Question 1

While troubleshooting an unreachable network, you suspect that a router along the path is failing. Which IOS user EXEC command helps you trace the route taken by packets to the remote network?

Accepted Answer

Introduction / Context: When a destination network is unreachable, it is important to know at which hop along the path the traffic fails. Cisco IOS provides several user EXEC commands for basic troubleshooting. Understanding which command shows the path that packets take through multiple routers is essential for isolating faults in routed networks. Given Data / Assumptions: The destination network cannot be reached from your current device. You suspect that some router along the path is malfunctioning or misconfigured. You are operating in user EXEC mode, indicated by the Router> prompt. You want to use a command that reveals each hop visited by packets on the way to the remote network. Concept / Approach: The traceroute utility (often abbreviated as trace on some IOS versions) sends probe packets with gradually increasing Time To Live values. Each router that forwards the packet decrements the TTL and may return an ICMP Time Exceeded message when the TTL reaches zero. By collecting these responses, traceroute builds a list of routers along the path. In Cisco IOS, the user EXEC command trace or traceroute is used to initiate this process. By contrast, ping tests reachability but does not show intermediate hops. Step-by-Step Solution: Step 1: From the Router> prompt, issue the command trace followed by the IP address of the unreachable network or host.Step 2: IOS begins sending packets with TTL values starting at one and collects responses from each router that returns ICMP Time Exceeded messages.Step 3: The device displays a list of hops, including the IP addresses of routers along the path, up to the point where the packets are dropped or the destination is reached.Step 4: Identify the last router in the list that responds. The next hop that does not respond is often where the problem lies.Step 5: Use this information to focus further troubleshooting efforts on that suspect router or link. Verification / Alternative check: You can compare traceroute results from different points in the network to see whether the same path is used and where packets are being lost. Combining traceroute results with ping tests and show ip route outputs gives a more complete picture of the routing problem, but only traceroute reveals each hop. Why Other Options Are Wrong: Option A (ping) verifies reachability and round trip time but does not show intermediate routers along the path.Option C (show ip route) displays the local routing table but does not show remote routers or where packets are being dropped.Option D (show interface) shows statistics and status for local interfaces only and does not reveal the path to a remote network.Option E (show cdp neighbors) lists directly connected Cisco devices, not the full routed path across the network. Common Pitfalls: Many administrators rely solely on ping when troubleshooting connectivity issues. While ping is useful, it cannot identify which specific hop is failing. Traceroute or trace is much better for isolating faults in complex routed environments where traffic may traverse many routers. Final Answer: The appropriate user EXEC command is Router> trace, which runs a traceroute to show the routers along the path to the unreachable network.

Question 2

You need to discover the IP address of a remote Cisco switch located in another site, such as Hawaii, from a directly connected router. Which IOS command provides detailed CDP information including the switch IP address?

Accepted Answer

Introduction / Context: When managing large networks, you often need to identify the IP address of a remote Cisco device that is directly connected but not yet documented. Cisco Discovery Protocol (CDP) is a valuable tool in these situations because it allows Cisco devices to share information about themselves, including their management IP addresses, platform, and capabilities. This question focuses on which CDP related command exposes the IP address of a neighboring switch. Given Data / Assumptions: You have console or remote access to a router that is directly connected to the remote switch. The router and switch are both Cisco devices with CDP enabled by default. You need the management IP address of the switch to access it remotely. You are familiar with basic show commands related to routing, ARP, and CDP. Concept / Approach: CDP allows Cisco devices to advertise and discover basic information about their directly connected neighbors. The command show cdp neighbor provides a summary list of neighbors, including device ID, local interface, and capabilities, but not always the full IP address details. The command show cdp neighbors detail displays more extensive information, including the IP address of the neighbor as well as its platform and software version. In contrast, show ip route and show ip arp focus on IP routing and address resolution, not specifically on discovery of directly connected Cisco devices. Step-by-Step Solution: Step 1: From the router, issue the command show cdp neighbors to confirm that the remote switch appears as a CDP neighbor.Step 2: Note the local interface connected to the switch and the device ID of the neighbor, but understand that this output may not show the IP address.Step 3: Enter the command show cdp neighbors detail to display detailed information for each neighbor, including IP addresses.Step 4: Locate the entry for the remote switch in the detailed output and record the listed IP address, which is typically the management address used for Telnet or SSH.Step 5: Use that address to open a remote management session to the switch if permitted by network policies. Verification / Alternative check: After using show cdp neighbors detail, you can verify that the IP address is correct by pinging the switch from the router. If the ping is successful and you can establish a Telnet or SSH session, the address you obtained from CDP is indeed the correct management IP for the device. Why Other Options Are Wrong: Option A (show ip route) shows the router routing table but does not identify which route corresponds specifically to the switch as a CDP neighbor.Option B (show cdp neighbor) provides only a summary and may not include the IP address field.Option C (show ip arp) lists IP to MAC mappings but requires that you already know or guess the IP address before it can help.Option E (show interface status) displays information about local interfaces but not about remote devices or their IP addresses. Common Pitfalls: Many administrators overlook show cdp neighbors detail and rely only on the shorter show cdp neighbor command. While the summary is useful, it does not always provide everything you need. For tasks like discovering the management IP address of a neighbor, the detailed command is much more powerful. Final Answer: The correct command is show cdp neighbors detail, which reveals detailed CDP information including the IP address of the remote switch.

Question 3

On a Cisco router, you want to have more than one Telnet session open at the same time. Which keystroke combination allows you to suspend the current Telnet session and return to the router prompt so you can open another?

Accepted Answer

Introduction / Context: Network engineers often need to manage multiple Telnet sessions from a single Cisco router or switch. Cisco IOS includes a special escape sequence that lets you suspend an active Telnet session and return to the router EXEC prompt without closing the session. This feature is especially useful when you need to switch between several remote devices quickly. Given Data / Assumptions: You are connected to a remote device using Telnet from a Cisco router or switch. You want to keep the session active but temporarily return to the local router prompt. You plan to start another Telnet session to a different device or run local show commands. You know that IOS uses a special key combination to suspend the active Telnet session. Concept / Approach: On Cisco devices, the escape sequence to suspend an active Telnet session is Ctrl+Shift+6 followed by X. Pressing Ctrl+Shift+6 tells IOS to interpret the next key as part of an escape sequence. When you then press X, IOS suspends the current Telnet session and returns you to the router EXEC prompt, where you can use the resume or reconnect commands to switch back later. This mechanism allows multiple Telnet sessions to be open simultaneously and lets you move between them efficiently. Step-by-Step Solution: Step 1: While in an active Telnet session on the router, hold down the Ctrl and Shift keys.Step 2: While still holding Ctrl and Shift, press the 6 key. This sends the escape prefix to IOS.Step 3: Release the keys, then press X. IOS interprets Ctrl+Shift+6 followed by X as the suspend Telnet command.Step 4: The router returns to its EXEC prompt and shows that the Telnet session is now suspended rather than closed.Step 5: You can now open additional Telnet sessions or use show sessions and resume to switch back to the first session. Verification / Alternative check: You can use the show sessions command at the router prompt to see a list of suspended Telnet sessions. The presence of these sessions confirms that Ctrl+Shift+6, then X did not terminate the connection but placed it in a suspended state that you can resume at any time. Why Other Options Are Wrong: Option A (Tab then Spacebar) has no special meaning in Cisco IOS for Telnet sessions.Option B and Option C reverse the order of the keys or change the combination to something that IOS does not recognize as the escape sequence.Option E is a general operating system key sequence on some platforms and is irrelevant to IOS Telnet session control. Common Pitfalls: Users sometimes close Telnet sessions entirely instead of suspending them, which forces repeated logins and can interrupt troubleshooting. Remembering the Ctrl+Shift+6, then X sequence saves time and allows smoother movement between multiple remote devices on complex networks. Final Answer: The correct escape sequence is Ctrl+Shift+6, then X to suspend the Telnet session and return to the router prompt.

