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Question 61:
Which protocol dynamically discovers the MAC address of a device on a local network?
A) ARP
B) DHCP
C) ICMP
D) DNS
Answer:
A)
Explanation:
The correct answer is ARP (A). ARP, or Address Resolution Protocol, is a network protocol used to dynamically discover the Media Access Control (MAC) address of a device when its IP address is known, enabling proper communication within a local area network (LAN). At the data link layer, devices communicate using MAC addresses, which are unique hardware identifiers assigned to network interfaces. However, higher-level protocols such as IP operate using logical addresses. ARP acts as a bridge between these layers, resolving IP addresses into MAC addresses so that data packets can be correctly delivered to their intended destinations.
When a device wants to send a packet to another device on the same subnet, it first checks its ARP cache to see if the MAC address corresponding to the target IP address is already known. If the MAC address is not present, the device broadcasts an ARP request over the local network, essentially asking, “Who has this IP address? Respond with your MAC address.” The device on the network that owns that IP address responds with an ARP reply, providing its MAC address. Once this information is received, the sending device can encapsulate the data packet in a frame with the correct destination MAC address, allowing successful delivery. The MAC address is then stored in the ARP cache for future use, reducing the need for repeated broadcasts and improving network efficiency.
Unlike ARP, DHCP (B) is used for dynamically assigning IP addresses and network configuration parameters to devices on a network, but it does not resolve IP addresses to MAC addresses. DHCP helps devices join a network by providing valid IP addressing and subnet information but does not facilitate immediate MAC address discovery for packet delivery. ICMP (C), or Internet Control Message Protocol, is used for sending diagnostic or control messages, such as ping requests and replies, to report errors or test connectivity. While ICMP can indicate whether a device is reachable, it does not provide MAC address resolution. DNS (D), or Domain Name System, translates human-readable domain names into IP addresses, enabling devices to locate resources on a network or the internet. DNS operates at a different layer and does not involve MAC address discovery.
ARP operates exclusively on the local subnet, as MAC addresses are not routable across networks. For communication outside the local subnet, packets are forwarded to a gateway or router, and ARP is used only for resolving the MAC address of the next-hop device. ARP is fundamental for Ethernet and other broadcast-based LANs, ensuring that devices can communicate effectively without manual MAC address configuration. Additionally, ARP has a defined cache timeout to accommodate changes in network topology and reduce network traffic caused by constant broadcasts.
Question 62:
Which protocol is used to exchange routing information between autonomous systems?
A) BGP
B) OSPF
C) EIGRP
D) RIP
Answer:
A)
Explanation:
Border Gateway Protocol (BGP) is the primary exterior gateway protocol used to exchange routing information between autonomous systems (ASes) on the internet. BGP is a path-vector protocol that selects routes based on multiple attributes such as AS path, origin type, and local preference rather than traditional metrics like hop count or bandwidth.
BGP maintains a loop-free topology by preventing routes learned from one AS from being advertised back to the same AS. It supports policy-based routing, route aggregation, and route filtering, making it essential for large-scale networks.
OSPF, EIGRP, and RIP are interior gateway protocols (IGPs) used within a single AS. CCNA candidates must understand BGP attributes, neighbor establishment, and basic configuration for inter-AS communication, as it forms a foundational concept for internet routing and enterprise WAN connectivity.
Question 63:
Which command displays detailed OSPF neighbor information on a router?
A) show ip ospf neighbor
B) show ip route
C) show running-config
D) show interfaces
Answer:
A)
Explanation:
The show ip ospf neighbor command provides information about all OSPF neighbors, including their router IDs, state (Down, Init, 2-Way, Full), and priority. This is crucial for verifying adjacency establishment, ensuring correct link-state database synchronization, and troubleshooting routing issues.
OSPF routers form neighbor relationships using hello packets, and neighbor states indicate the progress of adjacency formation. Full adjacency is required for routers to exchange link-state advertisements (LSAs) and build a complete topology database.
