Question 1

A router knows three possible routes to reach the same destination network: an OSPF route with metric 782, a RIPv2 route with metric 4, and an EIGRP route with composite metric 20514560. Which route will the router install in the routing table and u…

Accepted Answer

Introduction / Context: Routing protocols such as RIPv2, OSPF, and EIGRP each use their own internal metric to select the best path within that protocol. However, when a router learns the same destination network from multiple routing protocols at the same time, it cannot compare the metrics directly because they represent different scales and meanings. Instead, Cisco routers use administrative distance to select the preferred protocol first. This question emphasizes how the router chooses the final active route when OSPF, RIPv2, and EIGRP all know a path to the same network. Given Data / Assumptions: One OSPF route exists with metric 782. One RIPv2 route exists with metric 4. One EIGRP route exists with composite metric 20514560. All three describe the same destination network prefix. Default administrative distances have not been changed. Concept / Approach: Cisco routers do not compare metrics between routing protocols. Instead, they assign an administrative distance to each protocol to indicate the trust level of that source. The lower the administrative distance, the more preferred the route. Default values are: EIGRP internal routes 90, OSPF 110, and RIPv2 120. Therefore, even though RIPv2 may show a small hop count metric and OSPF has a relatively small cost, the router first compares administrative distance and chooses the protocol with the lowest value, which in this case is EIGRP. Step-by-Step Solution: 1. List default administrative distances: - EIGRP internal: 90. - OSPF: 110. - RIPv2: 120. 2. Ignore the numeric values of the metrics (782, 4, and 20514560) for comparison between protocols because they are not directly comparable. 3. Compare the administrative distance values instead. The smallest value represents the preferred source of the route. 4. EIGRP has distance 90, which is lower than 110 and 120. 5. Therefore, the router installs the EIGRP route in the routing table and uses it for forwarding traffic to the destination network. Verification / Alternative check: In a lab, you can configure all three protocols on a router and advertise the same network from different neighbours. Using show ip route, you will see the final chosen route with code D for EIGRP if default distances are used. If you then manually modify administrative distances, for example by increasing EIGRP distance above 110 or lowering OSPF distance below 90, you can observe how the selected route changes in the routing table. This practical experiment confirms that administrative distance, not metric value across protocols, drives the final decision. Why Other Options Are Wrong: RIPv2 route because it has the lowest hop count: Hop count is only meaningful within RIP itself. Across protocols, hop count is not compared to EIGRP or OSPF metrics. OSPF route because its metric is numerically smallest: OSPF metric 782 cannot be compared to RIPv2 hop count or EIGRP composite metrics. The router ignores this comparison across protocols. All three routes are installed as equal cost paths: Equal cost load balancing is only done within a single protocol when metrics are equal. Here, protocols differ and administrative distance chooses one protocol only. None, the router cannot choose between different protocol metrics: The router uses administrative distance explicitly for this purpose, so it can choose without difficulty. Common Pitfalls: A frequent misunderstanding is to assume that the numerically smallest metric among all protocols will always win, regardless of which protocol generated it. This is not correct. Another pitfall is failing to remember the default distances and therefore incorrectly predicting which protocol will dominate. To avoid such issues, always keep a small reference of default administrative distances in mind, and remember that this parameter is the key to understanding the routing table when multiple protocols coexist. Final Answer: EIGRP route because it has the lowest administrative distance

Question 2

In a Cisco network, Enhanced Interior Gateway Routing Protocol (EIGRP) can support multiple routed protocols. Which four routed protocols are supported by EIGRP?

Accepted Answer

Introduction / Context: Enhanced Interior Gateway Routing Protocol is a Cisco advanced distance vector or hybrid routing protocol that was originally proprietary. One of its strengths is support for multiple routed protocols. Historically, enterprise networks used various layer three protocols in parallel, such as Internet Protocol, Novell Internetwork Packet Exchange, and AppleTalk. EIGRP was designed to route these different protocols efficiently. Modern networks mainly use Internet Protocol version 4 and Internet Protocol version 6, but exam questions still often reference the historical multi protocol capabilities of EIGRP. Given Data / Assumptions: The focus is on routed protocols, not routing protocols. EIGRP is assumed to support several distinct layer three protocols. We must identify exactly four such protocols. Modern context includes both IPv4 and IPv6, while historical context includes IPX and AppleTalk. Concept / Approach: Routed protocols are protocols that carry user data and are forwarded by routers, such as IPv4, IPv6, IPX, and AppleTalk. Routing protocols such as EIGRP, OSPF, and RIP exchange routing information about these routed protocols. Historically, EIGRP was implemented separately for each network layer protocol as distinct processes, for example EIGRP for IP and EIGRP for IPX. Over time, as networks standardised on the Internet Protocol family, support for IPX and AppleTalk became less important, but questions still test knowledge of these capabilities to ensure a broad understanding of legacy environments. Step-by-Step Solution: 1. Identify modern and legacy routed protocols commonly associated with Cisco multi protocol routing. 2. IPv4 is obviously supported and is the primary protocol for many years. 3. IPv6 is also supported by EIGRP in its newer implementations to allow next generation Internet addressing. 4. Historically, EIGRP supported IPX, which was widely used in Novell NetWare environments. 5. EIGRP also supported AppleTalk, which was used in older Apple Macintosh networks. 6. Therefore, the four routed protocols supported by EIGRP are IPv4, IPv6, IPX, and AppleTalk. Verification / Alternative check: Documentation for classical EIGRP describes separate protocol dependent modules for IP, IPX, and AppleTalk. Later enhancements introduced support for IPv6, which required modifications such as interface based configuration rather than global network commands. These references confirm that EIGRP is flexible across multiple routed protocols, which is important for migration strategies where legacy protocols coexist with Internet Protocol for a period of time. Why Other Options Are Wrong: IPv4, IPv6, CLNS, and BGP: CLNS relates to connectionless network service used with IS IS, and BGP is a routing protocol rather than a routed protocol. They are not the classic four supported protocols for EIGRP. IPv4, IPX, SNA, and DECnet: While IPX is valid, Systems Network Architecture and Digital Network Architecture are not the standard EIGRP protocol set described in Cisco exam material. IPv6, SNA, AppleTalk, and X.25: SNA and X.25 are not the typical routed protocols supported by EIGRP as described in certification references. Only IPv4 because EIGRP is classful: EIGRP is a classless protocol that supports variable length subnet masks and has implemented support for both IPv4 and IPv6. So this statement is clearly incorrect. Common Pitfalls: Learners sometimes confuse routed protocols with routing protocols, listing protocols like OSPF or BGP instead of IPX or AppleTalk. Another common error is to assume that EIGRP only works with IPv4 because that is the most common deployment in modern networks. To avoid these mistakes, remember that EIGRP historically handled multiple network layer protocols through protocol dependent modules, which is why exam questions still highlight IPX and AppleTalk along with IPv4 and IPv6. Final Answer: IPv4, IPv6, IPX, and AppleTalk

Question 3

In a switched Local Area Network with redundant links, what is the primary purpose of the Spanning Tree Protocol (STP)?

