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Understanding How Layer 2 Switches Handle Unknown MAC Addresses

Switches operate at Layer 2 (Data Link Layer) of the OSI model and maintain a MAC address table (CAM table) to forward frames efficiently.

When a switch receives a frame, it checks its MAC address table to see if it knows the destination MAC address.

If the destination MAC address is not in the table (meaning the switch does not know which port leads to PC-C), the switch follows the flooding behavior.

What Happens When Switch1 Receives a Frame from PC-A to PC-C?

Switch1 checks its MAC table:

The source MAC (PC-A) is recorded in the table on port G0/1.

The destination MAC (PC-C) is not in the table.

Switch1 does not know where PC-C is:

It floods the frame out of all active ports except the port it was received on (G0/1).

This means the frame is sent to:

Switch2 (via G0/23)

PC-B (via G0/2)

Switch2 receives the frame and follows the same process:

If Switch2 has PC-C's MAC in its table, it forwards the frame appropriately.

If not, it floods the frame again until PC-C replies.

Once PC-C responds, Switch1 and Switch2 learn its MAC address and update their tables.

Why Other Options Are Incorrect:

❌ A. Switch1 queries Switch2 for the MAC address of PC-C.

Incorrect: Switches do not query other switches directly for MAC addresses. Instead, they rely on learning MAC addresses dynamically through frame forwarding.

❌ B. Switch1 drops the frame and sends an error message back to PC-A.

Incorrect: Switches do not drop frames for unknown MAC addresses. Instead, they flood the frames out all ports except the incoming port.

❌ D. Switch1 sends an ARP request to obtain the MAC address of PC-C.

Incorrect:

ARP (Address Resolution Protocol) is only used to resolve IP addresses to MAC addresses.

Since PC-A is sending a frame (Layer 2), not an IP packet (Layer 3), ARP is not involved here.

Conclusion

Since Switch1 does not know the destination MAC address, it floods the frame out all active ports except the port it was received on. This is the default behavior of Layer 2 switches when they encounter an unknown MAC address.

Thus, the correct answer is:✔ C. Switch1 floods the frame out all active ports except port G0/1.

References

Cisco CCNA 200-301 Official Guide – MAC Address Table & Frame Forwarding

RFC 894 – Standard for Ethernet Frame Forwarding

Cisco Networking Essentials – Switch Flooding Behavior

The correct matching of the protocols to their examples is as follows:

DHCP: Assign the reserved IP address 10.10.10.200 to a web server at your company.

DNS: Perform a query to translate companypro.net to an IP address.

ICMP: Perform a ping to ensure that a server is responding to network connections.

Here’s how each protocol corresponds to its example:

DHCP (Dynamic Host Configuration Protocol) is used to assign IP addresses to devices on a network. In this case, DHCP would be used to assign the reserved IP address 10.10.10.200 to a web server.

DNS (Domain Name System) is used to translate domain names into IP addresses. Therefore, to translate companypro.net to an IP address, DNS would be utilized.

ICMP (Internet Control Message Protocol) is used for sending error messages and operational information indicating success or failure when communicating with another IP address. An example of this is using the ping command to check if a server is responding to network connections.

These protocols are essential for the smooth operation of networks and the internet.

Perform a query to translate companypro.net to an IP address.

DNS (Domain Name System): DNS is used to resolve domain names to IP addresses.

Assign the reserved IP address 10.10.10.200 to a web server at your company.

DHCP (Dynamic Host Configuration Protocol): DHCP is used to assign IP addresses to devices on a network.

Perform a ping to ensure that a server is responding to network connections.

ICMP (Internet Control Message Protocol): ICMP is used by network devices to send error messages and operational information, and it is the protocol used by the ping command.

DNS (Domain Name System): DNS translates human-friendly domain names like "companypro.net" into IP addresses that computers use to identify each other on the network.

DHCP (Dynamic Host Configuration Protocol): DHCP automatically assigns IP addresses to devices on a network, ensuring that no two devices have the same IP address.

ICMP (Internet Control Message Protocol): ICMP is used for diagnostic or control purposes, and the ping command uses ICMP to test the reachability of a host on an IP network.