Show ip route displays active routes but not neighbor details. Show running-config shows configuration, and show interfaces displays interface status. CCNA candidates must understand OSPF neighbor relationships, adjacency states, and related troubleshooting steps for reliable routing.
Question 64:
Which protocol uses 802.1Q tagging to carry multiple VLANs over a single link?
A) Trunking
B) Access
C) EtherChannel
D) STP
Answer:
A)
Explanation:
VLAN trunking using IEEE 802.1Q allows multiple VLANs to be transported over a single physical link. Each frame is tagged with a VLAN identifier, enabling switches to distinguish traffic from different VLANs. The native VLAN is sent untagged, while other VLANs are encapsulated with a 4-byte tag.
Trunking is essential for inter-switch connectivity, especially in enterprise networks with many VLANs. Access ports carry a single VLAN, EtherChannel aggregates links for bandwidth, and STP prevents loops but does not carry multiple VLANs.
CCNA candidates must understand trunk configuration, verification using commands like show interfaces trunk, and how trunking interacts with VLANs, spanning tree, and Layer 2 forwarding.
Question 65:
Which IP address range is reserved for APIPA?
A) 169.254.0.0 – 169.254.255.255
B) 192.168.0.0 – 192.168.255.255
C) 10.0.0.0 – 10.255.255.255
D) 172.16.0.0 – 172.31.255.255
Answer:
A)
Explanation:
The correct answer is 169.254.0.0 – 169.254.255.255 (A). This range of IP addresses is reserved for APIPA, which stands for Automatic Private IP Addressing. APIPA is a feature built into Windows and some other operating systems that allows a device to automatically assign itself an IP address in the absence of a DHCP server. This ensures that basic network functionality, such as local communication between devices on the same subnet, remains possible even if dynamic address assignment fails.
APIPA addresses fall within the 169.254.0.0/16 subnet, giving devices the ability to generate a unique address automatically without requiring manual configuration. When a device boots up and attempts to obtain an IP address via DHCP, it sends a broadcast request to a DHCP server. If no DHCP server responds, the device assigns itself an APIPA address from the 169.254.0.0 – 169.254.255.255 range. This allows the device to communicate with other devices that have also assigned themselves APIPA addresses on the same subnet. However, communication is limited to the local network; APIPA addresses are not routable across different networks or the internet.
The APIPA process includes a conflict detection mechanism. Before finalizing an IP address, the device performs an ARP (Address Resolution Protocol) check to ensure that no other device on the network is using the same IP address. If a conflict is detected, the device selects another IP address from the APIPA range and repeats the verification process. This automatic conflict resolution helps maintain network stability and reduces the chances of IP address collisions when DHCP services are temporarily unavailable.
The purpose of APIPA is primarily to provide continuity of local network operations. Without APIPA, devices that fail to receive a DHCP-assigned address would have no IP configuration and therefore would be unable to participate in any network communication. This would hinder tasks such as file sharing, printer access, or communication with other devices on the same LAN. APIPA is particularly useful in small networks, temporary setups, or during DHCP server outages, providing a fallback mechanism for basic network connectivity.
The other options are private IP address ranges defined by RFC 1918 and are used for manually assigned or DHCP-assigned private networks. 192.168.0.0 – 192.168.255.255 (B), 10.0.0.0 – 10.255.255.255 (C), and 172.16.0.0 – 172.31.255.255 (D) are commonly used in enterprise and home networks but require either manual configuration or a functioning DHCP server for automatic assignment. Unlike APIPA, these ranges are designed to be routable within private networks and do not serve as a fallback mechanism for DHCP failures.
Question 66:
Which type of NAT allows multiple private IPs to share one public IP using ports?
A) PAT
B) Static NAT
C) Dynamic NAT
D) NAT64
Answer:
A)
Explanation:
The correct answer is PAT, which stands for Port Address Translation. PAT is a type of Network Address Translation (NAT) that allows multiple devices on a private network to share a single public IP address by differentiating traffic using unique port numbers. This method is widely used in modern networks to conserve public IP addresses and enable private networks to communicate with external networks such as the Internet.
