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Cisco CCNA Switching and VLANs

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Question 1

Which of the following are Layer 2 attack types that target switch infrastructure? (Select THREE)

Answer choices

  • A. MAC address flooding (CAM table overflow attack) (Correct)

  • B. VLAN hopping via double-tagging (Correct)

  • C. IP spoofing via forged source addresses

  • D. ARP poisoning/ARP spoofing (Correct)

  • E. Route redistribution manipulation

Explanation

Layer 2 switch attacks: (A) MAC flooding — attacker sends millions of frames with random source MACs, filling the CAM table. The switch then floods all frames (unknown unicast flooding) — attacker can capture all traffic. Mitigate with port security (limit MACs per port). (B) VLAN hopping (double-tagging) — attacker on native VLAN adds extra 802.1Q tag; first switch removes outer tag, second switch uses inner tag to forward to another VLAN. Mitigate by changing native VLAN to unused VLAN. (D) ARP poisoning — attacker sends fake ARP replies, mapping their MAC to another device's IP — enables MitM attack. Mitigate with DAI. (C) IP spoofing is Layer 3. (E) Route redistribution is a routing protocol attack.

Question 2

What is EtherChannel (LACP/PAgP), and what problem does it solve?

Answer choices

  • A. A method for encrypting traffic between switches

  • B. Bundling multiple physical links into a single logical link to increase bandwidth and provide redundancy while avoiding STP blocking (Correct)

  • C. A way to run multiple VLANs on a single trunk port

  • D. A protocol for automatically assigning IP addresses to switch management interfaces

Explanation

EtherChannel bundles 2–8 parallel physical Ethernet links into one logical link (port-channel). Benefits: combined bandwidth (e.g., 4×1Gbps links = 4Gbps logical link), link redundancy (if one physical link fails, traffic continues on remaining links), and STP sees only one logical link (no blocked ports — all bandwidth is used). LACP (802.3ad) is the IEEE standard for EtherChannel negotiation. PAgP is Cisco's proprietary EtherChannel negotiation protocol. Both sides must agree on settings (speed, duplex, VLAN configuration) for EtherChannel to form.

Question 3

Cisco's Rapid PVST+ (802.1w per-VLAN) eliminates the need for PortFast because RSTP convergence is fast enough that end device connections are not delayed.

Answer choices

  • A. True

  • B. False (Correct)

Explanation

While Rapid PVST+ (RSTP 802.1w) converges much faster than classic STP (sub-second vs. 30–50 seconds for topology changes), PortFast is still used and recommended on access ports. RSTP still requires a brief negotiation period for new port connections (link comes up → edge port proposal/agreement). PortFast (called "edge port" in RSTP terminology) immediately transitions to forwarding for access ports connecting to end devices, eliminating any delay. PortFast + BPDU Guard remains best practice even with Rapid PVST+ — any PortFast/edge port that receives a BPDU indicates a switch was connected where only an end device should be.

Question 4

By default, all ports on a Cisco switch belong to VLAN 1.

Answer choices

  • A. True (Correct)

  • B. False

Explanation

On Cisco switches, all ports are in VLAN 1 by default (the default VLAN). VLAN 1 is also the default native VLAN for trunk links and the default management VLAN. However, best practices recommend: moving all user/device ports off VLAN 1 (change native VLAN), creating purpose-specific VLANs (data, voice, management, native), and leaving VLAN 1 unused. VLAN 1 traffic on trunks is untagged (no 802.1Q tag) and changing the native VLAN is important for security.

Question 5

In a standard 802.1D STP topology, the designated port on each network segment is the port with the lowest path cost to the Root Bridge. If path costs are equal, the port with the highest bridge ID wins.

Answer choices

  • A. True

  • B. False (Correct)

Explanation

In STP port election: each segment has one designated port (the port that forwards frames on that segment). The port selected is the one with the lowest cumulative path cost to the Root Bridge. If path costs are equal, the port belonging to the switch with the LOWEST Bridge ID wins (not highest). If Bridge IDs are also equal (unlikely in practice), the port with the lowest port ID (port priority + port number) wins. All STP tiebreakers favor LOWER values — lower bridge ID, lower port priority, lower port number.