuration
3. show ospf neighbor
4. show system processes extensive

Correct Answer: 1

Explanation:

The show interfaces terse command provides a concise overview of interface status. It is useful for quickly checking whether interfaces are administratively enabled and operationally up or down. The command can display logical and physical interface information in a compact format, making it convenient during initial troubleshooting. For more detailed information such as interface counters, errors, and physical details, an administrator can use a more extensive interface command. show configuration displays configuration statements, while show ospf neighbor focuses on OSPF relationships. A quick interface-status check with show interfaces terse is often a useful first step when investigating connectivity problems.

Question 400:

Which Junos operational command can be used to verify the system’s current uptime?

1. show system uptime
2. show system users
3. show chassis hardware
4. show route

Correct Answer: 1

Explanation:

The Junos show system uptime command displays information about how long the system has been running since its most recent boot or restart. This information can be useful when troubleshooting unexpected device restarts, checking whether a recent reboot occurred, or correlating system uptime with network events. Administrators can compare the uptime information with system logs to determine whether an outage or protocol change occurred around a reboot. show system users provides user-session information, show chassis hardware displays hardware details, and show route provides routing information. System uptime is therefore a simple but useful operational metric during Junos troubleshooting.