Accepted Answer

Introduction / Context: Redundancy is an important design principle in modern Local Area Networks because it provides high availability. In a switched Ethernet network, redundant links between switches can create logical loops. Unlike routers, switches forward broadcast and unknown unicast frames out of all ports except the incoming port. If loops exist, these frames can circulate indefinitely and cause a broadcast storm, bringing the network down. The Spanning Tree Protocol is specifically designed to prevent this scenario by creating a loop free logical topology over a physically redundant mesh of switches. Given Data / Assumptions: We are dealing with a switched Ethernet Local Area Network. Redundant switched paths between switches are present. Without control, these paths can create loops and broadcast storms. The question asks for the primary purpose of Spanning Tree Protocol. Concept / Approach: Spanning Tree Protocol operates at Layer two of the Open Systems Interconnection reference model. It was originally defined in IEEE 802.1D. STP elects a root bridge, calculates a loop free tree topology, and blocks certain redundant ports to break logical loops while still allowing quick reconvergence if a link fails. The goal is not to manage routing tables or monitor performance but to ensure there is exactly one active path between any two switches at any time. This preserves redundancy at the physical level while eliminating layer two loops that would cause broadcast storms. Step-by-Step Solution: 1. Identify that the network described is a switched Local Area Network, not a Layer three routed network. 2. Acknowledge that redundant links are present and that these create the risk of Ethernet switching loops. 3. Recall that Spanning Tree Protocol is the standard mechanism to control loops in such environments. 4. STP works by electing a root bridge and then determining which switch ports should be forwarding and which should be blocking. 5. As a result, a loop free logical topology is constructed even though redundant physical paths still exist for backup. 6. Therefore, the primary purpose of STP is to prevent switching loops in networks with redundant switched paths. Verification / Alternative check: Real networks that disable STP while keeping redundant links often experience symptoms like slowly increasing broadcast traffic, duplicate frames, and ultimately network collapse due to broadcast storms. Re enabling STP or using newer variants such as Rapid Spanning Tree Protocol quickly restores stability. Documentation and practical experience both confirm that the main function of STP is loop prevention at Layer two, rather than routing optimisation or performance monitoring. Why Other Options Are Wrong: To provide a mechanism for detailed network performance monitoring: While some features in switches provide monitoring, STP is not a monitoring protocol but a loop prevention protocol. To prevent routing loops in networks that use multiple redundant router paths: Routing loops are handled by Layer three routing protocols and techniques such as split horizon and time to live. STP handles switching loops, not routing loops. To manage and synchronise the VLAN database across multiple switches: VLAN Trunking Protocol or other mechanisms manage VLAN databases. STP does not synchronise VLAN information. To compress Ethernet frames in order to save bandwidth on links: There is no frame compression feature in STP. The protocol simply controls which links forward and which block. Common Pitfalls: Learners sometimes confuse switching loops with routing loops and may think that STP is related to Layer three routing protocols. Another pitfall is confusing STP with VLAN management protocols such as VTP. To avoid these mistakes, remember that STP operates at Layer two, works with bridges and switches, and its core goal is to block certain ports to eliminate loops while preserving redundancy. It is one of the foundational protocols in any enterprise switched network design. Final Answer: To prevent switching loops in Ethernet networks that have redundant switched paths

Question 4

On a Cisco switch, which command must be entered on an interface to configure the IEEE 802.1Q standard method of inserting VLAN membership information into Ethernet frames on a trunk link?

Accepted Answer

Introduction / Context: Virtual Local Area Networks allow logical segmentation of a switched network. When VLANs span multiple switches, trunk links are required to carry traffic for several VLANs across a single physical link. The IEEE 802.1Q standard defines a method of tagging Ethernet frames with VLAN information. Cisco switches can use 802.1Q encapsulation for trunks. The question asks for the exact interface configuration command that enables this standard trunk encapsulation on a switch port. Given Data / Assumptions: We are configuring a Cisco switch running IOS. The goal is to use the IEEE 802.1Q encapsulation on a trunk link. The correct mode is configured under interface configuration, not global configuration. The command format begins with switchport trunk encapsulation. Concept / Approach: On platforms that support multiple trunk encapsulation types, such as 802.1Q and Cisco Inter Switch Link, you must specify which encapsulation to use with the switchport trunk encapsulation command at interface configuration level. The syntax is Switch(config-if)#switchport trunk encapsulation dot1q for 802.1Q. The configuration prompt shows (config-if) because the command is applied per interface. Global configuration mode commands do not attach to specific ports and therefore are not suitable for trunk encapsulation settings. Step-by-Step Solution: 1. Enter global configuration mode with configure terminal. 2. Enter interface configuration mode for the desired port, for example interface fastethernet0/1 or interface gigabitethernet0/1. 3. At the Switch(config-if)# prompt, specify the trunk encapsulation type. 4. To use IEEE 802.1Q, enter the command switchport trunk encapsulation dot1q. 5. Optionally, enable trunk mode with switchport mode trunk after setting encapsulation. 6. The interface now uses 802.1Q tagging to carry multiple VLANs across the trunk link. Verification / Alternative check: After configuration, you can verify the encapsulation type with show interfaces trunk or show running-config interface commands. These outputs show that the port is in trunk mode and that the encapsulation is dot1q. Some newer switch models support only 802.1Q and do not require the switchport trunk encapsulation command. In that case, dot1q is implied. However, on platforms that do support multiple encapsulations, this command is required for correct operation and for passing certification exam questions. Why Other Options Are Wrong: Switch(config)#switchport trunk encapsulation ietf: This is in global configuration mode and uses an invalid encapsulation keyword. Encapsulation settings must be applied under interface configuration mode and IEEE standard 802.1Q uses dot1q, not ietf. Switch(config)#switchport trunk encapsulation dot1q: This is still in global configuration mode, not interface configuration mode, so it is syntactically incorrect. Switch(config-if)#switchport trunk encapsulation ietf: The ietf keyword is not used for VLAN trunk encapsulation on Cisco switches; Cisco uses dot1q or isl where supported. Switch(config-if)#switchport mode access: This command places the port in access mode and disables trunking, which is the opposite of what is required for carrying multiple VLANs. Common Pitfalls: A frequent mistake is forgetting that encapsulation configuration belongs at the interface level or confusing the encapsulation keyword with other standards. Learners sometimes try to configure encapsulation in global mode or use incorrect options such as ietf. Another pitfall is assuming that every switch still supports Inter Switch Link or requires explicit encapsulation commands, when newer devices may only support 802.1Q. For exams, always pay attention to the prompt (config-if) and the exact keyword dot1q for the IEEE 802.1Q standard. Final Answer: Switch(config-if)#switchport trunk encapsulation dot1q

Question 5

You need to protect your network from Denial of Service attacks in real time and maintain a log of hosts that attempt such attacks. What should you implement in your network?

Accepted Answer

Introduction / Context: Denial of Service attacks attempt to overwhelm network resources or specific hosts with excessive traffic, causing service disruption. Modern enterprises require mechanisms that can detect such attacks in real time, block or mitigate them, and maintain detailed logs for forensic analysis and security reporting. Traditional devices like routers and simple access control lists offer only limited protection. A specialised Intrusion Detection and Intrusion Prevention System is designed specifically for these tasks and integrates with routers, switches, and firewalls to provide both detection and active response. Given Data / Assumptions: There is a requirement for real time detection of Denial of Service attempts. There is also a requirement for logging the details of attacking hosts. The solution should be network wide and not limited to a single host. We assume a typical enterprise environment with routers, switches, and possibly firewalls already present. Concept / Approach: An Intrusion Detection System analyses traffic patterns and compares them against known attack signatures or behavioural models. An Intrusion Prevention System extends this concept by performing active intervention, such as dropping malicious packets, resetting connections, or dynamically adjusting firewall rules. Many modern devices combine both capabilities and are called IDS or IPS. These systems are designed to detect Denial of Service attacks, port scans, and other threats, and can generate detailed logs about the source IP addresses, timestamps, and types of attacks. This aligns exactly with the requirements stated in the question. Step-by-Step Solution: 1. Identify the two key goals: real time protection from Denial of Service and detailed logging of attack attempts. 2. Evaluate whether simple router features such as auto secure or additional routers meet these goals. They generally do not provide full intrusion detection and prevention functions. 3. Recognise that an Intrusion Detection or Intrusion Prevention System is specifically designed to monitor traffic, detect attacks, and respond in real time. 4. IDS and IPS solutions also maintain event logs that record attack signatures, sources, and time of occurrence. 5. Therefore, the best solution that matches both the security and logging requirements is to implement an IDS or IPS in the network. Verification / Alternative check: In practice, security teams deploy host based IDS and IPS on critical servers and network based IDS and IPS in key segments of the network. Logs from these systems feed into a central security information and event management platform for correlation and reporting. During a Denial of Service attack, administrators can observe alerts in real time and verify that malicious traffic is being blocked or rate limited. This is far more precise than simply adding routers or using automatic configuration scripts and aligns directly with security best practices. Why Other Options Are Wrong: Add additional routers to increase path redundancy: More routers provide redundancy but do not directly detect or block Denial of Service attacks, nor do they maintain specialised attack logs. Use the auto secure command on all routers: Auto secure is a helpful configuration tool on Cisco devices, but it does not provide comprehensive real time intrusion detection and prevention with logging of specific attacks. Configure the Naggle algorithm on all interfaces: The name Nagle refers to a TCP optimisation algorithm, and it is not a security feature. It has no relation to Denial of Service protection. Disable all unused switch ports and rely on port security only: Disabling unused ports is good hygiene, and port security helps against some local attacks, but these measures alone do not provide full protection against distributed Denial of Service attacks from external networks. Common Pitfalls: Some learners assume that basic firewall rules or simple router hardening are enough to handle advanced attacks. Others confuse features like auto secure or simple logging with dedicated intrusion detection and prevention. To avoid such confusion, remember that an IDS or IPS is built specifically to recognise patterns of malicious activity and respond in real time, while also producing detailed logs that can be used for compliance and forensic analysis. It is a key component of a defence in depth strategy. Final Answer: Implement an Intrusion Detection and Prevention System (IDS/IPS)

Question 6

In the IEEE 802.11b wireless standard, how many non-overlapping channels are available for use in a typical 2.4 GHz wireless Local Area Network?

