2.3 - Switching features: VLANs, port security and LACP

Network+ N10-009 objective 2.3 covers switch configuration and features. Port security limits how many MAC addresses a port will learn, blocking rogue devices or MAC flooding. On an 802.1Q trunk, the native VLAN carries untagged frames, and mismatched native VLANs cause problems. Jumbo frames are Ethernet frames larger than the standard 1500-byte MTU, up to about 9000 bytes, which improve efficiency for storage and backups when enabled end to end. Port aggregation with LACP bonds two switch uplinks to double bandwidth and add redundancy. You should also know VLAN tagging, trunking, Spanning Tree Protocol, and MTU. Expect scenario questions that describe limiting MACs on a port, untagged trunk traffic, oversized frames or combining uplinks, and ask which switching feature - port security, native VLAN, jumbo frames or LACP - is being described.

Memory hook
Limit MACs learned on a port = port security. Untagged frames on an 802.1Q trunk = native VLAN. Frames beyond 1500 MTU (~9000) = jumbo frames. Bond two uplinks for speed + redundancy = LACP.

Practice questions

1. Which protocol prevents switching loops in a network with redundant links?

  • VTP
  • STP (correct answer)
  • LACP
  • CDP

STP (Spanning Tree Protocol) blocks redundant paths to prevent Layer 2 loops and broadcast storms. LACP bundles links, CDP/VTP are Cisco discovery/VLAN sync.

2. Which PoE standard (802.3bt) delivers the highest power to a device?

  • 802.3af (PoE)
  • 802.3bt (PoE++) (correct answer)
  • 802.3at (PoE+)
  • 802.3u (Fast Ethernet)

802.3bt (PoE++, Type 3/4) supplies up to ~60-100 W at the port. 802.3at (PoE+) is ~30 W, 802.3af (PoE) is ~15.4 W, 802.3u is Fast Ethernet (not PoE).

3. Which technology bundles several physical links into one logical link for more bandwidth?

  • Spanning Tree Protocol (STP)
  • VLAN trunking with 802.1Q
  • Port mirroring (SPAN)
  • Link aggregation (LACP) (correct answer)

Link aggregation (LACP/802.3ad) bonds multiple links into one logical channel for higher throughput and redundancy. STP blocks loops, trunking carries VLANs.

4. Which switch feature limits how many MAC addresses a port will learn?

  • DHCP snooping
  • Storm control
  • Port security (correct answer)
  • BPDU guard

Port security caps the MACs allowed on a port and can shut it down on violation, stopping MAC flooding and rogue devices. DHCP snooping targets rogue DHCP.

5. Which term describes a VLAN that carries untagged frames on an 802.1Q trunk?

  • Voice VLAN
  • Management VLAN
  • Native VLAN (correct answer)
  • Guest VLAN

The native VLAN carries untagged traffic on a trunk. Best practice sets it to an unused VLAN to reduce VLAN hopping risk.

6. Which STP enhancement lets an access port skip to forwarding for end devices?

  • Root guard
  • BPDU guard
  • Loop guard
  • PortFast (correct answer)

PortFast moves an access port straight to forwarding, skipping listening/learning for hosts. BPDU guard disables the port if a BPDU arrives there.

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