References:

DNS Basics: What is DNS?

DHCP Overview: What is DHCP?

ICMP and Ping: Understanding ICMP

Based on the image description provided, here are the cable types matched with the appropriate connection descriptions:

Connects Switch S1 to Router R1 Gi0/0/1 interface Cable Type: = Straight-through UTP Cable

Connects Router R2 Gi0/0/0 to Router R3 Gi0/0/0 via underground conduit Cable Type: = Fiber Optic Cable

Connects Router R1 Gi0/0/0 to Router R2 Gi0/0/1 Cable Type: = Crossover UTP Cable

Connects Switch S3 to Server0 network interface card Cable Type: = Straight-through UTP Cable

The choices are based on standard networking practices where:

Straight-through UTP cables are typically used to connect a switch to a router or a network interface card.

Fiber optic cables are ideal for long-distance, high-speed data transmission, such as connections through an underground conduit.

Crossover UTP cables are used to connect similar devices, such as router-to-router connections.

These matches are consistent with the color-coded cables in the image: green for switch connections, yellow for router-to-router connections within the same rack, and blue for inter-rack connections. The use of these cables follows the Ethernet cabling standards.

Connects Switch S1 to Router R1 Gi0/0/1 interface:

Cable Type: Straight-through UTP Cable

Explanation: A straight-through UTP cable is typically used to connect different types of devices, such as a switch to a router.

Connects Router R2 Gi0/0/0 to Router R3 Gi0/0/0 via underground conduit:

Cable Type: Fiber Optic Cable

Explanation: Fiber optic cables are used for long-distance connections, such as those through an underground conduit between buildings.

Connects Router R1 Gi0/0/0 to Router R2 Gi0/0/1:

Cable Type: Crossover UTP Cable

Explanation: A crossover UTP cable is typically used to connect similar devices directly, such as router to router connections.

Connects Switch S3 to Server0 network interface card:

Cable Type: Straight-through UTP Cable

Explanation: A straight-through UTP cable is typically used to connect a switch to an end device, such as a server.

Straight-through UTP Cable: Used to connect different devices (e.g., switch to router, switch to server).

Crossover UTP Cable: Used to connect similar devices directly (e.g., router to router, switch to switch).

Fiber Optic Cable: Used for long-distance and high-speed connections, often between buildings or data centers.

References:

Network Cable Types and Uses: Cisco Network Cables

Understanding Ethernet Cabling: Ethernet Cable Guide

The command that will display the output provided, which includes capability codes, local interface details, device IDs, hold times, and platform port ID capabilities, is the show cdp neighbor command. This command is used in Cisco devices to display current information about neighboring devices detected by Cisco Discovery Protocol (CDP), which includes details such as the interface through which the neighbor is connected, the type of device, and the port ID of the device1.

References :=

•Cisco - show cdp neighbors

The provided output is from the Cisco Discovery Protocol (CDP) neighbor table. The show cdp neighbor command displays information about directly connected Cisco devices, including Device ID, Local Interface, Holdtime, Capability, Platform, and Port ID.

•A. show mac-address-table: Displays the MAC address table on the switch.

•C. show inventory: Displays information about the hardware inventory of the device.

•D. show ip interface: Displays IP interface status and configuration.

Thus, the correct answer is B. show cdp neighbor.

References :=

•Cisco CDP Neighbor Command

•Understanding CDP

When prioritizing trouble tickets, the most critical issues affecting business operations or high-impact activities should be addressed first. Here's a breakdown of the tickets:

Ticket 1: Relocation of a printer, while necessary, is not urgent and does not impact critical operations.

Ticket 2: An ongoing webinar losing internet access is critical, especially if the webinar is time-sensitive and involves multiple participants.

Ticket 3: Slower response time for a cloud-based application is important but typically not as urgent as a complete loss of internet access for a live event.

Ticket 4: Wireless access down in the cafeteria affects users but does not have the same immediate impact as a live webinar losing connectivity.

Thus, the correct answer is B. Ticket 2: An online webinar is taking place in the conference room. The video conferencing equipment lost internet access.