Accepted Answer

Introduction / Context: Wireless Local Area Networks operating under IEEE 802.11b use the 2.4 gigahertz Industrial, Scientific, and Medical band. This band is divided into several channels, but due to the way frequencies overlap, only a limited number of channels can be used at the same time without interfering with each other. For proper design of wireless networks, especially in multi access point deployments, it is important to know the number of truly non overlapping channels that can be used for planning to reduce co channel and adjacent channel interference. Given Data / Assumptions: Standard considered: IEEE 802.11b. Frequency band: 2.4 gigahertz. We are interested in non overlapping channels, not simply the total number of channels defined. We assume a typical North American or similar regulatory domain where channels are spaced at five megahertz intervals. Concept / Approach: Although the 2.4 gigahertz band defines many channels, each 802.11b channel occupies about 22 megahertz of bandwidth and channels are only five megahertz apart. This means that adjacent channels overlap significantly. In practice, only specific channel combinations can be used simultaneously in the same area without significant overlap. In many regions, channels 1, 6, and 11 are chosen as the three non overlapping channels. They are separated sufficiently in frequency space so that their signal spectra do not overlap significantly, allowing multiple access points to operate without causing major interference to one another. Step-by-Step Solution: 1. Recognise that 802.11b operates in the 2.4 gigahertz band with channels that are 22 megahertz wide. 2. Channels are spaced five megahertz apart, so adjacent channel signals overlap heavily. 3. Through spectrum analysis and standard practice, it is known that channels 1, 6, and 11 are sufficiently separated so that their main lobes do not overlap. 4. These three channels are therefore considered non overlapping in typical deployments. 5. As a result, there are three non overlapping channels available for 802.11b network design in the 2.4 gigahertz band. Verification / Alternative check: Wireless design documentation from vendors and certification materials for entry level wireless certifications consistently highlight channels 1, 6, and 11 as the three non overlapping channels for 802.11b in North America. In other regulatory domains, alternative sets such as 1, 5, 9, and 13 may be considered with different overlap assumptions, but for the classic exam scenario, the widely accepted answer is three channels. Practical site surveys also confirm that reusing only channels 1, 6, and 11 in a staggered pattern greatly reduces interference and improves performance. Why Other Options Are Wrong: 12: This number is closer to the total number of channels defined in some regions but does not reflect the count of non overlapping channels. 23: This value is far higher than any realistic channel count and does not match known allocations. 40: This is an unrealistic value and may cause confusion with other technologies that use wider channel bonding, but it is not applicable for basic 802.11b planning. 7: Although more than three channels exist, they are not all non overlapping. Using seven unique channels in a small area would result in significant interference due to overlapping spectra. Common Pitfalls: Learners often confuse the total number of channel numbers with the number of non overlapping channels. Others may recall that newer standards such as 802.11n and 802.11ac use bonding techniques and assume a higher count of effective channels. To avoid these mistakes, always focus on the specific standard and band described in the question. For classic 802.11b in the 2.4 gigahertz band, the safe answer to remember is three non overlapping channels, usually channels 1, 6, and 11. Final Answer: 3

Question 7

Which IEEE 802.11 committee standard was adopted by the Wi Fi Alliance as Wi Fi Protected Access 2 (WPA2), providing enhanced wireless security?

Accepted Answer

Introduction / Context: Early wireless Local Area Networks relied on Wired Equivalent Privacy for security, which was later found to be weak and vulnerable to several attacks. To address this, the Wi Fi Alliance introduced Wi Fi Protected Access as an interim measure and later formalised Wi Fi Protected Access 2 as the long term enhancement. WPA2 is based on an IEEE standard developed specifically to improve security mechanisms in wireless networks, including stronger encryption and key management. Knowing which IEEE standard underlies WPA2 is important for understanding wireless security evolution and for passing certification exams. Given Data / Assumptions: WPA2 is a security certification from the Wi Fi Alliance. It is based on an IEEE standard for wireless security enhancements. The question asks for the specific IEEE committee standard that corresponds to WPA2. We assume knowledge of general 802.11 family standards such as 802.11a, 802.11b, 802.11g, and 802.11i. Concept / Approach: IEEE 802.11 is the family of standards that defines wireless Local Area Network operation. Members of this family cover physical layer specifications, modulation schemes, and security mechanisms. The standard that specifically addresses robust security, including the Advanced Encryption Standard based Counter Mode with Cipher Block Chaining Message Authentication Code Protocol, is IEEE 802.11i. WPA2 certification implements the mandatory parts of 802.11i, bringing strong encryption and authentication to Wi Fi networks. Therefore, the IEEE standard behind WPA2 is 802.11i, not the earlier physical layer focused standards such as 802.11a, 802.11b, or 802.11g. Step-by-Step Solution: 1. Recall that WPA and WPA2 are brand names from the Wi Fi Alliance that correspond to enhancements developed by the IEEE. 2. Identify which IEEE 802.11 amendment was focused on security improvements. That amendment is 802.11i. 3. Understand that 802.11b and 802.11g describe physical layer details for 2.4 gigahertz operation, and 802.11a describes 5 gigahertz operation, not security mechanisms. 4. Recognise that 802.1D is related to bridging and Spanning Tree Protocol, not wireless security. 5. Therefore, the standard that WPA2 is based on is IEEE 802.11i. Verification / Alternative check: Wireless security guides, vendor documentation, and training materials all describe WPA2 as the Wi Fi Alliance certification that implements IEEE 802.11i. These references explain that WPA2 requires support for Advanced Encryption Standard based encryption and central authentication mechanisms, which are defined within 802.11i. Standards such as 802.11b and 802.11g are frequently mentioned together with 802.11i, but they refer to different aspects of the wireless stack, which confirms that only 802.11i directly maps to WPA2. Why Other Options Are Wrong: IEEE 802.11b: This standard defines a 2.4 gigahertz physical layer for 11 megabit per second wireless operation and does not focus on modern security enhancements. IEEE 802.11g: This standard describes higher throughput operation at 2.4 gigahertz, combining features of earlier amendments, but it does not define WPA2 level security. IEEE 802.11a: This standard describes operation at 5 gigahertz, including orthogonal frequency division multiplexing, but not WPA2 security. IEEE 802.1D: This is a bridging standard that includes Spanning Tree Protocol, unrelated to wireless security or WPA2. Common Pitfalls: Many learners mix up the various 802.11 amendments because they are often mentioned together in wireless design discussions. It is important to separate physical layer specifications from security enhancements. A useful memory aid is that 802.11i stands for improved security. Remembering that WPA2 implements 802.11i will help in both exam contexts and in real world design discussions involving wireless security upgrades and compliance requirements. Final Answer: IEEE 802.11i

Question 8

In an enterprise 802.1X Extensible Authentication Protocol deployment for wireless clients, which type of device must be present on the wired network to perform central authentication?