PAT operates by mapping each private IP address and its associated source port to the public IP address with a unique port number. For example, if multiple devices within a home or enterprise network initiate connections to the internet, PAT ensures that the return traffic can be correctly routed back to the originating device by using the combination of public IP and port numbers. This enables efficient utilization of a limited number of public IP addresses while maintaining connectivity for multiple hosts simultaneously. Without PAT, each private IP would require a separate public IP for external communication, which is not practical in most networks due to the scarcity of IPv4 addresses.
Other types of NAT function differently and do not provide the same port-based sharing mechanism. Static NAT maps a single private IP address to a single public IP address, creating a one-to-one relationship. This is useful for devices that need a consistent public IP, such as web servers or mail servers, but it does not allow multiple hosts to share a single IP. Dynamic NAT maps private IP addresses to available public IP addresses from a pool, but still maintains a one-to-one mapping during each session. While dynamic NAT helps address the shortage of public IPs to some extent, it cannot handle scenarios where many private hosts need to share a single public IP simultaneously. NAT64 is a specific NAT implementation designed to translate IPv6 addresses to IPv4 addresses, enabling communication between IPv6-only and IPv4-only networks. It is not typically used for general IPv4 private-to-public translation with port sharing.
PAT is sometimes referred to as NAT overload because it allows more private IP addresses than there are available public IP addresses. By leveraging unique port numbers, it can support hundreds or even thousands of private hosts using a single public IP. This is particularly important in enterprise networks, internet service providers, and home networks where IP address conservation is critical. PAT also maintains session tracking, ensuring that responses from external servers are correctly matched to the originating internal host.
Additionally, PAT enhances network security by hiding internal network structures. External devices only see the public IP and cannot directly access internal IP addresses unless port forwarding or other rules are explicitly configured. This reduces the attack surface and helps prevent unauthorized access to internal hosts.
Question 67:
Which command displays interface bandwidth and delay for EIGRP metrics?
A) show interfaces
B) show ip route
C) show running-config
D) show ip protocols
Answer:
A)
Explanation:
The correct answer is show interfaces (A). The show interfaces command on a Cisco router or switch is a powerful diagnostic tool that provides detailed information about each network interface, including operational status, IP addressing, error statistics, bandwidth, delay, and other Layer 1 and Layer 2 parameters. When it comes to EIGRP (Enhanced Interior Gateway Routing Protocol) metrics, bandwidth and delay values are critical because EIGRP uses these values to calculate the composite metric for routing decisions. Understanding and verifying these parameters is essential for ensuring optimal path selection and network performance.
EIGRP calculates its routing metric using a formula that considers several parameters: bandwidth, delay, reliability, load, and sometimes MTU. Bandwidth and delay are the primary factors in the calculation. Bandwidth represents the maximum transmission capacity of the interface, typically in kilobits per second, while delay represents the time taken for a packet to traverse the interface, usually measured in microseconds. By using the show interfaces command, an administrator can check the configured bandwidth and the observed delay values for each interface. These values are then used by EIGRP to determine the best path to a destination network. Misconfigured bandwidth or unexpected delays can lead to suboptimal routing, which can degrade network performance.
The show ip route (B) command, while important for verifying routing table entries and understanding which routes are active and learned via different protocols, does not provide the detailed interface-specific information such as bandwidth or delay values. It shows the result of EIGRP calculations but not the underlying metrics. Similarly, show running-config (C) displays the current device configuration, including interface settings, routing protocols enabled, and static routes, but it does not provide real-time operational data like interface bandwidth or delay. Lastly, show ip protocols (D) provides information about the routing protocols running on the device, including EIGRP configuration, autonomous system numbers, networks advertised, and timers. While this command shows which interfaces are participating in EIGRP, it does not provide the detailed interface metrics necessary for route metric calculation.
