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When would a switch record multiple entries for a single switch port in its MAC address table?

• when another switch is connected to the switch port
• when multiple ARP broadcasts have been forwarded
• when a router is connected to the switch port
• when the switch is configured for Layer 3 switching

The correct answer is:

“when another switch is connected to the switch port.”

Detailed Explanation:

Understanding the MAC Address Table

Before diving into the specific scenario, it’s important to understand how a switch operates in terms of MAC address management. A switch is a Layer 2 device in the OSI model that operates primarily using MAC addresses (Media Access Control addresses) to forward frames between devices on a local network. Each switch maintains a MAC address table, which is a dynamic database that maps MAC addresses to specific switch ports.

The MAC address table is built as the switch learns the source MAC addresses of frames that it receives on its ports. When a frame arrives at a switch, the switch examines the source MAC address and records it in its MAC address table along with the port number from which the frame was received. This allows the switch to efficiently forward frames destined for that MAC address by directing them out of the correct port.

Scenario: When Another Switch is Connected to the Switch Port

When another switch is connected to a port on the first switch, the situation changes compared to when a single end device (like a computer or server) is connected to that port. In this scenario, the switch port connected to the other switch becomes a trunk port or an access port carrying traffic for multiple devices or multiple VLANs.

Here’s why this leads to multiple entries in the MAC address table for that port:

1. Multiple Devices Behind the Port:
   • When a switch is connected to another switch, the traffic from multiple devices connected to the second switch can pass through the port on the first switch. For each device connected to the second switch that sends traffic, the first switch will learn the MAC addresses of those devices. Since all these devices are behind the single switch port (connected to the second switch), the first switch will record multiple MAC addresses mapped to that one port.
2. Forwarding Decisions:
   • The first switch uses its MAC address table to make forwarding decisions. When it receives a frame destined for a MAC address that it has learned is behind the connected switch, it forwards the frame out of the specific port that connects to the second switch.
3. Dynamic MAC Address Learning:
   • The MAC address table is dynamic, meaning it continuously updates as new MAC addresses are discovered and old ones time out. As new devices communicate through the connected switch, their MAC addresses are added to the MAC address table under the same port, leading to multiple entries associated with that single port.

Other Scenarios:

Let’s briefly analyze why the other scenarios provided in the question would not lead to multiple entries for a single port in the MAC address table:

1. When Multiple ARP Broadcasts Have Been Forwarded:
   • ARP (Address Resolution Protocol) broadcasts are used to map IP addresses to MAC addresses. When an ARP broadcast is sent, it is flooded out all switch ports, but it does not result in multiple MAC addresses being associated with a single port. Instead, it typically results in the switch learning the MAC address of the device that responds to the ARP request, and this MAC address is mapped to the port from which the ARP reply is received.
2. When a Router is Connected to the Switch Port:
   • A router connected to a switch port typically results in only one MAC address being recorded for that port – the MAC address of the router’s interface connected to the switch. Routers generally serve as gateways between different networks and don’t have multiple MAC addresses on a single interface (unless using subinterfaces with different VLANs, but that still doesn’t change the fundamental port-to-MAC address mapping for each VLAN).
3. When the Switch is Configured for Layer 3 Switching:
   • Layer 3 switching involves routing packets based on IP addresses, but it doesn’t directly affect the MAC address table in terms of multiple entries for a single port. Even when Layer 3 functionality is enabled, the MAC address table operates at Layer 2, mapping MAC addresses to specific ports. The switch might route traffic between VLANs, but each VLAN would still have its own set of MAC addresses mapped to the relevant ports.

Practical Implications of Multiple MAC Addresses on a Single Port

1. Network Segmentation and Design:
   • In a typical enterprise network, switches are often connected to other switches to extend the network and segment traffic into different VLANs or departments. The MAC address table on each switch helps ensure that traffic is only forwarded to the appropriate ports, which helps in efficient network segmentation and reduces unnecessary traffic.
2. Troubleshooting and Security:
   • Knowing that multiple MAC addresses can be associated with a single switch port is crucial during network troubleshooting. For instance, if an administrator sees many MAC addresses associated with a single port, they can deduce that another switch or a hub is connected to that port. This could also indicate a potential security risk, such as a rogue switch being connected to the network, or it could be part of the normal network design.
3. Port Security:
   • Cisco switches offer a feature called Port Security that can limit the number of MAC addresses learned on a specific port. This is particularly useful for preventing unauthorized devices from connecting to the network. In cases where multiple MAC addresses are detected on a port that is supposed to be connected to a single device, Port Security can shut down the port or take other predefined actions.
4. Spanning Tree Protocol (STP):
   • When multiple switches are connected, Spanning Tree Protocol (STP) is often used to prevent loops in the network. Multiple MAC addresses on a single port could indicate that the port is part of a larger switch topology that requires STP to maintain a loop-free network.
5. Network Monitoring:
   • Network monitoring tools can track the MAC address table entries and alert administrators to changes, such as the appearance of multiple MAC addresses on a single port. This can help in detecting unauthorized devices or potential configuration issues in the network.

Conclusion

In conclusion, a switch would record multiple entries for a single switch port in its MAC address table when another switch is connected to that port. This is because the connected switch forwards traffic from multiple devices through the single port on the first switch, leading to the learning and recording of multiple MAC addresses. Understanding this behavior is fundamental to effective network design, troubleshooting, and security management. It highlights the importance of proper network segmentation, the use of VLANs, and the need for security features like Port Security to maintain the integrity and performance of the network.