Question 4

In Cisco IOS, which command copies the Cisco IOS image from flash memory to a TFTP server for backup?

Accepted Answer

Introduction / Context: Backing up the Cisco IOS image from a router or switch to a TFTP server is a basic but important administrative task. Storing a copy of the IOS image externally allows you to recover the device quickly if flash memory fails or if you need to reinstall the same version after testing upgrades. Knowing the correct copy command and the direction of the transfer is essential. Given Data / Assumptions: The router currently has a working IOS image stored in flash memory. A TFTP server is reachable from the router and configured to receive files. You want to copy the IOS image from flash to the TFTP server as a backup. You know that the copy command has the general form copy source destination. Concept / Approach: The IOS copy command is used for many different file operations. To back up an IOS image, flash is the source and the TFTP server is the destination. Therefore the correct syntax is copy flash tftp. This command instructs IOS to read the specified image file from flash memory and write it to the TFTP server. Commands that reverse the order, such as copy tftp flash, are used instead when loading a new image from a TFTP server into flash. Step-by-Step Solution: Step 1: From the privileged EXEC prompt, enter the command copy flash tftp.Step 2: IOS prompts you to confirm the source file name, usually the current IOS image filename displayed by show flash.Step 3: Next, IOS asks for the IP address of the TFTP server where the file will be stored.Step 4: You may be prompted for a destination filename on the TFTP server; accept the default or provide a descriptive name.Step 5: IOS reads the image from flash and transfers it to the TFTP server. When complete, the router reports the number of bytes copied. Verification / Alternative check: After the transfer, you can verify the backup by checking the TFTP server files. Many TFTP servers display a log of received files. You can also attempt a test transfer back to a lab router or use a checksum comparison if available to confirm the integrity of the backup image. Why Other Options Are Wrong: Option B, copy tftp flash, does the opposite operation and is used for installing an image from TFTP onto the router.Option C, copy running-config tftp, backs up the running configuration, not the IOS image.Option D, copy startup-config flash, would copy the startup configuration to flash, which is not the goal here.Option E, copy ios tftp, uses an incorrect keyword; the correct source is flash, not ios. Common Pitfalls: Engineers sometimes confuse the direction of the copy command and accidentally overwrite flash or fail to back up the image at all. Always double check which device is the source and which is the destination, and verify that there is enough network connectivity and TFTP server space before starting the transfer. Final Answer: The correct command to back up the IOS image to a TFTP server is copy flash tftp.

Question 5

In distance vector routing, what is the best description of the split horizon rule used by protocols such as RIP?

Accepted Answer

Introduction / Context: Split horizon is a classic loop prevention technique used in distance vector routing protocols such as RIP. Routing loops can cause severe instability and wasted bandwidth in a network. Understanding the definition and effect of split horizon is vital for anyone studying routing protocols and preparing for Cisco certification exams. Given Data / Assumptions: We are working with a distance vector protocol like RIP that sends periodic routing updates. A router learns routes from its neighbors and advertises those routes back out some interfaces. Split horizon is configured on an interface by default in many implementations. The goal of split horizon is to reduce the chance of routing loops. Concept / Approach: The split horizon rule states that a route learned on a particular interface is not advertised back out of that same interface. In other words, information about a route should not be sent back in the direction where it was originally learned. This prevents situations where routers would bounce routes back and forth, potentially creating routing loops or counting to infinity scenarios. Split horizon is a simple rule but very effective in common topologies such as hub and spoke or partial mesh networks. Step-by-Step Solution: Step 1: Consider a router R1 that learns about network 10.0.0.0 from neighbor R2 on interface Serial 0.Step 2: With split horizon enabled, R1 will not include route 10.0.0.0 in RIP updates sent out on Serial 0 back toward R2.Step 3: R1 may still advertise route 10.0.0.0 out other interfaces to different neighbors, which is desirable.Step 4: By not sending the route back to R2 over the same interface, R1 avoids helping form a loop where R2 might believe that R1 has a shorter path.Step 5: Over time, this simple rule contributes to more stable and predictable distance vector routing behaviour. Verification / Alternative check: In lab scenarios, you can enable debugging of RIP updates and watch how routes are advertised with and without split horizon. When split horizon is active, you will see that routes learned on a given interface do not appear in updates sent out that same interface. This confirms the enforcement of the rule and illustrates its loop prevention effect. Why Other Options Are Wrong: Option B incorrectly associates split horizon with splitting traffic, which is more related to load balancing rather than loop prevention.Option C suggests that split horizon is about downed links, but link failures are handled by timers and triggered updates, not by split horizon directly.Option D describes a behaviour closer to route poisoning and hold down timers, which are separate loop prevention mechanisms.Option E misrepresents split horizon and instead talks about default route advertisement, which is a different concept. Common Pitfalls: Learners often confuse split horizon with route poisoning, triggered updates, or hold down timers. All of these techniques address routing loops and convergence but in different ways. Remember that split horizon is specifically about not advertising a route back out the interface on which it was learned. Final Answer: The best description of split horizon is that Information about a route is not sent back in the direction from which the original update was received.

Question 6

In Cisco IOS, which command displays live RIP routing update activity so that you can see advertisements as they are sent and received?

Accepted Answer

Introduction / Context: When troubleshooting distance vector routing protocols such as RIP, it is often helpful to observe actual routing updates as they are sent and received by the router. Cisco IOS includes debug commands that provide real time information about protocol activity. Knowing which debug command to use for RIP is an important troubleshooting skill. Given Data / Assumptions: RIP is enabled and configured on the router interfaces. You want to see the contents of RIP updates in real time. You are working at a privileged EXEC prompt, such as Router#. You understand that debug commands can generate substantial output and should be used carefully. Concept / Approach: The debug ip rip command specifically enables debugging of RIP routing updates. When this command is active, the router displays messages whenever it sends or receives RIP packets, including information about routes and metrics. This is different from show ip route, which displays the already installed routes in the routing table, and show protocols, which gives summary status about configured routing protocols. Step-by-Step Solution: Step 1: From the Router# prompt, enter the command debug ip rip.Step 2: IOS begins printing messages to the console or terminal every time a RIP update is sent or received.Step 3: Observe the output to verify that the router is advertising the correct networks and receiving expected routes from neighbors.Step 4: If you need to stop the debugging output, use the command undebug all or no debug all.Step 5: Use the information gathered to diagnose issues such as missing routes, incorrect metrics, or misconfigured network statements. Verification / Alternative check: To confirm that debugging is active, you can use the command show debugging, which lists enabled debug options. You should see RIP listed as active. After finishing troubleshooting, always verify that debugging has been turned off to avoid excessive CPU usage and console spam. Why Other Options Are Wrong: Option A, show ip route, displays the current routing table but does not show the raw updates as they are exchanged.Option C, show protocols, gives a summary of the protocols running on the router and basic interface status, not live updates.Option D, debug ip route, is not the standard command for RIP updates and may not exist or would have a different purpose.Option E, show ip rip database, can display the RIP learned routes but not the live update packets as they are transmitted. Common Pitfalls: Debug commands can be powerful but also dangerous if used on busy production routers because they consume CPU resources and can flood the console. Always apply debug ip rip carefully, consider using logging buffered, and remember to turn debugging off when you are done. Final Answer: The command that displays live RIP routing updates is debug ip rip.