By reviewing the output of show interfaces, administrators can also identify other critical details affecting EIGRP operation, such as duplex mismatches, interface errors, packet drops, and physical connectivity issues. Bandwidth and delay can be manually adjusted on interfaces to influence EIGRP metric calculations, which is often used in network optimization scenarios to manipulate routing decisions for load balancing or traffic engineering purposes. For example, artificially lowering the bandwidth on a high-capacity link can cause EIGRP to prefer alternative paths.
Show interfaces (A) is the correct command for displaying the interface-specific bandwidth and delay values that directly impact EIGRP metric calculation. While show ip route, show running-config, and show ip protocols provide valuable network information, they do not offer the detailed operational metrics required to understand or manipulate EIGRP path selection. Proper use of show interfaces ensures accurate monitoring, troubleshooting, and optimization of EIGRP-based networks, making it an indispensable tool for network administrators managing dynamic routing in complex enterprise environments.
Question 68:
Which VLAN is commonly used for switch management?
A) VLAN 1
B) VLAN 10
C) VLAN 20
D) VLAN 30
Answer:
A)
Explanation:
The correct answer is VLAN 1, which is the default VLAN configured on most Cisco switches and is commonly used for switch management purposes. VLANs, or Virtual Local Area Networks, are logical subdivisions within a switch that separate network traffic for performance, security, or organizational reasons. VLAN 1 is automatically created on every Cisco switch and serves as the default VLAN for all switch ports unless configured otherwise. Historically, it has been used to carry management traffic, control plane traffic, and untagged traffic for administrative tasks.
Switch management involves accessing and configuring the switch remotely or locally, monitoring its status, or performing network administration tasks. Using VLAN 1 allows administrators to communicate with the switch via management protocols such as Telnet, SSH, SNMP, or web interfaces. It provides a designated network path for administrative operations without interfering with user or production traffic, which is typically assigned to other VLANs. By default, all switch ports belong to VLAN 1, making initial switch configuration and connectivity straightforward for administrators.
However, while VLAN 1 is the default and widely used, best practices in modern network design recommend creating a separate, dedicated management VLAN for administrative access. Using VLAN 1 for management can pose security risks because it is also used for default untagged traffic and protocol communications such as CDP (Cisco Discovery Protocol). Attackers could potentially exploit VLAN 1 if it is accessible from untrusted segments of the network. Network engineers often create a unique VLAN (e.g., VLAN 10 or 99) specifically for management, isolate it, and restrict access to trusted administrative hosts. Despite this, VLAN 1 remains the conventional starting point for initial switch setup and is often referenced in documentation and legacy configurations.
Other options listed, such as VLAN 10, VLAN 20, or VLAN 30, may be used in networks for user data, voice, or application segmentation, but they are not default VLANs for management on Cisco switches. Administrators may assign these VLANs for specific departmental traffic, voice traffic for IP phones, or isolated application networks, depending on design requirements. Using custom VLANs for management improves security and helps enforce network segmentation policies, but VLAN 1 continues to serve as the default administrative VLAN until explicitly changed.
Understanding VLAN 1’s role in switch management is critical for network troubleshooting, deployment, and auditing. For instance, if management access is lost or misconfigured, knowing that VLAN 1 is the default helps administrators restore connectivity or migrate management interfaces to a dedicated VLAN. Additionally, when configuring inter-switch links, trunking, or VLAN pruning, recognizing VLAN 1’s default presence ensures that essential control and management traffic is not inadvertently blocked, which could disrupt administrative access.
Question 69:
Which command verifies which VLANs are active on a switch?
A) show vlan brief
B) show ip route
C) show interfaces
D) show mac address-table
Answer:
A)
Explanation:
The correct answer is show vlan brief, which is a commonly used command on Cisco switches to display information about VLANs configured and active on the device. VLANs, or Virtual Local Area Networks, are a critical component of modern network design, allowing logical segmentation of network traffic within a single physical switch or across multiple switches. VLANs improve network performance, enhance security by isolating traffic, and facilitate management of broadcast domains.