References :=

IT Help Desk Best Practices

Prioritizing IT Support Tickets

In the provided image of the Cisco PoE switch, the ports identified with number 6 are the standard RJ-45 Ethernet ports typically found on switches that provide both data connectivity and Power over Ethernet (PoE). PoE ports are designed to supply power to devices such as IP phones, wireless access points, and other PoE-enabled devices directly through the Ethernet cable.

Ports:

•2: Console port (for management and configuration)

•3 and 4: Specific function ports (often for management)

•6: RJ-45 Ethernet ports (capable of providing PoE)

•7: SFP ports (for fiber connections, typically do not provide PoE)

Thus, the correct answer is C. Ports identified with number 6.

References :=

•Cisco Catalyst 2960-L Series Switches Data Sheet

•Cisco PoE Overview

OSI model

During the data encapsulation process, the Data Link layer of the OSI model is responsible for adding a header that contains MAC addressing information and a trailer used for error checking. The header typically includes the source and destination MAC addresses, while the trailer contains a Frame Check Sequence (FCS) which is used for error detection1.

The Data Link layer ensures that messages are delivered to the proper device on a LAN using hardware addresses and translates messages from the Network layer into bits for the Physical layer to transmit. It also controls how data is placed onto the medium and is received from the medium through the physical hardware.

References :=

The OSI Model – The 7 Layers of Networking Explained in Plain English

OSI Model - Network Direction

Which layer adds both header and trailer to the data?

What is OSI Model | 7 Layers Explained - GeeksforGeeks

Statement 1: Low bandwidth can increase network latency.

True: Low bandwidth can result in increased network latency because the network may become congested, leading to delays in data transmission.

Statement 2: High levels of network latency decrease network bandwidth.

False: High levels of network latency do not decrease the available network bandwidth, but they do affect the perceived performance and throughput of the network.

Statement 3: You can increase throughput by decreasing network latency.

True: Decreasing network latency can increase throughput because data can be transmitted more quickly and efficiently without delays.

Bandwidth vs. Latency: Bandwidth refers to the maximum rate at which data can be transferred over a network path. Latency is the time it takes for a data packet to travel from the source to the destination.

Low bandwidth can cause network congestion, which can increase latency as packets wait to be transmitted.

High latency does not reduce the actual bandwidth but can affect the overall performance and efficiency of data transmission.

Reducing latency can lead to higher throughput because the network can handle more data in a given period without delays.

References:

Network Performance Metrics: Cisco Network Performance

Understanding Bandwidth and Latency: Bandwidth vs. Latency

Out-of-band management

When a Cisco switch is not accessible from the network, the recommended out-of-band method to access its running configuration is through the console port. Out-of-band management involves accessing the network device through a dedicated management channel that is not part of the data network. The console port provides direct access to the switch’s Command Line Interface (CLI) without using the network, which is essential when the switch cannot be accessed remotely via the network12.

References :=

Out-of-band (OOB) network interface configuration guidelines

Out of band management configuration

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The tracert command output shows the path taken to reach the destination IP address, 192.168.1.10. The command output indicates:

•Hops 1 and 2 are successfully reached.

•Hop 3 times out, meaning the router at hop 3 did not respond to the tracert request. However, this does not necessarily indicate a problem with forwarding packets, as some routers may be configured to block or not respond to ICMP requests.

•Hops 4 and 5 are successfully reached, with hop 5 being the destination IP 192.168.1.10, indicating that the server is reachable.

Thus, the correct answer is C. The server with the address 192.168.1.10 is reachable over the network.

References :=

•Cisco Traceroute Command

•Understanding Traceroute

The tracert command output indicates that the server with the address 192.168.1.10 is reachable over the network. The asterisk (*) at hop 3 suggests that the probe sent to that hop did not return a response, which could be due to a variety of reasons such as a firewall blocking ICMP packets or the router at that hop being configured not to respond to ICMP requests. However, since the subsequent hops (4 and 5) are showing response times, it means that the packets are indeed getting through and the server is reachable12.

References :=

•How to Use Traceroute Command to Read Its Results

•How to Use the Tracert Command in Windows