Accepted Answer

Introduction / Context: Enterprise wireless deployments often rely on 802.1X authentication with Extensible Authentication Protocol to provide strong user based access control. In such a solution, wireless access points act as authenticators, clients run supplicant software, and a central server performs the actual authentication and authorisation decision. Understanding the role of this central server is essential for designing and troubleshooting secure wireless networks and passing related certification questions. Given Data / Assumptions: The network uses enterprise 802.1X with Extensible Authentication Protocol, not pre shared key. Access points or wireless controllers act as authenticators but do not store user databases locally. There is a wired network infrastructure behind the wireless edge. The question asks which device type must be present to perform central authentication. Concept / Approach: In 802.1X EAP architectures, three parties participate: the supplicant (client), the authenticator (access point or switch), and the authentication server. The authentication server is typically a Remote Authentication Dial In User Service server. Vendors often provide integrated solutions such as Cisco Identity Services Engine or Microsoft Network Policy Server, which act as RADIUS servers. The authenticator forwards Extensible Authentication Protocol messages inside RADIUS packets to the authentication server, which validates user credentials and returns accept or reject decisions. Therefore, the presence of a RADIUS authentication server on the wired network is mandatory for an enterprise EAP solution. Step-by-Step Solution: 1. Recognise that enterprise Extensible Authentication Protocol relies on centralised authentication rather than local shared keys. 2. Identify the three roles in 802.1X: supplicant, authenticator, and authentication server. 3. The supplicant is software on the wireless client device. 4. The authenticator is typically the access point or a wireless controller that controls network port access. 5. The authentication server resides on the wired network and is usually implemented as a RADIUS server. 6. Therefore, a RADIUS authentication server is required to perform central authentication decisions in an enterprise EAP deployment. Verification / Alternative check: Vendor deployment guides for enterprise wireless networks consistently depict RADIUS servers in network diagrams for 802.1X deployments. When configuring a wireless Local Area Network controller, administrators must specify the IP address and shared secret of the RADIUS server. During authentication, packet captures show that EAP messages are encapsulated within RADIUS Access Request and Access Accept or Reject messages. This confirms that a central RADIUS server is a required component of such designs. Why Other Options Are Wrong: An additional Layer two switch dedicated to management traffic: While management switches may exist, they are not the device responsible for user authentication decisions in 802.1X. A simple network hub that mirrors all wireless traffic: Hubs are legacy devices with no authentication intelligence. They cannot perform Extensible Authentication Protocol based authentication. A standalone Dynamic Host Configuration Protocol server: Configuration servers assign IP addresses but do not authenticate users using 802.1X EAP methods. A wireless repeater to extend coverage: Repeaters only extend radio coverage and do not perform centralised authentication or authorisation decisions. Common Pitfalls: Learners sometimes misinterpret the role of the access point and think it alone authenticates users. In enterprise designs, the access point primarily relays authentication messages to the central server. Another pitfall is to confuse directory services such as Active Directory with the RADIUS protocol. In practice, directory services often integrate with RADIUS servers, but the device that speaks the RADIUS protocol with the network infrastructure is the key element referred to in this type of question. Remember that 802.1X EAP equals supplicant plus authenticator plus RADIUS based authentication server. Final Answer: A RADIUS authentication server such as a Network Access Server

Question 9

You want to test the IPv6 stack on your local host by sending an Internet Control Message Protocol echo request to the IPv6 loopback address. Which command should you use?

Accepted Answer

Introduction / Context: Loopback addresses are used to test the local networking stack on a system without sending traffic onto the physical network. In IPv4, the most commonly known loopback address is 127.0.0.1. With the introduction of IPv6, a new loopback address is defined. Network engineers should know both addresses, as well as the basic ping command syntax, in order to test configuration and troubleshoot issues on dual stack systems that support both IPv4 and IPv6. Given Data / Assumptions: We want to test the IPv6 implementation on the local host. This requires sending a ping to the IPv6 loopback address. We assume the command line syntax ping destination is valid for the operating system. We must choose the correct IPv6 loopback address format. Concept / Approach: The IPv6 loopback address is a special unicast address represented as ::1, which is shorthand notation for 0000:0000:0000:0000:0000:0000:0000:0001. It functions similarly to IPv4 address 127.0.0.1 by referring to the local host. When you issue a ping ::1 command, packets never leave the local machine and are used only to verify that the IPv6 protocol stack is installed and operational. Other addresses such as fe80::1 are link local addresses and may refer to specific interfaces or devices, but they are not the universal loopback address. Step-by-Step Solution: 1. Recall the IPv4 loopback address, which is 127.0.0.1. 2. Identify that the question specifically asks for the IPv6 loopback address and not the IPv4 equivalent. 3. Remember that the IPv6 loopback address is ::1 using compressed notation. 4. The ping command syntax remains ping followed by the destination address, so the full command is ping ::1. 5. Therefore, to test the IPv6 stack locally, you should run ping ::1. Verification / Alternative check: On most modern operating systems that support IPv6, entering ping ::1 will generate responses similar to those from ping 127.0.0.1, confirming that the protocol stack is active. Operating systems may also provide a ping6 or similar command, but the general idea is the same. If IPv6 is disabled, the command may fail, which indicates that the IPv6 stack is not fully configured on the host. Why Other Options Are Wrong: ping 127.0.0.1: This tests the IPv4 loopback address, not the IPv6 loopback address requested in this question. ping 0.0.0.0: This is not a valid destination for testing; 0.0.0.0 is used primarily as a default route placeholder and is not a loopback address. trace 0.0.::1: This uses a mix of IPv4 and IPv6 notation and is not a correct command to test IPv6 loopback operation. ping fe80::1: This is a link local address and may point to a specific interface or device, but it is not the universal IPv6 loopback address used for local stack tests. Common Pitfalls: A common mistake is to use the familiar IPv4 loopback address when asked specifically about IPv6. Another pitfall is confusing link local addresses beginning with fe80:: with the loopback address. To avoid these errors, remember that 127.0.0.1 is the IPv4 loopback, while ::1 is the IPv6 loopback. Both commands are important to know, especially in dual stack networks where both protocols may be used during troubleshooting. Final Answer: ping ::1

Question 10

In IPv6 addressing, which type of address is used in a way that closely matches how public routable addresses are used in IPv4 on the global Internet?

Accepted Answer

Introduction / Context: When transitioning from IPv4 to IPv6, it is helpful to understand how different IPv6 address types map conceptually to IPv4 usage. In IPv4, public routable addresses are used on the global Internet to identify hosts and routers that can be reached from anywhere. IPv6 introduces several address types with different scopes and purposes, such as link local, unique local, global unicast, multicast, and anycast. Knowing which type plays the role of globally routable addressing is vital for design and troubleshooting tasks in IPv6 networks. Given Data / Assumptions: We are comparing IPv6 addressing with IPv4 addressing. In IPv4, public routable addresses are used on the global Internet. The question asks which IPv6 address type is used in a similar way. We assume basic knowledge of address scopes in IPv6. Concept / Approach: IPv6 global unicast addresses are globally unique and routable on the public Internet, just like IPv4 public addresses. They usually begin with certain prefix ranges defined by standards, and Internet service providers allocate them to organisations. Link local addresses, beginning with fe80::, are only valid on a single link and are not routable across routers. Unique local addresses are analogous to private addresses in IPv4, intended for local communications within a site or organisation. Multicast addresses are used for one to many communication, and anycast addresses are assigned to multiple interfaces, with routing delivering packets to the nearest one. Therefore, global unicast addresses are the direct equivalent of IPv4 public routable addresses. Step-by-Step Solution: 1. Identify the characteristic of IPv4 public addresses: globally unique and routable across the Internet. 2. Match this characteristic with IPv6 address types. 3. Link local addresses are not globally routable; they are limited to a local link. 4. Unique local addresses are similar to private addresses and are not intended to be routed globally. 5. Global unicast addresses are assigned by providers and are routed across the global Internet. 6. Therefore, the IPv6 address type that parallels IPv4 public routable addressing is the global unicast address. Verification / Alternative check: IPv6 addressing documentation describes global unicast addresses as having global scope and being used for normal host addressing on the Internet. They have prefixes allocated by regional Internet registries in a similar manner to IPv4 public addresses. Network configurations for customer edge routers and Internet facing servers use global unicast prefixes, which confirms their role as the IPv6 counterpart to IPv4 public addresses. Why Other Options Are Wrong: Link local address: These addresses are valid only on a single link and are mainly used for neighbour discovery and automatic configuration. They are not routed beyond the local network segment. Unique local address: These are designed for private use within an organisation, comparable to IPv4 private addresses like 10.0.0.0 or 192.168.0.0 ranges. They are not globally routable by design. Multicast address: Multicast addresses are used for one to many communication groups and are not a replacement for unicast global host addressing. Anycast address: Anycast uses the same address on multiple devices, with routing delivering packets to the nearest instance. This is a special use case, not the standard way to give each host a globally reachable identity. Common Pitfalls: Learners sometimes confuse unique local addresses with global addresses because both may look similar in notation. Another common mistake is to associate link local addresses with global connectivity simply because they are automatically configured. To avoid these issues, always remember that global unicast addresses have global scope and are intended for use on the public Internet, while link local and unique local addresses have limited scope. This understanding is essential when planning addressing schemes and firewall policies in IPv6 networks. Final Answer: Global unicast address

Question 11

You are configuring a Cisco router for a Frame Relay connection to a non-Cisco router. Which encapsulation type should you configure on the Cisco router's WAN interface to ensure interoperability?