When the show vlan brief command is executed, it provides a summary of all VLANs on the switch, including the VLAN ID, name, status, and the ports associated with each VLAN. The status field indicates whether a VLAN is active or suspended, helping administrators determine which VLANs are operational and currently carrying traffic. This information is essential for validating network configuration and ensuring that VLANs are correctly implemented according to design requirements. By seeing which VLANs are active and the interfaces assigned to them, administrators can verify proper segmentation and troubleshoot connectivity issues effectively.
The show vlan brief command is particularly useful for monitoring and troubleshooting VLAN configuration problems. For example, if a host cannot communicate with other devices in its VLAN, the command can help determine whether the VLAN is active and whether the switch port is correctly assigned. It also helps identify misconfigurations such as VLAN mismatches between switches, which can prevent proper trunking and traffic flow. Understanding the VLAN structure of a switch is crucial in large networks, where hundreds of VLANs may be implemented across multiple switches, and maintaining an organized VLAN assignment is key to efficient network operation.
Other commands listed do not provide the same VLAN-specific information. The show ip route command displays Layer 3 routing table entries, including network destinations and next-hop information, but it does not give insight into Layer 2 VLAN configurations. The show interfaces command shows interface status, IP addresses, and traffic statistics but does not summarize VLAN membership or activation status. The show mac address-table command displays learned MAC addresses and associated switch ports, which is helpful for Layer 2 traffic analysis but does not indicate which VLANs are active or assigned to specific interfaces.
In addition to troubleshooting, show vlan brief is also important for network planning and auditing. It allows administrators to confirm that VLAN naming conventions are consistent, unused VLANs are removed, and critical VLANs are properly active. This helps maintain network security and operational efficiency. For example, keeping track of active VLANs ensures that sensitive traffic remains isolated from other parts of the network, reducing the risk of unauthorized access.
Question 70:
Which routing protocol is classless and supports VLSM?
A) RIPv2
B) RIP v1
C) IGRP
D) BGP
Answer:
A)
Explanation:
The correct answer is RIPv2, which is a classless routing protocol that supports Variable Length Subnet Masking (VLSM). RIPv2 is an enhancement of the original RIP version 1, addressing several limitations that existed in RIP v1. One of the critical limitations of RIP v1 is that it is a classful protocol, meaning it does not send subnet mask information with its routing updates and therefore cannot support VLSM or route summarization effectively. RIPv2, in contrast, includes subnet mask information in its updates, allowing administrators to implement more efficient IP addressing schemes and optimize the utilization of IP address space.
Classless routing protocols, such as RIPv2, OSPF, and EIGRP, send both the network address and the subnet mask in their routing updates. This allows for more granular subnetting and supports hierarchical network designs, which are important in modern enterprise environments. With VLSM support, RIPv2 enables network administrators to divide IP address spaces into subnets of varying sizes according to the number of hosts needed on each subnet. This flexibility reduces IP address waste compared to classful routing, which requires all subnets within a major network to have the same mask.
RIPv2 uses hop count as its metric, with a maximum limit of 15 hops, meaning that any network beyond 15 hops is considered unreachable. Although RIPv2 is relatively simple and easy to configure, its limitations, such as slow convergence and scalability issues in large networks, must be considered when designing enterprise routing architectures. Despite these limitations, RIPv2 remains a valuable protocol for small to medium-sized networks that require simple configuration, support for VLSM, and classless routing capabilities.
Other options are not suitable for this question. RIP v1 does not support classless routing or VLSM, which restricts flexibility in subnetting and leads to inefficient IP address utilization. IGRP, a Cisco proprietary protocol, is also classful and does not support VLSM, making it less suitable for modern networks that require flexible subnetting. BGP, while a classless protocol and widely used for interdomain routing on the internet, is primarily designed for large-scale, autonomous system-level routing and is far more complex than RIPv2. It is not typically considered for simple VLSM implementations within enterprise LANs or small networks.