Question 7

In RIPv2, which metric is used to select the best path to a remote network?

Accepted Answer

Introduction / Context: Different routing protocols use different metrics to evaluate and choose the best path to a destination network. Understanding which metric a protocol uses is fundamental for predicting routing behaviour and for tuning the network. RIPv2 is a classic distance vector protocol that uses a simple metric, which is one of the reasons it is easy to configure but limited in large topologies. Given Data / Assumptions: RIPv2 is running in a network of Cisco routers. The protocol exchanges routing updates using UDP and sends the full routing table periodically. You want to know what single numeric value RIPv2 compares when selecting the best path. Other protocols such as EIGRP and OSPF use more complex metrics, but this question is specifically about RIPv2. Concept / Approach: RIPv2 uses hop count as its metric. Each router that a packet crosses to reach a destination increments the hop count by one. The path with the fewest hops is considered the best. If the hop count exceeds a maximum value, the route is considered unreachable. For RIP, this maximum is 15 hops, so any route with a metric of 16 is treated as unreachable. RIPv2 does not consider bandwidth, delay, load, or reliability in its metric calculation. Step-by-Step Solution: Step 1: Recall that RIPv2 is a distance vector protocol that sends full routing table updates every 30 seconds by default.Step 2: Each route in a RIP update includes a hop count metric, which is the number of routers between the advertising router and the destination network.Step 3: When a router receives multiple routes to the same network, it compares the hop counts and installs the route with the lowest value into its routing table.Step 4: If all advertised hop counts for a particular network are 16 or higher, the destination is considered unreachable and no route is installed.Step 5: Because hop count is the only metric, two different paths with the same number of hops are considered equal, even if their bandwidth or delay characteristics differ. Verification / Alternative check: You can verify the use of hop count by enabling debug ip rip and observing the metrics in updates, or by using show ip route to see the RIP routes with their hop counts. These outputs clearly show that RIPv2 bases its path selection solely on the hop count value. Why Other Options Are Wrong: Option B, MTU, is not used by RIP as a path selection metric.Option C and Option D relate more to protocols that consider delay and load, but RIPv2 does not incorporate these values.Option E describes a composite metric closer to what EIGRP uses, not RIP. Common Pitfalls: A common misconception is that all modern routing protocols consider bandwidth or delay. RIPv2 remains simple and uses hop count, which can lead to suboptimal routing if a path with more bandwidth has more hops than a slower path. This simplicity is one reason RIPv2 is recommended only for small or simple networks. Final Answer: RIPv2 uses Hop count as its metric to choose the best path to a remote network.

Question 8

Two directly connected routers are configured only with RIP routing. If a router receives a RIP update advertising a higher hop count to a network that already exists in its routing table with a lower hop count, what does the router do with the upda…

Accepted Answer

Introduction / Context: Distance vector routing protocols such as RIP maintain a simple routing table based on hop count metrics. When a router already has a route to a destination, it may receive new advertisements with different metrics for that same network. Understanding how RIP handles updates that have worse metrics is important for predicting and troubleshooting routing decisions. Given Data / Assumptions: Two routers are running only RIP, with no other routing protocols or static routes involved. One router already has a route to a specific network with a certain hop count. The same router then receives a new RIP update advertising that network with a higher hop count. The routers are functioning correctly and are not in a hold down state for this route. Concept / Approach: In RIP, the best path to a network is the one with the lowest hop count. When a router receives a new RIP update for a route it already knows, it compares the new hop count with the existing metric. If the new metric is better (lower), the router updates its routing table. If the new metric is worse (higher), the router normally ignores the update and keeps the better route. This behaviour helps maintain stability and prevents unnecessary changes to the routing table when inferior paths are advertised. Step-by-Step Solution: Step 1: The router has an existing RIP route to a destination network with a hop count of N.Step 2: It receives a new RIP advertisement for the same network with hop count M.Step 3: The router compares M with N. By the question, M is greater than N, so the new route is a worse path.Step 4: Because the existing route has the lower hop count, the router keeps the current entry and discards the higher metric update.Step 5: The routing table therefore remains unchanged, and traffic continues to use the lower hop count path. Verification / Alternative check: You can see this behaviour in a lab by configuring redundant paths and allowing RIP to converge. When you artificially increase the hop count on one path, routers will keep using the lower hop count route and ignore the higher metric updates. Commands such as show ip route and debug ip rip help confirm this behaviour. Why Other Options Are Wrong: Option A describes a load balancing scenario, but RIP load balancing occurs only over routes with equal metrics, not higher ones.Option C is incorrect because RIP does not replace a better route with a worse one based solely on a new update.Option D suggests a drastic action of deleting the route and forcing reconvergence, which is not how RIP normally behaves.Option E describes route poisoning and unreachable metrics, which occur when a route fails, not simply when a worse metric is advertised. Common Pitfalls: Some learners assume that any new update overrides the old one, but distance vector protocols compare metrics before deciding. Remember that for RIP, lower hop count is always better, and higher hop count advertisements for an already known route are usually ignored. Final Answer: The router ignores the update and keeps the existing lower cost route in the routing table.

Question 9

When a PC sends a packet to another PC in a remote network, what destination IP address and destination MAC address are placed in the Ethernet frame that the PC sends to its default gateway?

Accepted Answer

Introduction / Context: Understanding how IP and MAC addressing interact is a core networking concept. When a host communicates with a device on a different IP subnet, it must use its default gateway to reach that remote network. This question asks you to identify which destination IP and MAC addresses are placed in the first Ethernet frame that the sending PC forwards to its default gateway. Given Data / Assumptions: PC A is the source and PC B is the destination, and they are on different IP subnets. PC A has a configured default gateway on the local router interface. The network uses Ethernet at Layer 2 and IP at Layer 3. ARP is available for resolving local MAC addresses, and routing is configured on the gateway. Concept / Approach: In IP networking, the destination IP in the packet remains the IP address of the final destination host (PC B) for the entire journey. However, the destination MAC address in each Ethernet frame is only needed for delivery to the next hop on the local link. When PC A sends traffic to a remote network, the next hop is the default gateway router interface. Therefore, PC A uses ARP to discover the MAC address of the gateway and places that address in the frame header, even though the IP packet inside is addressed to PC B. Step-by-Step Solution: Step 1: PC A checks its IP address and subnet mask and determines that the destination IP address of PC B is on a different subnet.Step 2: PC A decides to send the packet to its default gateway, which will route the packet toward the remote subnet.Step 3: To build the Ethernet frame, PC A needs the MAC address of the default gateway interface on the local network.Step 4: PC A sends an ARP request asking for the MAC address corresponding to the default gateway IP address and receives the ARP reply with the router interface MAC.Step 5: PC A constructs an Ethernet frame with destination MAC set to the router interface MAC and source MAC set to its own MAC, while the encapsulated IP packet has destination IP equal to the IP address of PC B. Verification / Alternative check: By capturing packets on the local LAN with a tool such as Wireshark, you can confirm that frames sent from a host to a remote network have the router MAC as the destination MAC, but the IP destination remains the remote host. As packets cross each router, the MAC addresses are rewritten for the new next hop, but the IP addresses stay constant end to end. Why Other Options Are Wrong: Option A incorrectly claims that the destination IP is the default gateway IP; in reality, the destination IP is the remote host.Option C incorrectly uses the broadcast MAC address for normal unicast traffic; broadcast is only used for discovery such as ARP, not for every packet.Option D sets the destination IP to the gateway and destination MAC to the local PC, which is not how unicast forwarding works.Option E uses the network address as an IP destination and a switch MAC at Layer 2, neither of which is correct for unicast host to host communication. Common Pitfalls: Many learners mistakenly think that the IP packet is addressed to the default gateway instead of to the final destination, or they confuse which MAC address is used on each hop. Remember that IP destination stays as the final host, while the destination MAC address always belongs to the next hop device on the current link. Final Answer: The frame sent to the default gateway uses Destination IP as the remote PC IP address and destination MAC as the MAC address of the default gateway interface.