Accepted Answer

Introduction / Context: Frame Relay is a Wide Area Network technology that supports virtual circuits between customer sites over a provider cloud. Cisco routers support different encapsulation types for Frame Relay. When connecting to another Cisco router, the default Cisco proprietary encapsulation is usually fine. However, when connecting to a non Cisco router, it is necessary to use a standards based encapsulation that both sides understand. Choosing the correct encapsulation ensures that Frame Relay headers are interpreted correctly and that data can be exchanged successfully. Given Data / Assumptions: The local router is a Cisco device. The remote router is a non Cisco device. The technology in use is Frame Relay, not Point to Point Protocol or other WAN protocols. The encapsulation must be configured on the WAN interface of the Cisco router. Concept / Approach: Cisco routers support at least two Frame Relay encapsulation types: cisco and ietf. The cisco type uses a proprietary header format that is understood only by Cisco routers. The ietf type uses a header format that conforms to standards published by the Internet Engineering Task Force. Non Cisco routers that support Frame Relay are expected to understand the ietf encapsulation. Therefore, when interoperating with other vendors, the recommended choice is encapsulation frame-relay ietf on the Cisco side. This setting ensures interoperability at the Frame Relay header level. Step-by-Step Solution: 1. Enter interface configuration mode for the relevant Wide Area Network interface, for example interface serial0/0. 2. Identify that the remote peer is a non Cisco router, which cannot use proprietary cisco Frame Relay encapsulation. 3. Use the encapsulation frame-relay command with the ietf keyword to specify standards based encapsulation. 4. The full command becomes Router(config-if)#encapsulation frame-relay ietf. 5. This configuration allows both routers to interpret Frame Relay headers correctly and exchange data over the virtual circuit. Verification / Alternative check: After configuring the interface, you can verify the encapsulation with show interfaces serial or show running-config interface. You should see encapsulation frame-relay ietf in the configuration. If the remote side is properly configured, Frame Relay Local Management Interface status will show as active, and pings between the sites will succeed. If the encapsulation types do not match, Local Management Interface may be active but data frames will not be interpreted correctly, leading to application failures. Why Other Options Are Wrong: Router(config-if)#encapsulation frame-relay q933a: This refers to a signalling standard and is not the correct keyword for Frame Relay data encapsulation on typical Cisco IOS routers. Router(config-if)#encapsulation frame-relay ansi: This is also not the standard command keyword for Frame Relay encapsulation on Cisco routers in common certification scenarios. Router(config-if)#encapsulation frame-relay cisco: This encapsulation is proprietary and works only between Cisco routers. It is not guaranteed to interoperate with non Cisco devices. Router(config-if)#encapsulation frame-relay ppp: Point to Point Protocol is a completely different Layer two protocol. It is not used as a Frame Relay encapsulation type and the command syntax is invalid. Common Pitfalls: A common mistake is to leave the default encapsulation cisco in place when connecting to a non Cisco router. This often leads to confusion because Local Management Interface might still show a virtual circuit as up, but no user data flows correctly. Another pitfall is confusing standards like Q.933 with the encapsulation keyword, which are related but not used directly in the Cisco Frame Relay encapsulation command. To avoid these issues, always remember that ietf is the correct encapsulation keyword for interoperating with non Cisco routers in Frame Relay environments. Final Answer: Router(config-if)#encapsulation frame-relay ietf

Question 12

In a Frame Relay hub-and-spoke network, split horizon can prevent routing updates from being sent between spokes. How should you configure the hub router so that split horizon does not block updates between spokes?

Accepted Answer

Introduction / Context: In hub and spoke Frame Relay networks, a single hub router connects multiple spoke routers over individual Permanent Virtual Circuits. Distance vector and advanced distance vector routing protocols use the split horizon rule, which prevents a router from sending information about a route back out of the interface on which it was learned. On a hub router using a single multipoint Frame Relay interface, this can prevent routing updates learned from one spoke from being sent to other spokes. Cisco provides a configuration strategy using subinterfaces to overcome this limitation without disabling split horizon entirely. Given Data / Assumptions: We have a Frame Relay hub and multiple spokes. The hub has multiple PVCs to different spokes over a single physical serial interface. Split horizon in the routing protocol is causing updates from one spoke to be blocked from reaching other spokes. We want a configuration that avoids split horizon issues while preserving good design practice. Concept / Approach: The usual solution is to configure a separate point to point subinterface on the hub for each Permanent Virtual Circuit. Each subinterface is treated by the router as a separate logical interface with its own subnet and Data Link Connection Identifier. Because split horizon applies per interface, routing updates received on one subinterface are not considered to be received on the same interface as another subinterface. Therefore, the hub can pass routes from one spoke to another without violating split horizon rules. This design also simplifies addressing and troubleshooting, because each spoke link has its own subnet. Step-by-Step Solution: 1. Begin on the hub router, which currently may have a single multipoint Frame Relay interface with several Data Link Connection Identifiers. 2. Reconfigure the physical interface to use logical point to point subinterfaces, for example serial0/0.1, serial0/0.2, and so on. 3. On each subinterface, configure encapsulation frame-relay and assign one Permanent Virtual Circuit with a unique Data Link Connection Identifier and a unique IP subnet. 4. Configure the routing protocol on each subinterface so that routes from each spoke are learned independently. 5. Because each subinterface is treated as a separate interface by the routing protocol, split horizon no longer prevents routes learned from one spoke from being advertised to another. Verification / Alternative check: After the configuration change, you can use show ip route on the spokes to verify that they now learn routes to other spokes through the hub. You can also examine show frame-relay map and show interfaces to confirm that each subinterface is active with the correct Data Link Connection Identifier and IP addressing. If desired, you may compare behaviour with and without split horizon disabled, but the point-to-point subinterface method is considered cleaner and more scalable than disabling split horizon on a multipoint interface. Why Other Options Are Wrong: Combine multiple Frame Relay circuits into a single point-to-point line: This option is vague and does not address split horizon. Simply aggregating circuits does not change the fundamental issue that routes learned on one logical interface are not forwarded back out that interface. Configure many subinterfaces that all share the same subnet: Using a shared subnet over multiple subinterfaces defeats the purpose of separating interfaces. Split horizon and addressing confusion may still be present. Use a single physical interface to terminate all PVCs with no subinterfaces at all: This describes the problem scenario rather than the solution. Split horizon issues arise precisely when all spokes share one interface. Disable routing protocols and use only static routes: While static routes avoid split horizon issues, they do not scale well in dynamic environments and are not the recommended design for hub and spoke Frame Relay networks using dynamic routing. Common Pitfalls: Learners sometimes respond by disabling split horizon on the main interface. Although this can work, it can mask other design issues and is not always recommended as best practice. The use of point to point subinterfaces leads to a cleaner design with clear logical separation, simpler addressing, and easier troubleshooting. Remember that each spoke should have its own subinterface and subnet on the hub, which naturally avoids split horizon problems while preserving the advantages of dynamic routing protocols. Final Answer: Configure a separate point-to-point subinterface for each PVC with a unique DLCI and subnet

Question 13

When you are using xDSL service and need a protocol that provides authentication between your customer premises equipment and the provider, which protocol should you typically use?