RIPv2’s classless behavior also supports route summarization, which can reduce the size of routing tables and minimize unnecessary routing updates. By using VLSM, administrators can create subnets that exactly match the number of hosts required per segment, improving address efficiency and supporting hierarchical addressing plans. This capability is essential for conserving IP address space, especially when dealing with limited private IPv4 networks.
Question 71:
Which protocol translates hostnames to IP addresses?
A) DNS
B) DHCP
C) ARP
D) ICMP
Answer:
A)
Explanation:
The correct answer is DNS, which stands for Domain Name System. DNS is a fundamental protocol in networking that translates human-readable hostnames into numerical IP addresses required for communication between devices over a network. Computers and other devices use IP addresses to locate and communicate with each other, but remembering numeric addresses is impractical for humans. DNS provides a hierarchical and distributed naming system that allows users to access websites and network resources using familiar domain names such as www.example.com instead of an IP address like 192.168.1.10.
When a user types a hostname into a browser or application, the device sends a DNS query to a DNS server. The server resolves the hostname to the corresponding IP address and returns it to the requesting client. This resolution allows the client to establish a TCP/IP connection with the server hosting the resource. DNS uses a hierarchical structure that includes root servers, top-level domain (TLD) servers, and authoritative name servers. Root servers direct queries to the appropriate TLD servers, which in turn point to authoritative servers responsible for the requested domain. This hierarchical resolution ensures scalability and reliability across the global internet.
DNS also supports different types of records, including A (IPv4 address), AAAA (IPv6 address), CNAME (canonical name for aliasing), MX (mail exchange), and TXT (textual information for verification or configuration). By using these records, DNS can provide not only address resolution but also additional services, such as mail routing and service location. For example, when an email is sent, the DNS MX record directs the message to the appropriate mail server based on the recipient’s domain.
Other options listed are incorrect because they serve different functions. DHCP (Dynamic Host Configuration Protocol) assigns IP addresses and network configuration parameters to devices dynamically but does not perform hostname-to-IP resolution. ARP (Address Resolution Protocol) translates IP addresses to MAC addresses within a local network, enabling communication at the data link layer, but it does not handle human-readable hostnames. ICMP (Internet Control Message Protocol) is used for network diagnostics, error reporting, and communication status checks, such as with ping or traceroute, but it is not responsible for translating names to addresses.
DNS is crucial for both internal and external network operations. In enterprise networks, internal DNS servers provide name resolution for hosts, printers, and services, allowing employees to access resources efficiently. Externally, DNS ensures that users can reach websites, cloud services, and other internet-based applications reliably. Without DNS, users would need to memorize IP addresses for every server, which is not practical and would significantly hinder usability and network navigation.
In summary, DNS is the correct protocol because it translates hostnames to IP addresses, enabling intuitive and efficient network communication. It is a cornerstone of modern networking, supporting the resolution of domain names, enhancing user experience, and facilitating the connectivity of devices across both local and global networks. DNS ensures seamless access to resources while providing scalability, reliability, and flexibility for large-scale network environments.
Question 72:
Which command shows active EIGRP neighbors?
A) show ip eigrp neighbors
B) show ip route
C) show running-config
D) show interfaces
Answer:
A)
Explanation:
The correct answer is show ip eigrp neighbors, which is a key command in Cisco networking used to display the current active neighbors of the EIGRP (Enhanced Interior Gateway Routing Protocol) process. EIGRP is a dynamic routing protocol that enables routers to exchange routing information within an autonomous system efficiently. Establishing neighbor relationships is fundamental for EIGRP operation, as routers share routing updates only with active neighbors to ensure proper route propagation and network convergence.
When the show ip eigrp neighbors command is executed, it provides detailed information about all directly connected EIGRP neighbors. The output includes important fields such as the neighbor’s IP address, the interface through which the neighbor is reachable, the hold time (time remaining before the neighbor is considered down if no messages are received), the uptime of the neighbor relationship, and the sequence number of the last message exchanged. This information allows network administrators to verify that EIGRP adjacencies are correctly established and operational, which is crucial for maintaining network stability and optimal routing.