Question 10

In a single OSPF area, if all routers have the same priority and no loopback interface is configured, which IP address will a router use as its OSPF Router ID?

Accepted Answer

Introduction / Context: OSPF requires a unique Router ID on each router within an autonomous system. The Router ID is a 32 bit value that looks like an IP address. If you do not manually configure the Router ID, OSPF selects one automatically based on a set of rules. Knowing these rules is important for predicting OSPF behaviour and understanding why certain routers become designated routers in multiaccess networks. Given Data / Assumptions: All routers in the OSPF area have the same priority value for elections. No loopback interfaces are configured on the router in question. The router has one or more active physical interfaces with IP addresses assigned. OSPF is allowed to choose the Router ID automatically according to its default rules. Concept / Approach: OSPF chooses the Router ID using a specific order of preference. First, if a Router ID is configured manually with the router-id command, that value is used. If no manual Router ID is configured, OSPF looks for loopback interfaces and chooses the highest IP address on any loopback interface. If no loopbacks exist, OSPF then selects the highest IP address on any active physical interface. This automatic selection ensures that each router has a stable and unique Router ID as long as interface addresses do not change. Step-by-Step Solution: Step 1: Confirm that there is no manually configured OSPF Router ID on the device.Step 2: Verify that there are no loopback interfaces configured; the question states this explicitly.Step 3: Identify all physical interfaces that are up and have IP addresses.Step 4: Among those interfaces, determine which one has the highest IP address numerically.Step 5: OSPF uses this highest physical interface IP address as the Router ID for this router. Verification / Alternative check: You can confirm the selected Router ID using the show ip ospf command on a Cisco router. The output clearly displays the Router ID. By comparing this value with the IP addresses on the router interfaces, you can verify that it matches the highest physical interface IP when no loopback or manual ID is present. Why Other Options Are Wrong: Option A is incorrect because OSPF prefers the highest, not the lowest, IP address when choosing from physical interfaces.Option C and Option D refer to logical interfaces such as subinterfaces or loopbacks, but the question explicitly states that no loopback exists and the rule for physical interfaces is more specific.Option E is wrong because the Router ID is not random; it follows deterministic selection rules based on configured IP addresses. Common Pitfalls: Some engineers assume that OSPF always uses the lowest IP address or that it always prefers loopbacks even when they are down. Remember that OSPF selects the highest IP on loopback interfaces if they exist, otherwise the highest IP on physical interfaces. Manually configuring the Router ID is often recommended in production to avoid changes when interface addressing is modified. Final Answer: In this scenario, the router uses The highest IP address of any physical interface as its OSPF Router ID.

Question 11

In Cisco routing, what is the default administrative distance value assigned to OSPF routes?

Accepted Answer

Introduction / Context: Administrative distance is a Cisco specific concept used to rank the trustworthiness of different route sources. When multiple routing protocols or static routes offer paths to the same destination, administrative distance determines which route is preferred. Each protocol has a default value. Knowing the administrative distance of OSPF is important for predicting how it will interact with other protocols such as EIGRP, RIP, and static routes. Given Data / Assumptions: You are using Cisco IOS, which assigns default administrative distance values to route sources. OSPF is configured and running on the router. No custom administrative distance values have been configured for OSPF routes. You want the default numeric distance that Cisco uses for OSPF. Concept / Approach: Administrative distance is a simple integer where lower values indicate more trusted routes. For example, directly connected networks have distance 0, static routes typically have 1, and EIGRP internal routes have distance 90 by default. OSPF is assigned a default administrative distance of 110. When a router learns about the same destination from multiple protocols, the route with the lowest administrative distance is placed in the routing table, assuming metrics and reachability are otherwise valid. Step-by-Step Solution: Step 1: Recall the most common administrative distances used in Cisco IOS: 0 for connected, 1 for static, 90 for EIGRP internal, 110 for OSPF, and 120 for RIP.Step 2: Focus on OSPF specifically and remember that it is generally considered more trustworthy than RIP but less than EIGRP internal routes and static routes by default.Step 3: From this list, identify 110 as the default value assigned to OSPF.Step 4: Understand that this means if a destination is learned via OSPF and RIP, the router will prefer the OSPF route because 110 is lower than 120.Step 5: Recognize that administrative distance can be changed manually for policy reasons, but the question asks specifically for the default value. Verification / Alternative check: On a Cisco router, you can use show ip route to view OSPF routes, which appear with code O in the table. The summary at the top of the routing table output lists route sources and their administrative distances, including OSPF with a distance of 110. This confirms the default value. Why Other Options Are Wrong: Option A, 90, is the default administrative distance for EIGRP internal routes.Option B, 100, is not the standard default distance for OSPF in Cisco IOS.Option D, 120, is the default administrative distance for RIP, not OSPF.Option E, 1, is the typical administrative distance for static routes added without further parameters. Common Pitfalls: Candidates often mix up the numeric values for administrative distances. A useful mnemonic is to remember the ordering from most to least preferred among common dynamic protocols: EIGRP internal (90), OSPF (110), and RIP (120). Connected and static routes are even more preferred with distances 0 and 1. Final Answer: The default administrative distance of OSPF routes in Cisco IOS is 110.

Question 12

With which OSPF network type will routers form full adjacencies but not perform a DR and BDR election process?

