A switch's MAC table has entries for ports 1–3. A frame arrives on Port 1 with source MAC AA:AA:AA:AA:AA:01 and destination MAC DD:DD:DD:DD:DD:04 (not in the table). What does the switch do?
ADrops the frame, because it cannot forward to an unknown destination
BSends it back out Port 1, because that is where the frame arrived from
CFloods the frame out every port except Port 1, to ensure delivery to the unknown destination
DQueries a central directory service to look up the location of DD:DD:DD:DD:DD:04
When a switch receives a frame destined for a MAC address not in its table, it floods — sends the frame out all ports except the one it arrived on. This guarantees delivery even without complete knowledge. Flooding also occurs for broadcast frames (destination FF:FF:FF:FF:FF:FF). As the destination device responds, its source MAC address is learned and the switch adds it to the table. Subsequent frames to that destination will be forwarded precisely, not flooded. The key rule: unknown destination → flood; known destination → forward to specific port only.
Question 2 Multiple Choice
When a new device connects to a switch and sends its first frame, how does the switch learn the device's MAC address?
AThe switch broadcasts a discovery query and the device responds with its MAC address
BThe switch reads the source MAC address of the arriving frame and records which port it arrived on
CThe network administrator manually enters the MAC-to-port mapping in the switch configuration
DThe device sends a special MAC registration frame that the switch stores separately
Switch learning is entirely passive and automatic. Every incoming frame contains a source MAC address, and the switch records 'this MAC address is reachable via this port' in its CAM table. No special registration or configuration is required. This is why MAC learning is called a 'transparent' process — devices have no idea a switch is observing their traffic. Table entries age out after ~300 seconds so the switch adapts as devices move or disconnect. The switch only learns from SOURCE addresses; it uses DESTINATION addresses for forwarding decisions.
Question 3 True / False
A hub and a switch both connect multiple devices on a network. When a frame arrives, both devices forward it to most connected ports, making them functionally equivalent for normal network operation.
TTrue
FFalse
Answer: False
This is the core distinction: a hub always floods every frame to every port — it has no intelligence and no MAC table. A switch floods only for unknown destinations and broadcasts; for known destinations, it forwards exclusively to the appropriate port. The practical consequences are dramatic: on a hub, all devices share the same collision domain and the same bandwidth. On a switch, each port is its own collision domain — devices on different ports can transmit simultaneously, and the switch's internal fabric can handle multiple conversations in parallel (microsegmentation), delivering full bandwidth to each port.
Question 4 True / False
Each port on a network switch is its own collision domain, which allows multiple devices on different ports to transmit simultaneously without interfering with each other.
TTrue
FFalse
Answer: True
Microsegmentation is the defining performance advantage of switches over hubs. On a hub, all ports share a single collision domain: if two devices transmit at the same time, the signals collide and both must retransmit (CSMA/CD). On a switch, each port connects to an independent collision domain. Two devices on different ports can transmit simultaneously; the switch's internal switching fabric handles both frames concurrently. A 24-port switch effectively gives each device a dedicated, full-bandwidth link — something impossible with a hub regardless of its port count.
Question 5 Short Answer
Explain what 'flooding' is in network switching, when it occurs, and what mechanism causes its frequency to decrease over time on an active network.
Think about your answer, then reveal below.
Model answer: Flooding is when a switch sends a received frame out every port except the one it arrived on. It occurs in two situations: (1) when the destination MAC address is not yet in the switch's MAC address table — the switch doesn't know which port leads to the destination; and (2) always, for broadcast frames with destination FF:FF:FF:FF:FF:FF. As devices communicate normally, every frame they send teaches the switch about their location: the switch records the source MAC address and ingress port, building its table. Once a destination MAC is in the table, the switch forwards precisely to that port instead of flooding. On an active network, the MAC table fills in quickly and steady-state flooding drops to only genuine broadcasts and occasional unicast floods for new or moved devices.
Understanding flooding is essential because it explains both the initial behavior of a new switch (it floods everything until it learns) and persistent flooding-related issues like broadcast storms. Flooding is a safety mechanism — it ensures delivery even with incomplete knowledge — but it generates traffic that all ports must process. Table aging (entries expire after ~300 seconds) is the mechanism that handles devices moving or disconnecting: the stale entry times out, and the next frame from that device teaches the switch its new location.