Understanding the status of EIGRP neighbors is essential for troubleshooting routing issues. For instance, if a router does not appear in the neighbor table, it may indicate a problem such as a misconfigured interface, an incorrect network statement, mismatched EIGRP autonomous system numbers, or a Layer 2 connectivity issue. By checking the neighbor table, administrators can quickly pinpoint the source of routing problems, ensuring that routers can properly exchange updates and calculate routes using the DUAL (Diffusing Update Algorithm) mechanism, which EIGRP employs to guarantee loop-free and efficient path selection.
Other commands listed do not provide the same neighbor-specific insight. The show ip route command displays the routing table, showing reachable networks and best paths, but does not give information about neighbor relationships. The show running-config command reveals the router’s current configuration, including EIGRP settings, but it does not confirm whether neighbors are actively exchanging routing information. The show interfaces command provides interface status, IP addresses, and statistics but does not indicate which routers are forming EIGRP neighbor relationships.
EIGRP neighbor information is also important for network monitoring and optimization. Administrators can verify the number of neighbors, assess stability over time, and detect changes that may impact routing convergence. This command also helps ensure that EIGRP is correctly propagating routes only to authorized neighbors, which is vital for security and predictable routing behavior. In complex networks with multiple routers and VLANs, continuous monitoring of neighbor relationships helps maintain reliable network operation and reduces the risk of routing loops or blackholes.
Question 73:
Which command displays the Layer 2 MAC address table?
A) show mac address-table
B) show ip route
C) show vlan brief
D) show interfaces
Answer:
A)
Explanation:
The correct answer is show mac address-table, which is a fundamental command used on Cisco switches to view the Layer 2 MAC address table. This table, also known as the forwarding table or CAM (Content Addressable Memory) table, contains mappings between MAC addresses and the switch ports where they were learned. The MAC address table is essential for proper Layer 2 switching because it allows the switch to forward frames to the correct destination port based on the destination MAC address in each Ethernet frame.
When a switch receives a frame on a port, it examines the source MAC address and records it in the MAC address table along with the interface where it was received. When a frame arrives with a destination MAC address, the switch looks it up in the MAC address table and forwards the frame to the appropriate port. If the destination MAC is unknown, the switch floods the frame to all other ports within the same VLAN, ensuring it reaches the correct destination. The show mac address-table command allows administrators to verify these mappings and ensure the switch is forwarding traffic correctly.
This command is particularly useful for troubleshooting network connectivity issues. For example, if a host is unable to communicate with another device, administrators can check the MAC address table to determine whether the switch has learned the host’s MAC address and if it is associated with the correct port. It also helps identify potential issues such as duplicate MAC addresses, port flapping, or misconfigured VLANs. By monitoring the MAC address table, network operators can confirm that devices are connected to the expected ports and verify network segmentation.
Other commands listed do not provide the same Layer 2 forwarding information. The show ip route command displays the routing table, which contains Layer 3 network routes and next-hop information, useful for verifying network reachability but not for MAC-level traffic forwarding. The show vlan brief command lists VLANs and associated ports on the switch, providing Layer 2 segmentation information but not the actual MAC address-to-port mappings. The show interfaces command displays interface status, IP addresses, and traffic statistics but does not reveal MAC address associations.
Using show mac address-table is essential in enterprise networks where switches handle hundreds or thousands of devices. It allows administrators to validate VLAN configurations, ensure correct connectivity, and monitor how MAC addresses are being learned. It is also critical when troubleshooting issues such as broadcast storms, VLAN mismatches, or unknown unicast flooding. The MAC address table can also be used in conjunction with features like port security, which limits the number of MAC addresses per port, to enhance network security.
Question 74:
Which IPv6 address type allows one-to-one communication?