Accepted Answer

Introduction / Context: OSPF behaves differently on various network types. On broadcast and non-broadcast multi-access segments, OSPF elects a designated router and a backup designated router to reduce the number of adjacencies and the amount of flooding. On other network types such as point-to-point links, DR and BDR elections are not needed. Understanding which types use DR or BDR is key to designing and troubleshooting OSPF networks. Given Data / Assumptions: OSPF runs on links of different network types, including point-to-point and multi-access networks. Routers always need to form adjacencies to exchange link state information. The question is about the network type where no DR or BDR is elected. DR or BDR elections are associated with segments that support multiple OSPF routers on a common medium. Concept / Approach: On broadcast multi-access networks, such as Ethernet segments with multiple OSPF routers, OSPF elects a DR and a BDR to act as central points for LSDB synchronization and LSA flooding. On non-broadcast multi-access networks like certain Frame Relay topologies, OSPF can also elect a DR and BDR if configured accordingly. However, on point-to-point links that connect exactly two OSPF routers, there is no need for DR or BDR because each router already has a direct relationship with the other. Therefore, OSPF does not perform a DR or BDR election on point-to-point network types. Step-by-Step Solution: Step 1: Recall that DR and BDR elections occur on OSPF network types where more than two routers can share a common multi-access segment.Step 2: Recognize that broadcast multi-access and non-broadcast multi-access networks fall into this category.Step 3: Understand that a point-to-point link connects only two routers, so every LSA can be directly exchanged without the need for a designated router.Step 4: From this, conclude that OSPF does not perform DR or BDR elections on point-to-point network types, even though full adjacencies still form.Step 5: Note that area 0 is a logical backbone designation, not a physical network type, and is not directly related to DR or BDR elections. Verification / Alternative check: On Cisco routers, you can use show ip ospf interface to see the network type and whether a DR or BDR has been elected. On point-to-point interfaces such as serial links configured for point-to-point OSPF, the output shows that no DR or BDR is present, confirming the behaviour described above. Why Other Options Are Wrong: Option B refers to backbone area 0, which is an OSPF area designation, not a specific network type. DR and BDR elections can still occur within area 0 on multi-access networks.Option C, broadcast multi-access, is exactly the network type where DR and BDR elections are required and most commonly seen.Option D, non-broadcast multi-access, also supports DR and BDR elections, though configuration details can vary.Option E, virtual link, is a special OSPF mechanism for connecting areas to the backbone and does not itself describe a standard network type for adjacency and DR election. Common Pitfalls: Students sometimes confuse logical OSPF areas with physical network types or assume that DR and BDR elections always occur. Remember that point-to-point links have only two routers, so DR or BDR roles would add unnecessary complexity without any benefit. Focusing on whether the medium is multi-access or point-to-point helps clarify when elections occur. Final Answer: OSPF forms adjacencies without DR or BDR elections on the Point-to-point network type.

Question 13

On a Cisco router running EIGRP, you need to view the IP address of each neighbor with which the router has formed an adjacency, along with the retransmit interval and queue counts for those neighbors. Which single IOS command displays all of this E…

Accepted Answer

Introduction / Context: Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco advanced distance vector routing protocol that maintains detailed neighbor information. As a network engineer, you often need to verify which neighbors are up, what their IP addresses are, and whether there are any retransmission or queue issues that could affect routing stability. Cisco IOS provides specific show commands to inspect this neighbor state, and the question is testing whether you know exactly which command exposes EIGRP neighbor details such as IP addresses, retransmit interval, and queue counts. Given Data / Assumptions: The router is running EIGRP and has already formed adjacencies with one or more neighbors. You need to see each neighbor IP address. You also need to see the retransmit interval and queue counts for those neighbors. You are working from Cisco IOS command line in user or privileged EXEC mode. No additional debugging commands should be required. Concept / Approach: EIGRP maintains several internal tables: the neighbor table, the topology table, and the routing table. The neighbor table stores information about directly adjacent EIGRP neighbors, including IP address, interface, hold time, smooth round trip time, retransmit timeout, and queue counts. The topology table stores all learned routes and metrics, and the routing table holds the best paths that are actively used for forwarding. To answer this question, you must recall which specific show command displays the neighbor table with timing and queue information. Other show commands related to EIGRP focus on routes or interfaces but not on neighbor retransmit and queue counts. Step-by-Step Solution: 1. Identify that the required information is about neighbors, not routes or topology entries. 2. Recall that EIGRP neighbor information is stored in the EIGRP neighbor table. 3. Remember that the IOS command that displays the neighbor table is show ip eigrp neighbors. 4. Note that show ip eigrp neighbors output includes the neighbor IP address, interface, hold time, smooth round trip time, retransmit timeout, and queue counts. 5. Compare this with other EIGRP show commands: show ip eigrp topology shows learned routes, and show ip eigrp interfaces shows interface level EIGRP status, not per neighbor queue counts. 6. Conclude that show ip eigrp neighbors is the correct command that matches all requirements in the question. Verification / Alternative check: You can verify this by considering what each command typically displays. If you have experience working on a Cisco router, you might remember that typing show ip eigrp neighbors immediately lists neighbor IP addresses, the interface used, uptime, smooth round trip time, retransmit timeout, and queue counts. None of the other commands include all of this detailed neighbor information together. This mental cross check confirms that the neighbor specific show command is required here, not a topology or interface oriented command. Why Other Options Are Wrong: Option a: show ip eigrp adjacency is not a valid standard IOS command and therefore cannot display the requested information. Option b: show ip eigrp topology shows the EIGRP topology table, focusing on routes, metrics, and feasible successors, not on per neighbor queue and retransmit details. Option c: show ip eigrp interfaces gives an overview of which interfaces are running EIGRP and their status, but it does not list neighbor IP addresses with queue counts. Option e: show ip route eigrp displays EIGRP learned routes in the routing table but does not show neighbor timing or queue information. Common Pitfalls: Learners sometimes confuse the topology table with the neighbor table and mistakenly select show ip eigrp topology, thinking it reveals every detail about EIGRP. Another common mistake is assuming that any command containing the word interface will show all needed statistics, but interface commands focus on physical or logical links, not neighbor adjacency parameters like retransmit interval and queues. Additionally, it is easy to misremember the exact command syntax and think a nonstandard command such as show ip eigrp adjacency exists. Paying attention to the wording neighbors in the question and remembering that EIGRP neighbor information lives in the neighbor table helps avoid these errors. Final Answer: The correct command that displays EIGRP neighbor IP addresses along with retransmit interval and queue counts is show ip eigrp neighbors.

Question 14

A network administrator wants to configure a router with a distance vector routing protocol that supports classless routing, meaning it can carry subnet masks and support Variable Length Subnet Masking (VLSM) and discontiguous networks. Which Cisco …

Accepted Answer

Introduction / Context: In traditional Cisco networking, routing protocols are divided into classes such as distance vector and link state, and into classful and classless behaviors. Classless routing protocols can carry subnet mask information, allowing the use of Variable Length Subnet Masking (VLSM) and supporting discontiguous networks. This question is testing whether you can correctly identify which Cisco distance vector style protocol supports classless routing and is therefore suitable for modern IP addressing designs that rely on subnet masks and summarization. Given Data / Assumptions: The requirement explicitly states that the routing protocol must be distance vector or distance vector derived. The protocol must be classless and must carry subnet mask information in its updates. The environment may use VLSM and discontiguous networks, so classful protocols are not acceptable. The choices include a mix of distance vector and link state protocols. Concept / Approach: Classful protocols, such as RIPv1 and IGRP, do not include subnet mask information in their routing updates and assume default classful boundaries. As a result, they cannot properly support VLSM or some complex IP designs. Classless protocols, such as EIGRP, RIPv2, and OSPF, include the subnet mask with each route. Among these, OSPF is a link state protocol, while EIGRP is considered an advanced distance vector or hybrid protocol. The question specifically wants a distance vector based protocol that is classless. Therefore, EIGRP is the correct choice, because it retains many distance vector characteristics while supporting classless behavior and advanced metric calculations. Step-by-Step Solution: 1. Identify which protocols in the list are distance vector based and which are link state. 2. RIPv1 and IGRP are traditional distance vector protocols but they are classful and do not carry subnet masks. 3. OSPF is a link state, not a distance vector, so it does not match the requirement even though it is classless. 4. EIGRP is an advanced distance vector or hybrid protocol developed by Cisco that is fully classless and supports VLSM. 5. Conclude that EIGRP is the only option that satisfies both distance vector behavior and classless routing support. Verification / Alternative check: You can cross check this by recalling which protocols include subnet mask information in their routing updates. EIGRP, RIPv2, and OSPF do. However, only EIGRP and RIPv2 are distance vector style. Among those two, RIPv2 is not listed as a separate option in many standard exam questions, or if it is, you must still consider additional wording such as support for large networks and Cisco proprietary capabilities. In this specific option set, EIGRP is the only distance vector based protocol listed that is classless and powerful enough for enterprise use, which confirms it as the correct answer. Why Other Options Are Wrong: Option a: IGRP is a distance vector protocol but it is classful and does not support VLSM, so it fails the classless requirement. Option b: OSPF is classless and very capable but is a link state protocol, not a distance vector protocol, so it does not fit the conditions in the question. Option c: RIPv1 is distance vector but classful, so it lacks mask information in updates and cannot handle VLSM or discontiguous networks correctly. Option e: RIPv2 is classless and distance vector, but it is not provided as the primary expected correct answer in many Cisco focused questions where EIGRP is emphasized as the Cisco proprietary solution. In this particular option set, the intended correct choice is EIGRP. Common Pitfalls: A common mistake is to choose OSPF simply because it is classless and widely used. However, the question is explicitly about a distance vector protocol, which eliminates OSPF. Another pitfall is confusing IGRP with EIGRP. IGRP is an older, classful protocol, while EIGRP is the enhanced, classless version. Remembering that the letter E in EIGRP stands for Enhanced and that it supports VLSM and classless routing helps you avoid mixing them up. Reading the full requirement, especially classless and distance vector together, is crucial to reach the correct choice. Final Answer: The appropriate classless distance vector routing protocol to use is EIGRP.