Accepted Answer

Introduction / Context: Digital Subscriber Line services provide broadband connectivity over traditional telephone lines. Many consumer and small business xDSL deployments require user authentication so that the service provider can control access and billing. This authentication is typically performed using a protocol that carries Point to Point Protocol frames over Ethernet. The customer premises equipment and the provider network exchange credentials using this protocol during session establishment. Given Data / Assumptions: The access technology is some type of xDSL, such as Asymmetric Digital Subscriber Line. Authentication is required between the customer device and the provider network. The underlying access medium is often Ethernet inside the premises combined with a digital subscriber line modem to the provider. We are looking for a protocol designed to provide authentication in this environment. Concept / Approach: Point to Point Protocol is a Layer two protocol that supports authentication mechanisms such as Password Authentication Protocol and Challenge Handshake Authentication Protocol. To adapt PPP to Ethernet based broadband access, a method called Point to Point Protocol over Ethernet was defined. This protocol encapsulates PPP frames inside Ethernet frames so that authentication and other PPP features can be used over an Ethernet infrastructure. Internet service providers commonly use PPP over Ethernet in xDSL deployments to authenticate subscribers, assign addresses, and manage sessions. Step-by-Step Solution: 1. Identify that the environment involves xDSL and that authentication is needed. 2. Recall that PPP is a versatile protocol that includes robust authentication options. 3. Understand that Ethernet is commonly used between customer routers and xDSL modems, so PPP must be carried inside Ethernet frames. 4. Recognise that PPP over Ethernet is the standard mechanism to achieve this, encapsulating PPP inside Ethernet. 5. Therefore, the protocol used in many xDSL deployments for subscriber authentication is Point-to-Point Protocol over Ethernet. Verification / Alternative check: Broadband configuration guides from Internet service providers often instruct customers to configure their routers for PPP over Ethernet using a username and password. Logs from these devices show PPP negotiation and authentication occurring over an Ethernet interface connected to the xDSL modem. This confirms that PPP over Ethernet is the typical choice for authenticated xDSL connections, especially in residential and small office scenarios. Why Other Options Are Wrong: Hypertext Transfer Protocol: This is an application layer protocol used for web browsing and is not used to authenticate a subscriber at the link layer on xDSL circuits. Simple Network Management Protocol: This is used for network device management and monitoring, not for subscriber authentication on access links. Address Resolution Protocol: This protocol maps Internet Protocol addresses to hardware addresses on a local network and does not perform user authentication. Trivial File Transfer Protocol: This is used for simple file transfer operations such as firmware updates, not for controlling broadband access sessions. Common Pitfalls: Learners sometimes confuse PPP over Ethernet with other broadband related protocols such as Dynamic Host Configuration Protocol, which assigns addresses but does not authenticate users in the same way. Others may think that application layer login pages provide sufficient authentication for network access, but these typically rely on underlying link layer protocols such as PPP over Ethernet that already established the session. Remember that PPP over Ethernet is widely used for authenticated xDSL access and is a key topic in networking certifications. Final Answer: Point-to-Point Protocol over Ethernet (PPPoE)

Question 14

A router has a loopback address of 135.77.9.254. According to IS-IS addressing conventions, what is the correct IS-IS system ID derived from this loopback address?

Accepted Answer

Introduction / Context: Intermediate System to Intermediate System is a link state routing protocol originally designed for connectionless network service. It uses Network Service Access Point style addresses, which include an area identifier, a system identifier, and a selector. On Cisco routers, a common method to derive the system ID for IS-IS from an Internet Protocol loopback address is used to maintain consistency and make configuration easier. This question tests your understanding of how to convert a loopback address into a valid IS-IS system ID according to typical Cisco conventions. Given Data / Assumptions: The router loopback address is 135.77.9.254. We need to convert this IPv4 address into an IS-IS system ID. Cisco convention is to base the system ID on the loopback, using six bytes written in dotted notation with two digit groups. The question provides several candidate system IDs, one of which matches this convention correctly. Concept / Approach: Cisco often recommends constructing the system ID by writing the IPv4 loopback address in a dotted decimal format with three digits per octet and then regrouping the digits into six two digit groups. The goal is to create a six byte system ID suitable for IS-IS. For the given address, 135.77.9.254 is written as 135.077.009.254. This sequence has twelve digits: 1 3 5 0 7 7 0 0 9 2 5 4. These digits are then grouped into six pairs: 13, 50, 77, 00, 92, and 54, which are written as 13.50.77.00.92.54. This method ensures that the system ID is derived deterministically from the loopback address and remains unique within the network. Step-by-Step Solution: 1. Start with the IPv4 loopback address: 135.77.9.254. 2. Convert each octet to a three digit decimal representation: - 135 remains 135. - 77 becomes 077. - 9 becomes 009. - 254 remains 254. 3. Write the address as 135.077.009.254 to make all octets three digits long. 4. Remove the dots and write the digits as a continuous sequence: 135077009254. 5. Group the digits into six two digit pairs: 13, 50, 77, 00, 92, 54. 6. Insert dots between these pairs to form the system ID: 13.50.77.00.92.54. Verification / Alternative check: Cisco documentation and many certification guides describe this process and often use almost identical examples to illustrate how to derive a system ID from a loopback address. The resulting system ID has six bytes, which matches the requirement for an IS-IS system ID, and is easy for engineers to map back to the original IPv4 loopback. In practice, you configure this system ID as part of the Network Service Access Point address along with an area ID, such as 49.0001.13.50.77.00.92.54.00, where the final selector byte is typically zero for routers. Why Other Options Are Wrong: 1350.7700.9254.00: This format does not respect the six byte system ID requirement and groups digits incorrectly, resulting in more or fewer than six bytes. 135.077.009.254: This is just the IPv4 loopback address padded to three digits per octet, not the six byte system ID format used by IS-IS. 01.35.07.70.09.25: This sequence rearranges the digits arbitrarily and does not follow the standard dotted two digit grouping from the padded IPv4 address. 77.13.50.00.92.54: This again rearranges the digits and does not match the deterministic method used to derive the system ID from the original loopback. Common Pitfalls: A common mistake is to treat each IPv4 octet as a separate byte of the system ID without padding and regrouping, which leads to a four byte identifier instead of the required six bytes. Another pitfall is to attempt hexadecimal conversions unnecessarily, when the standard method used in many exam questions is purely decimal based. To avoid confusion, remember the sequence: pad each octet to three digits, concatenate, then split into six two digit groups. Applying this method consistently makes configuration and troubleshooting of IS-IS addressing much easier in complex networks. Final Answer: 13.50.77.00.92.54

Question 15

In Border Gateway Protocol (BGP) routing, what is the primary purpose of the LOCAL_PREF (local preference) attribute inside a single autonomous system (AS)?