A) Unicast
B) Multicast
C) Anycast
D) Link-local
Answer:
A)
Explanation:
The correct answer is Unicast. In IPv6 networks, unicast addresses are used for one-to-one communication, meaning that a packet sent from a source device is delivered to a single, specific destination device. Each unicast address identifies a unique interface on a networked device. This is the most common form of communication in IP networking because it allows direct data transmission between two devices, ensuring precise delivery of packets and reliable connectivity.
IPv6 unicast addresses can be globally routable, such as global unicast addresses (similar to public IPv4 addresses), or link-local, which are automatically configured on all IPv6-enabled interfaces and used for communication within a single link or subnet. Global unicast addresses enable devices to communicate across the internet, while link-local unicast addresses are critical for local network operations, such as neighbor discovery and routing protocol exchanges. Every IPv6-enabled interface has a link-local address, even if no global unicast address is assigned.
Unicast addresses contrast with multicast and anycast. Multicast addresses allow one-to-many communication, delivering packets to all interfaces in a specified group. This is useful for applications like video streaming, group messaging, or service advertisements but does not provide direct, individual communication. Anycast addresses, on the other hand, allow one-to-nearest communication, where packets sent to the address are delivered to the nearest interface in a group of devices that share the same anycast address. While anycast can optimize routing and service distribution, it is not designed for direct, one-to-one device communication.
Link-local addresses, although technically unicast in function, are restricted to a single subnet and cannot be routed beyond the local link. They are automatically generated using the interface identifier and the prefix fe80::/10, allowing network devices to communicate within the same subnet without manual configuration. These addresses are essential for functions like neighbor discovery, automatic address configuration, and routing protocol operations in IPv6 networks.
Using unicast addresses ensures that traffic is directed exactly to the intended recipient without ambiguity, which is essential for applications requiring guaranteed delivery, such as file transfers, remote management, and client-server communications. In routing, unicast addresses are also used to establish next-hop paths, determine interface reachability, and configure access control lists to secure communications between specific devices.
Question 75:
Which command verifies that an interface is operational and has an IP address?
A) show ip interface brief
B) show running-config
C) show vlan brief
D) show mac address-table
Answer:
A)
Explanation:
The correct answer is show ip interface brief, which is a commonly used command on Cisco network devices to quickly verify the status of interfaces and their IP address assignments. This command provides a concise summary of all interfaces on a router or switch, displaying key information such as the interface name, IP address, interface status (administratively up or down), and protocol status (up or down). This output allows network administrators to immediately determine whether an interface is operational and correctly configured with an IP address.
When troubleshooting network connectivity issues, the show ip interface brief command is often the first diagnostic step. By viewing this summary, administrators can identify interfaces that are administratively shut down, physically disconnected, or misconfigured. For instance, if an interface shows as administratively down, it indicates that the interface has been disabled manually using the shutdown command. If the protocol status is down, but the interface is administratively up, it often points to a physical layer issue, such as a disconnected cable or faulty hardware.
In addition to showing interface status, this command displays the assigned IP addresses for each interface. This is critical for ensuring that devices are reachable across the network. If an interface does not have an IP address or has been assigned an incorrect address, connectivity issues will arise. By using show ip interface brief, administrators can quickly verify that each interface has a valid IP configuration that aligns with the network addressing scheme.
Other commands listed do not provide the same concise, operational overview. The show running-config command displays the full device configuration, which includes interface configurations, routing protocols, and other settings, but it requires manual inspection to determine interface status and IP assignments. The show vlan brief command lists VLANs and their associated ports, which is useful for Layer 2 segmentation but does not provide IP information or operational status of interfaces. The show mac address-table command displays the MAC addresses learned on switch ports but does not indicate interface status or IP address assignments.
Using show ip interface brief is especially valuable in large networks with many interfaces. Instead of manually checking each interface configuration, the command provides a single, easy-to-read summary. It allows administrators to spot errors quickly, verify that new devices are connected correctly, and ensure that routing protocols will function properly since operational interfaces with valid IP addresses are required for protocol adjacency.