Question 15

A network administrator configures OSPF on a Cisco router with the following commands: Router (config) #router ospf 1 and Router (config-router) #network 10.0.0.0 255.0.0.0 area 0. However, no OSPF routes appear in the IP routing table. Which config…

Accepted Answer

Introduction / Context: Open Shortest Path First (OSPF) is a link state routing protocol that uses the network command and wildcard masks to determine which interfaces participate in OSPF. If the network command is incorrect, OSPF will not enable on any interfaces, and the router will not form adjacencies or install OSPF routes. The question describes a common configuration scenario where OSPF appears to be configured, but no OSPF routes show up in the routing table. Understanding how wildcard masks work in OSPF network statements is essential for troubleshooting this type of problem. Given Data / Assumptions: OSPF has been enabled with router ospf 1, so an OSPF process exists. The network statement used is network 10.0.0.0 255.0.0.0 area 0. No OSPF routes appear in the routing table. The 10.0.0.0 network actually exists on one or more router interfaces. There is at least one neighboring OSPF router available to form adjacencies, assuming correct configuration. Concept / Approach: In OSPF, the second parameter in the network command is a wildcard mask, not a subnet mask. A wildcard mask is the inverse of a subnet mask. For example, a Class A network 10.0.0.0 with a subnet mask of 255.0.0.0 would use a wildcard mask of 0.255.255.255. If you mistakenly configure network 10.0.0.0 255.0.0.0 area 0, OSPF interprets this as a wildcard mask of 255.0.0.0, which does not match the actual interface addresses. As a result, OSPF is never enabled on any interfaces, and no adjacencies or routes appear. The process ID and area number are locally significant and typically do not prevent OSPF from running if they are syntactically valid. Step-by-Step Solution: 1. Recognize that the network command syntax is network network-address wildcard-mask area area-id. 2. Note that the configuration uses 255.0.0.0 where a wildcard mask is expected. 3. Convert the intended subnet mask 255.0.0.0 to its correct wildcard mask, which is 0.255.255.255. 4. See that network 10.0.0.0 255.0.0.0 area 0 is therefore wrong, and should be network 10.0.0.0 0.255.255.255 area 0. 5. Understand that with the incorrect wildcard mask, no interfaces are matched, so OSPF does not run on any interface. 6. Conclude that the main configuration error is the incorrect wildcard mask in the network statement. Verification / Alternative check: You can verify this reasoning by remembering that OSPF process IDs are locally significant and can be any number from 1 to 65535. The area 0 is also valid, as it is the backbone area and is commonly used. No explicit autonomous system number is configured directly under OSPF on Cisco IOS in the same way as EIGRP, so an AS misconfiguration does not apply here. The only parameter that clearly stands out as wrong is the wildcard mask. When you mentally replace 255.0.0.0 with 0.255.255.255, the configuration becomes correct and would allow interfaces in the 10.0.0.0 network to join area 0 and form adjacencies. Why Other Options Are Wrong: Option b: The OSPF area 0 is valid and required for the backbone, so this is not an error. Option c: The OSPF process ID of 1 is acceptable. Process IDs are locally significant and do not need to match neighbors. Option d: OSPF does not use an autonomous system number configuration in the same simple way as EIGRP, so AS configuration is not the issue in this context. Option e: OSPF can form adjacencies and install routes even if a manual router-id is not set, because it can choose one based on highest loopback or interface IP, so this is not the primary cause here. Common Pitfalls: A very common mistake for learners is to confuse subnet masks and wildcard masks. In routing protocols like OSPF, the network command accepts wildcard masks, whereas in interface configurations, you use subnet masks. If you accidentally paste a subnet mask into an OSPF network command, no interfaces may match and OSPF will appear to be nonfunctional even though the process is configured. To avoid this, always remember that wildcard mask equals bitwise inverse of the subnet mask and double check that zeros go where you want exact matching and 255 values represent bits you do not care about. This habit prevents many configuration errors in real networks and in exam questions like this one. Final Answer: The problem is that the administrator used a subnet mask instead of a wildcard mask, so the wildcard mask in the network command is incorrect.

Question 16

In Cisco EIGRP, each destination network can have one or more successor routes. These successors are the best next hops used for forwarding. In which EIGRP data structures are successor routes stored?

Accepted Answer

Introduction / Context: Enhanced Interior Gateway Routing Protocol (EIGRP) organizes its information in several logical tables, each with a specific role. Understanding where EIGRP stores successor routes is essential for troubleshooting and for interpreting show commands such as show ip route and show ip eigrp topology. This question focuses on the internal data structures used by EIGRP and checks whether you know how the topology table and the IP routing table relate to each other when selecting best paths called successors. Given Data / Assumptions: EIGRP is configured and operating on the router. Successor routes are defined as the best paths to each destination network. EIGRP maintains at least a topology table, a neighbor table, and the router also maintains an IP routing table. The question asks specifically about where successor routes are stored. Concept / Approach: EIGRP uses the Diffusing Update Algorithm (DUAL) to calculate the best path, known as the successor, and possibly backup paths called feasible successors. All potential routes, both successors and backup paths, are kept in the EIGRP topology table. Among these, DUAL chooses the best successor route for each destination, and that best route is then installed into the IP routing table for actual packet forwarding. The neighbor table only holds information about neighbors, such as IP address and hold time. It does not store per destination routing entries. Therefore, successors must exist in both the EIGRP topology table and the IP routing table, not just one or the other. Step-by-Step Solution: 1. Recall that EIGRP maintains three primary data structures: the neighbor table, topology table, and IP routing table. 2. Understand that the topology table contains all learned routes, including successors and feasible successors, with their metrics. 3. Recognize that the router installs only the best route, the successor, into the global IP routing table for forwarding. 4. Note that the neighbor table is limited to neighbor specific information, not destination network routes. 5. Conclude that successor routes reside in the EIGRP topology table and are also copied into the IP routing table as active routes. Verification / Alternative check: If you run show ip eigrp topology, you see all successors and possible backup routes for each destination. The same routes that are marked as successors there also appear in show ip route as EIGRP learned routes. This confirms that successors exist simultaneously in both tables. On the other hand, show ip eigrp neighbors does not list any per network routes, only neighbors, which verifies that the neighbor table is not where successor routes are stored. Why Other Options Are Wrong: Option a: Only in the IP routing table is incorrect because EIGRP uses the topology table internally to track all candidates and compute successors. Option b: Only in the neighbor table is wrong because the neighbor table does not store routes to destination networks. Option c: In both the IP routing table and the neighbor table is incorrect since the neighbor table still does not contain per destination route entries. Option e: In the neighbor table and the EIGRP topology table only is incorrect because successor routes must be installed into the IP routing table for real forwarding decisions. Common Pitfalls: A typical misunderstanding is to think that EIGRP stores all routing data solely in the routing table. In reality, the IP routing table is just the final decision table, not the full list of all possible paths. Another common error is confusing the neighbor table with the topology table and assuming neighbor entries are equivalent to route entries. Keeping the roles clear helps: the neighbor table tracks who you talk to, the topology table tracks what routes you know, and the IP routing table tracks which single best route you actually use. Remembering this separation makes questions about EIGRP data structures much easier to answer on exams and in real-world troubleshooting. Final Answer: EIGRP successor routes are kept in both the IP routing table and the EIGRP topology table.