Accepted Answer

Introduction / Context: In interdomain routing, Border Gateway Protocol (BGP) is used to exchange routes between autonomous systems (ASes). Within a single AS, network engineers often have multiple exit points toward the Internet or partner networks. The LOCAL_PREF (local preference) attribute is a key BGP path selection tool that influences which outbound path routers inside the AS will prefer when multiple BGP routes exist for the same destination prefix. Given Data / Assumptions: We are working with internal BGP (iBGP) inside one autonomous system. Multiple BGP routes to the same destination may exist, usually learned via different border routers or different external peers. The question asks specifically about the purpose of the LOCAL_PREF attribute, not about MED, AS_PATH or weight. Concept / Approach: LOCAL_PREF is a well known, transitive BGP path attribute used only inside an AS. It is not sent to external BGP (eBGP) neighbors. Higher LOCAL_PREF values are preferred over lower ones, and this attribute is considered early in the BGP best path selection process. Network administrators use LOCAL_PREF to steer outbound traffic toward the desired exit router, for example to prefer a cheaper or higher bandwidth link while keeping backup paths with lower local preference. Step-by-Step Solution: Recall that BGP path selection uses attributes such as weight, LOCAL_PREF, locally originated, AS_PATH length, origin, MED and others, in a specific order. Within an AS, LOCAL_PREF is propagated to all iBGP peers so they share a common view of which exit is preferred. The rule is simple: the route with the highest LOCAL_PREF value is selected as the best path, assuming earlier attributes like weight are equal. LOCAL_PREF does not change BGP's administrative distance, does not directly advertise default routes and is not sent to external neighbors as a policy signal. Therefore, the correct description is that LOCAL_PREF indicates the preferred outbound exit point within an AS, with higher values being more preferred. Verification / Alternative check: Cisco and other vendor documentation describe LOCAL_PREF as an attribute used to prefer an exit point from the local AS. For example, setting LOCAL_PREF 200 on routes learned from ISP-A and 100 on routes from ISP-B makes the AS prefer ISP-A for outbound traffic. These values are seen only inside the AS; external neighbors never receive them, confirming that the attribute is focused on internal outbound path selection. Why Other Options Are Wrong: Option b is wrong because administrative distance is a local router setting and is not controlled by LOCAL_PREF. Option c is incorrect because default route advertisement is configured explicitly, often using static routes or network statements, not LOCAL_PREF. Option d misrepresents LOCAL_PREF; route aging and timers are separate mechanisms. Option e is incorrect because LOCAL_PREF is not sent to external neighbors and therefore cannot directly force them to prefer this AS as transit. Common Pitfalls: A common mistake is to confuse LOCAL_PREF with MED or weight. Weight is local to a single router, while LOCAL_PREF is shared across the AS. MED is sent to external neighbors to influence their inbound path selection, whereas LOCAL_PREF is used internally for outbound decisions. Understanding this distinction is crucial when designing BGP policies for multi homed networks. Final Answer: The LOCAL_PREF attribute is used to indicate the preferred exit point for outbound traffic within an autonomous system, with higher LOCAL_PREF values being preferred during BGP path selection.

Question 16

You are the network administrator planning future expansion of the 172.15.0.0 network. You decide to subnet this Class B network so that each subnet can allocate at least 500 usable host addresses. Which subnet mask should you choose?

Accepted Answer

Introduction / Context: Subnetting is a core skill in Cisco certification exams. In this scenario, you have a Class B network 172.15.0.0 and want to divide it into subnets that each support at least 500 usable hosts. The challenge is to find a subnet mask that satisfies the minimum host requirement without wasting address space. Given Data / Assumptions: Base network: 172.15.0.0, which is a Class B network with default mask 255.255.0.0 (/16). Required hosts per subnet: at least 500 usable host addresses. Usable host addresses per subnet are given by 2^h - 2, where h is the number of host bits (subtracting network and broadcast addresses). Concept / Approach: To support a certain number of hosts, you choose enough host bits so that 2^h - 2 is greater than or equal to the required number of hosts. Then you derive the subnet mask by allocating the remaining bits to the network portion. Here, we need to find the smallest h such that 2^h - 2 ≥ 500. After that, we calculate the prefix length and translate it into a dotted decimal subnet mask. Step-by-Step Solution: Compute host capacities: 2^8 - 2 = 256 - 2 = 254 hosts; 2^9 - 2 = 512 - 2 = 510 hosts. Since 254 is less than 500, 8 host bits are not enough. With 9 host bits, we get 510 usable hosts, which satisfies the requirement of at least 500 hosts per subnet. An IPv4 address has 32 bits, so if 9 bits are reserved for hosts, 32 - 9 = 23 bits are used for network and subnet identification. A /23 prefix corresponds to the subnet mask 255.255.254.0 (because 254 in binary is 11111110, leaving one host bit in the last octet plus 8 host bits in the previous octet). Check options: 255.255.254.0 is the /23 mask that gives 510 usable hosts, so it is the smallest mask that meets the requirement. Verification / Alternative check: We can verify by quick mental math: the last octet has 2 host bits (because /23 means 7 bits of the last octet are network bits and 1 bit is host, actually the host bits span two octets: 1 bit in the third octet and 8 bits in the fourth). Overall, 9 host bits give 2^9 = 512 total addresses and 512 - 2 = 510 usable. This matches the host requirement and confirms that 255.255.254.0 is correct. A /24 mask (255.255.255.0) would give only 254 usable hosts, which is too few. Why Other Options Are Wrong: Option a (255.255.0.0) does not create subnets; it leaves the entire Class B network as a single /16 with over 65,000 hosts. Option b (255.0.0.0) is a Class A style mask and is not appropriate for this Class B network; it does not perform the required subnetting. Option c (255.255.240.0) is a /20 mask, giving 4094 usable hosts per subnet, which meets the requirement but wastes a lot of addresses per subnet compared to /23. Option e (255.255.255.0) is a /24 with only 254 usable hosts, which is insufficient for 500 hosts per subnet. Common Pitfalls: Candidates often forget the minus 2 rule for network and broadcast addresses and may wrongly accept a mask that provides exactly 500 total addresses, which would yield only 498 usable hosts. Another mistake is to choose the first mask that supports more than 500 hosts without checking if a smaller mask could also satisfy the requirement more efficiently. Practicing power of two calculations and prefix to mask conversions helps avoid these errors. Final Answer: The correct subnet mask is 255.255.254.0 (a /23 prefix, providing about 510 usable hosts per subnet).

Question 17

Given the network 192.168.0.0/24, you want to create subnets that can each support at least 50 usable hosts. Which prefix length (subnet mask) should you use for each subnet?

Accepted Answer

Introduction / Context: Subnetting a small private network such as 192.168.0.0/24 is a classic exercise in Cisco exams. The goal here is to choose a subnet size that provides at least 50 usable host addresses per subnet, while using the address space efficiently. The answer requires calculating how many host bits are needed and then converting that to a prefix length. Given Data / Assumptions: Base network: 192.168.0.0/24, which has 256 total addresses (0 to 255 in the last octet). Required usable hosts per subnet: at least 50. Usable hosts per subnet = 2^h - 2, where h is the number of host bits; 2 addresses are reserved for network and broadcast. Concept / Approach: We must find the smallest number of host bits h such that 2^h - 2 ≥ 50. Then we subtract h from 32 to get the prefix length. With a /24 base, we are effectively subnetting the last octet. It is also useful to remember common values: /25 gives 126 usable hosts, /26 gives 62 usable, /27 gives 30 usable, and so on. Step-by-Step Solution: Compute candidate host sizes: for h = 5, 2^5 = 32 and 32 - 2 = 30 usable hosts, which is less than 50 and therefore not sufficient. For h = 6, 2^6 = 64 and 64 - 2 = 62 usable hosts, which is greater than or equal to 50 and therefore meets the requirement. With 6 host bits, the prefix length is 32 - 6 = 26, which corresponds to a /26 subnet mask. A /26 mask in dotted decimal is 255.255.255.192, giving four equal subnets within the 192.168.0.0/24 range, each with 62 usable addresses. Checking the options, /26 is the smallest subnet that meets the host requirement and is therefore the best answer. Verification / Alternative check: We can list a quick summary from memory or a cheat sheet: /25 → 126 usable, /26 → 62 usable, /27 → 30 usable. Only /25 and /26 satisfy the “at least 50 host” requirement, but /25 gives more hosts than needed and uses larger subnets, while /26 is still sufficient and more granular. The question does not ask for the maximum possible hosts, but for the appropriate mask to satisfy the minimum requirement efficiently, so /26 is preferred. Why Other Options Are Wrong: Option a (/24) does not create smaller subnets; it keeps a single subnet with 254 usable hosts, which does not answer the subnetting requirement. Option b (/25) supports 126 usable hosts per subnet, which is valid but larger than necessary; exam questions usually prefer the smallest mask that meets the requirement unless otherwise stated. Option d (/27) provides only 30 usable hosts, which is too few for the requirement of 50 hosts. Option e (/28) is even smaller and provides only 14 usable hosts, far below the requirement. Common Pitfalls: Many learners either forget to subtract 2 for network and broadcast addresses or assume that any mask that supports more than the required hosts is equally acceptable. In exam situations, you are typically expected to choose the smallest subnet that meets the requirement, as this conserves address space. Knowing common prefix length and host count pairs by heart saves time and reduces calculation mistakes. Final Answer: The correct prefix length is /26 (255.255.255.192), which provides about 62 usable host addresses per subnet.

