200-301 - Network Fundamentals (20% of the exam) - Section 1.13
Describe switching concepts - MAC learning and aging, frame switching, frame flooding, and the MAC address table - and compare TCP to UDP.
Explain how a switch builds its MAC address table by learning source addresses, ages out stale entries, forwards known-unicast frames out a single port, and floods unknown-unicast, broadcast, and multicast frames out all ports in the VLAN. Compare connection-oriented TCP, with its three-way handshake, sequencing, and acknowledgements, against connectionless UDP, and choose the right transport for a given application.
MAC address tableFrame floodingMAC learning and agingTCP three-way handshakeTCP vs UDP
Practice question for this objective
lock_openFree sampleNetwork Fundamentalsmedium
An engineer is documenting how a Layer 2 switch builds and uses its MAC address table on a single VLAN with default settings. Which statements accurately describe the switch's behaviour as it learns addresses and forwards frames? Select THREE.
AIt records the source MAC of each arriving frame against the port on which the frame was received.check_circle Correct
BIt floods a unicast frame out every port in the VLAN except the ingress port when the destination MAC is unknown.check_circle Correct
CIt populates the table using the destination MAC of each frame and the port towards which the frame is sent.
DIt refreshes the ageing timer for an existing entry every time that source MAC sends another frame.check_circle Correct
EIt consults the destination IP address to choose the egress port for a frame within the VLAN.
A switch learns source MAC addresses against ingress ports, refreshes entries on activity, and floods unknown-unicast frames within the VLAN. Layer 2 forwarding is driven by the MAC address table: source addresses are learned per ingress port and their timers refresh on each frame, while a frame whose destination is not yet in the table is flooded to all other ports in the VLAN so the destination can still be reached. Destination MAC drives forwarding lookups, not learning, and IP plays no part in intra-VLAN switching.
Why A is correct: MAC learning works on the source address of inbound frames, mapping that address to the ingress port so future traffic can be sent there.
Why B is correct: An unknown-unicast frame is flooded to all other ports in the VLAN so it can reach a host the table has not yet learned, while the ingress port is excluded.
Why C is wrong: This is tempting because forwarding uses the destination MAC, but learning is based on the source MAC and the ingress port, not the destination.
Why D is correct: Each frame from a known source resets the entry's ageing timer, so an active host stays in the table and only idle entries are removed.
Why E is wrong: This confuses Layer 3 routing with Layer 2 switching, since a switch forwards inside a VLAN using the destination MAC, not the IP address.
Answers that look right on this material and are not. Each one is a distractor from a different question in the 200-301 bank for this domain.
It drops the frame and sends an ARP request back to the original sender to resolve the destination.
errorWhy it is wrong: Switches do not generate ARP on a host's behalf and do not drop unknown unicasts; ARP is an end-host process, so this confuses Layer 2 forwarding with Layer 3 address resolution.
It forwards the frame only out Gi0/2 and Gi0/3, the two ports already learned in VLAN 10.
errorWhy it is wrong: This looks reasonable because those are the known ports, but a switch does not restrict an unknown unicast to previously learned ports; it floods to every other port in the VLAN.
The entry is removed as soon as a frame destined to that host arrives on a different port.
errorWhy it is wrong: Receiving a frame addressed to the host does not delete its entry; entries are keyed on source addresses and aging, not on incoming destination traffic, so this is wrong.