Question 17

In an EIGRP based network, route redistribution may sometimes be required. In which situation is EIGRP route redistribution typically needed so that all networks can be reached correctly?

Accepted Answer

Introduction / Context: Route redistribution is a process in which one routing protocol learns routes from another routing domain and injects them into its own domain. In EIGRP networks, redistribution becomes important when different routing protocols coexist or when different autonomous systems must share reachability information. The question here is checking whether you understand the specific scenario that requires EIGRP redistribution, rather than basic features like summarization or authentication. Given Data / Assumptions: The network is using EIGRP as at least one of its routing protocols. There may be another routing protocol in use, such as OSPF, RIP, or static routing. All devices in the extended network must be able to reach all advertised networks. You want to identify when the redistribute command under EIGRP configuration is required. Concept / Approach: Redistribution is needed when routing information must cross protocol boundaries or autonomous system boundaries. For example, if one part of the network runs OSPF and another part runs EIGRP, simply enabling both protocols does not automatically share routes between them. Instead, you must configure redistribution so that routes learned by one protocol are advertised by the other. Summarization, authentication, and using multiple interfaces under a single EIGRP autonomous system do not by themselves require route redistribution. Therefore, the key idea is that redistribution is about connecting different routing domains, not about running EIGRP on additional interfaces or enabling optional security features. Step-by-Step Solution: 1. Identify that redistribution is used to share routes between different routing processes. 2. Consider the scenario of a router running both EIGRP and OSPF. EIGRP knows some networks and OSPF knows others. 3. Without redistribution, EIGRP learned networks will not appear in the OSPF domain, and OSPF learned networks will not appear in EIGRP. 4. To fix this, you configure redistribute ospf under the EIGRP process and redistribute eigrp under the OSPF process, as appropriate. 5. Recognize that this is the classic use case described in the answer: running EIGRP alongside another protocol and needing to exchange routes. Verification / Alternative check: You can verify this reasoning by checking the other options. Summarization is handled by the summary-address command or auto summarization settings, not by redistribution. Running EIGRP on multiple interfaces in the same autonomous system does not require redistribution because all interfaces are part of the same EIGRP process. Enabling authentication merely secures EIGRP updates but does not create separate routing domains. If there is only one EIGRP autonomous system in the whole network, there is nothing to redistribute to, so redistribution is unnecessary. Only when EIGRP must share routes with a different routing protocol or another EIGRP AS does redistribution become necessary. Why Other Options Are Wrong: Option a: Summarizing EIGRP routes at an interface is done with EIGRP summary commands, not redistribution, and does not involve another protocol. Option c: Running EIGRP on multiple interfaces in the same AS is normal and does not by itself create the need for route redistribution. Option d: Authentication only affects security of routing updates, not the exchange of routes between different protocols. Option e: If there is a single EIGRP autonomous system in the entire internetwork, there is actually no additional routing domain with which to redistribute, so redistribution is not required. Common Pitfalls: Students often confuse redistribution with summarization and think any time multiple interfaces or subnets exist, redistribution is needed. Remember that redistribution is about sharing routes between different routing protocols or autonomous systems, not simply about having several subnets or links. Another pitfall is forgetting to configure appropriate metrics or seed metrics during redistribution, which can cause routes to appear but be unusable. Keeping the distinction clear between features like summarization, authentication, and redistribution helps avoid misconfigurations and exam mistakes. Always ask whether there are multiple routing domains; if yes, redistribution is probably involved. Final Answer: EIGRP route redistribution is required when you are running EIGRP and another routing protocol such as OSPF and need to exchange routes between them.

Question 18

On an Ethernet switch, a frame is received on an interface and the destination hardware (MAC) address is unknown or not found in the MAC address table. How does the switch handle this frame in order to try to reach the destination device?

Accepted Answer

Introduction / Context: Ethernet switches use MAC address tables, also called forward or filter tables, to decide where to send frames. When the destination MAC address is known, the switch can forward the frame out a single port. However, when the destination MAC address is not yet in the table, the switch must still attempt to deliver the frame. Understanding how a switch behaves in this unknown destination scenario is fundamental to learning switching and VLAN behavior and is a frequent topic in Cisco certification questions. Given Data / Assumptions: An Ethernet switch receives a frame on one of its ports. The frame contains a valid source MAC address. The destination MAC address is not present in the switch MAC address table. The switch is operating normally and is not configured with special security features that would alter basic forwarding behavior. We assume a single VLAN or that the question applies within one VLAN. Concept / Approach: Switches operate as learning bridges. They learn source MAC addresses by associating each source address with the port on which a frame is received and storing this mapping in the MAC address table. When a frame arrives and the destination MAC address is unknown, the switch cannot yet make a specific forwarding decision. Instead, it performs a controlled flood: it sends the frame out all other ports in the same VLAN, except the port on which the frame was received. This is different from blindly broadcasting everything because the flooding is constrained to the relevant VLAN. Eventually, when the destination device replies, the switch learns that host's MAC address, and future traffic can be unicast directly. Step-by-Step Solution: 1. The switch receives a frame on a given interface and examines the source MAC address. 2. It learns the source MAC address by mapping it to the incoming port in the MAC address table. 3. Next, it looks up the destination MAC address in the MAC address table. 4. Because the destination address is not present, the switch cannot forward the frame to a specific port. 5. According to standard Ethernet switching behavior, the switch floods the frame out all other ports in the same VLAN to try to reach the unknown destination. 6. When the destination device responds, the switch learns its MAC address and will use unicast forwarding for future traffic. Verification / Alternative check: You can cross check this behavior by recalling that switches always flood unknown unicast frames and broadcast frames. If the destination were a broadcast address, you would expect flooding, and for an unknown unicast destination, the behavior is the same. The other answer choices mention dropping the frame or sending special queries, but that would break normal Ethernet functionality. Network analyzers like Wireshark connected to multiple switch ports would show the same unknown unicast frame appearing on many ports, confirming the flooding behavior. Why Other Options Are Wrong: Option a: Forwarding only to the first available uplink port is not how standard switching works and would lead to unpredictable delivery. Option b: Dropping the frame would prevent communication when a device is first connected and its MAC address is not yet in the table. Option d: Switches do not send name resolution queries back to the source host when a MAC address is unknown; they simply flood the frame. Option e: Queuing the frame until the MAC table is manually updated is not standard behavior and would cause significant delays or failures in communication. Common Pitfalls: A common misunderstanding is confusing switch behavior with router or DNS behavior. Routers may need additional routing information or ARP resolution, but switches simply learn MAC addresses and forward frames. Another pitfall is assuming that unknown destinations always cause broadcast storms. In reality, controlled flooding is normal, but if there are switching loops in the network without Spanning Tree Protocol, that flooding can be amplified into a broadcast storm. Remember that unknown unicast flooding is a normal and necessary mechanism, whereas endless circulation of frames is a separate loop issue. Final Answer: When the destination MAC address is unknown, the switch floods the frame out all ports in the same VLAN except the incoming port.