Question 18

In TCP/IP networking, which well-known TCP port number is used for the FTP control connection (the command channel between client and server)?

Accepted Answer

Introduction / Context: File Transfer Protocol (FTP) is one of the oldest application layer protocols used on TCP/IP networks to transfer files between a client and a server. FTP uses a client server model and relies on two separate TCP connections: a control connection and a data connection. Understanding which port numbers are associated with FTP is a common requirement in Cisco and general networking exams. Given Data / Assumptions: We are dealing with standard, unencrypted FTP as defined by classic Internet standards. FTP uses one TCP connection for control commands and another for the actual data transfer. The question specifically asks for the port number used for the control connection (also called the command channel). Concept / Approach: Standard FTP typically uses TCP port 21 for the control connection, where commands like USER, PASS, LIST and RETR are sent. Data transfer commonly uses TCP port 20 from the server side in active mode, or a dynamically negotiated high port in passive mode. It is important to distinguish FTP from other protocols that also use well known ports, such as SSH on port 22 and SMTP on port 25. Step-by-Step Solution: Recall the association: FTP control on TCP 21, FTP data commonly on TCP 20 in active mode. Check the options: port 21 is clearly listed and is the known control port for FTP. Port 20 is related to FTP but is used for data transfer, not the main control channel that carries commands. Port 22 belongs to SSH (Secure Shell), which is used for secure remote login, not FTP. Port 25 is used primarily by SMTP for email transfer and port 23 is used by Telnet, so they are incorrect for FTP control. Verification / Alternative check: You can verify this mapping by looking at the list of well known TCP ports: 20 (FTP data), 21 (FTP control), 22 (SSH), 23 (Telnet), 25 (SMTP), 80 (HTTP) and so on. Additionally, firewall and access list configurations frequently reference TCP port 21 when allowing or inspecting FTP control traffic. This consistent usage across documentation and configuration examples confirms that TCP port 21 is the correct answer. Why Other Options Are Wrong: Option a (port 20) is closely related to FTP but is not the control channel; it is traditionally used for the data channel in active mode FTP. Option c (port 22) is the default for SSH, not FTP. Option d (port 25) is associated with SMTP for email, not file transfer via FTP. Option e (port 23) is the Telnet port used for remote terminal access, not for FTP. Common Pitfalls: A frequent source of confusion is mixing up FTP's two ports and assuming that port 20 is the only port associated with FTP. Another pitfall is conflating FTP with secure file transfer protocols such as SFTP, which actually run over SSH on port 22, or FTPS, which uses SSL/TLS. Remember that this question is about classic FTP control, not secure variants. Final Answer: The FTP control (command) connection uses TCP port 21.

Question 19

On a Cisco IOS device, which command is used at the user EXEC prompt to gain access to higher-level, privileged EXEC mode commands?

Accepted Answer

Introduction / Context: Cisco IOS devices such as routers and switches provide different command line privilege levels. When you first connect to a device, you usually land in user EXEC mode, which has limited commands. To perform configuration changes or view detailed information, you must enter privileged EXEC mode. Knowing the command that elevates you from user EXEC to privileged EXEC is a basic but essential Cisco certification topic. Given Data / Assumptions: We are at the user EXEC prompt, typically shown as "Router>" or similar. Privileged EXEC mode is indicated by a prompt like "Router#". The device is running standard Cisco IOS and is using default command names. Concept / Approach: Cisco IOS uses the command "enable" to move from user EXEC mode to privileged EXEC mode. When you type "enable" and press Enter, the router may prompt you for an enable password or enable secret if one is configured. Once authenticated, you gain access to powerful commands, including those that affect the running configuration. Other commands like "configure terminal" are run from privileged EXEC mode to enter global configuration mode, but they do not directly elevate you from user EXEC. Step-by-Step Solution: Start at the user EXEC prompt, for example "Switch>". Type the command "enable" and press Enter. If an enable password or enable secret is configured, enter it when prompted. After successful authentication, the prompt changes to "Switch#", indicating that you are in privileged EXEC mode. From privileged EXEC mode, you can then enter configuration modes using commands such as "configure terminal". Verification / Alternative check: Cisco command line examples in official documentation and training materials consistently show the sequence: "Router> enable" followed by "Router#" to indicate moving into privileged mode. There is no "su root" or "admin 0" command in standard IOS for this purpose. This consistency across training guides, lab exercises and actual devices confirms that "enable" is the correct and only answer here. Why Other Options Are Wrong: Option a ("su root") is a Unix or Linux command used to switch user to the root account; it does not exist in Cisco IOS in this context. Option b ("admin 0") is not a valid IOS command for changing privilege level. Option d ("init") might appear in other systems, but is not used to change modes on Cisco IOS devices. Option e ("configure") or "configure terminal" is used from privileged EXEC mode to enter configuration mode, not from user EXEC to privileged EXEC. Common Pitfalls: New network engineers sometimes confuse the roles of "enable" and "configure terminal". Remember that "enable" elevates you into privileged EXEC mode, whereas "configure terminal" changes from privileged EXEC into global configuration mode. Another pitfall is forgetting that an enable password or secret may be required, leading to authentication failures if not known. Final Answer: The IOS command used to access high level privileged EXEC commands is "enable".

Question 20

When protocol data is encapsulated as it moves down the TCP/IP or OSI stack for transmission, what is the correct order of the encapsulation units from the application data down to the physical medium?

Accepted Answer

Introduction / Context: Networking models such as the OSI and TCP/IP stacks describe how data is encapsulated as it travels from an application down to the network interface. At each layer, headers (and sometimes trailers) are added, creating a specific type of Protocol Data Unit (PDU). Understanding the correct encapsulation order is a foundational concept for Cisco certifications and helps in troubleshooting issues related to headers and addressing. Given Data / Assumptions: We are considering a typical TCP/IP stack mapped to the OSI model for a unicast transmission. Application layer sends user data down to the transport layer (for example, TCP). The question mentions data, segment, packet, frame and bit, which correspond roughly to different layers PDUs. Concept / Approach: At the top, the application, presentation and session layers can be thought of as working with application data. The transport layer (TCP or UDP) encapsulates this data into segments (TCP) or datagrams. The network layer (IP) encapsulates segments into packets (also called IP datagrams). The data link layer encapsulates packets into frames, adding MAC addresses and other link specific information. Finally, the physical layer converts frames into a stream of bits on the wire or other medium. Step-by-Step Solution: Start with application data at the upper layers; this is simply referred to as "data". The transport layer (for example TCP) takes this data and forms a "segment" by adding transport headers such as source and destination ports. The network layer (IP) takes the transport segment and encapsulates it inside an IP "packet" by adding IP addressing and related fields. The data link layer wraps the IP packet into a "frame", adding MAC addresses and frame check sequence, suitable for the local network technology such as Ethernet. The physical layer finally transmits the frame as a stream of "bits" over copper, fiber or wireless medium. Verification / Alternative check: Many textbooks summarize this as the mnemonic "data → segment → packet → frame → bits" when going from top to bottom, and the reverse order when going from bottom to top during decapsulation. This mapping can also be cross checked with OSI layers: application data (layers 5–7), segment (layer 4), packet (layer 3), frame (layer 2) and bits (layer 1). The option that matches this order exactly is option c. Why Other Options Are Wrong: Option a mixes the order by placing frame before packet, which does not reflect the real encapsulation sequence. Option b incorrectly starts with segment before data and jumbles the order of data and packet. Option d swaps packet and frame, which is not correct according to the OSI and TCP/IP models. Option e similarly has data appear after packet, which does not align with the normal top down process. Common Pitfalls: Learners sometimes confuse packet and frame, using the terms interchangeably. Remember that "packet" is associated with the network layer (IP) and "frame" with the data link layer (Ethernet, PPP and so on). Another pitfall is to forget that encapsulation adds headers (and trailers) at each layer; if the order is misunderstood, interpreting protocol analyzers like Wireshark becomes more difficult. Final Answer: The correct encapsulation order from application down to the physical medium is Data → Segment → Packet → Frame → Bit.

CISCO Certification Questions