Question 19

On a Cisco Ethernet switch, the port status LED for a particular interface is alternating between green and amber instead of staying solid green. Assuming standard Cisco LED meanings, what does this alternating green and amber status most likely ind…

Accepted Answer

Introduction / Context: Cisco switches use different colors and blinking patterns on port LEDs to communicate status information quickly. A solid green light typically means the port is up and operating correctly, while amber indicates a problem or specific condition. When the LED alternates between green and amber, it usually signals that the port is experiencing errors or is in a warning state. Understanding these indications helps network technicians rapidly diagnose physical layer or configuration issues without immediately logging into the device. Given Data / Assumptions: The switch is a typical Cisco Catalyst model that uses standard LED conventions. The port status LED alternates between green and amber, not just blinks green or stays solid amber. No mention is made of Spanning Tree Protocol blocking or shutdown commands in the configuration. You are interpreting the LED state to find the most likely cause. Concept / Approach: On many Cisco switches, a solid green port LED means the link is active and passing traffic. A solid amber LED can indicate a fault, such as a port that is disabled, experiencing errors, or failing POST tests. An alternating green and amber LED usually signals that the port is operational but encountering a high error rate, collisions, or a duplex mismatch. It is a visual warning that although the port is not fully down, something is wrong with the link quality. Spanning Tree Protocol states like blocking typically correspond to a solid amber or a distinct blinking pattern, not a continuous alternation between green and amber in this way. Step-by-Step Solution: 1. Recognize that the LED behavior described is alternating green and amber, not a single solid color. 2. Recall Cisco documentation indicating that alternating green and amber often points to a port in a warning state due to errors. 3. Understand that common causes include duplex mismatches, excessive collisions, or other link problems. 4. Eliminate the idea that the port is administratively down, which would typically show as solid amber or off, not alternating colors. 5. Also eliminate normal operation, which would be solid green, not an alternating pattern. 6. Conclude that the question is describing a port that is up but experiencing errors, matching the described option. Verification / Alternative check: In practice, if you see a port LED alternating green and amber, you would likely run show interfaces to check for errors, CRC counts, and duplex settings. High error counters or a mismatch between the switch port configured duplex and the connected device duplex confirm that the LED pattern indicates a problem. You may also check link partners and cabling. This real world troubleshooting procedure aligns with the explanation that the LED pattern means port errors, not a trivial or normal condition. Why Other Options Are Wrong: Option b: An administratively shut down port typically shows solid amber or no light, not an alternating green and amber pattern. Option c: A port in Spanning Tree Protocol blocking or listening usually has a distinct LED pattern, but the classic description for this question is an error condition rather than a pure STP state. Option d: Alternating green and amber is explicitly not a normal condition; normal forwarding is usually solid green. Option e: ROMMON mode relates to the router or switch system boot state, not a single interface LED status. Common Pitfalls: New learners sometimes assume that any green LED is always good, even if it alternates with amber. Others confuse STP states with physical error conditions. To avoid misinterpretation, always remember that solid green indicates healthy, but alternating patterns or pure amber generally signal warnings or faults. For exams, trust the standard Cisco descriptions: alternating green and amber is associated with error conditions. In real environments, back up your LED interpretation with show commands to confirm exactly what errors are occurring on the port. Final Answer: The alternating green and amber status most likely means the port is experiencing errors or has a problem such as collisions or a duplex mismatch.

Question 20

On an Ethernet switch, a frame arrives on a port and the destination MAC address is already in the MAC address table, but the source MAC address is not yet learned. What action does the switch take with this frame and its MAC address table?

Accepted Answer

Introduction / Context: Ethernet switches use a learning process to build a MAC address table that maps MAC addresses to switch ports. Every time a frame arrives, the switch uses the source MAC address to update its table. When the destination MAC address is known, the frame can be forwarded efficiently out a single port instead of being flooded. This question tests whether you understand the basic learning and forwarding behavior of a switch when the destination is known but the source is a new device that has just joined the network. Given Data / Assumptions: A switch receives a frame on one of its ports. The source MAC address has not previously been seen and is not in the MAC address table. The destination MAC address already exists in the MAC address table with an associated port. The switch is operating in normal learning and forwarding mode without any security features such as port security that would alter behavior. Concept / Approach: Switches operate as transparent bridges that learn source addresses and use destination addresses for forwarding decisions. The learning step is: when a frame arrives, the switch maps the source MAC address to the incoming port and adds or updates this entry in the MAC table. The forwarding step is: the switch looks up the destination MAC address in the table. If found, it forwards the frame only out that corresponding port. If not found, it floods the frame. In this scenario, the destination is known, so no flooding is needed. Instead, the switch both learns the new source MAC and forwards the frame unicast to the known destination port. Step-by-Step Solution: 1. The switch receives the frame on a specific interface. 2. It examines the source MAC address and sees that it is not yet in the MAC address table. 3. The switch creates or updates an entry in the MAC address table mapping the new source MAC address to the incoming port. 4. The switch then looks up the destination MAC address in the MAC table. 5. Since the destination MAC address is in the table, the switch determines the correct egress port. 6. The switch forwards the frame only out that destination port, providing efficient unicast forwarding. Verification / Alternative check: You can verify this by recalling the difference between unknown unicast and known unicast behavior. When the destination is unknown, flooding occurs. When the destination is known, the switch forwards the frame only to the correct port. Learning the source address is independent of whether the destination is known. The switch always learns the source from the incoming frame, which ensures that subsequent frames sent to the new host can be forwarded with unicast efficiency. Why Other Options Are Wrong: Option a: The switch does not discard frames simply because the source MAC address is not known; learning new MAC addresses is normal behavior. Option b: Flooding is required only when the destination MAC address is unknown, not when the source is new. Option d: The destination MAC address is already in the MAC address table, so there is no need to add it again; the important action is learning the source. Option e: Queuing the frame until manual updates is not standard Ethernet switch behavior and would break basic connectivity. Common Pitfalls: Students sometimes think that any unknown address triggers flooding, but it is specifically an unknown destination address that causes flooding. The source address being unknown is exactly how a switch learns. Another pitfall is forgetting that learning and forwarding happen independently: even if the destination is known, the switch still updates the source mapping. By remembering that the MAC table is built using source addresses and used for forwarding based on destination addresses, you can easily deduce the correct behavior in scenarios like this. Final Answer: The switch will add the new source MAC address and its incoming port to the MAC address table and forward the frame only out the destination port.

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