This objective covers how switches organize traffic, manage performance, and prevent disastrous loops inside a local area network (LAN). As an IT professional running a sophisticated home lab with multiple VLANs and OPNsense, you likely have hands-on familiarity with some of these concepts, but let’s review them through the exact lens CompTIA uses for the exam.
1. VLANs & Layer 3 Switching
Virtual Local Area Network (VLAN)
- What it does: Break up a single physical switch into multiple isolated, logical networks. Devices on different VLANs cannot talk to each other without a router or a Layer 3 switch.
- VLAN Database: The file stored on the switch (traditionally
vlan.datin Cisco environments) that keeps track of all the VLAN IDs and names configured on that device.
SVI (Switch Virtual Interface)
- What it is: A virtual Layer 3 interface configured on a switch to give a specific VLAN an IP address.
- Why use it: It serves two main purposes: it provides an IP address so you can remotely manage the switch (e.g., SSH into VLAN 1), or it acts as the default gateway to route traffic between different local VLANs right on the switch itself (Layer 3 switching), bypassing an external router.
2. Interface Configuration & Trunking
802.1Q Tagging & Native VLAN
When a cable connects two switches, it must carry traffic for all VLANs. This link is called a Trunk Port.
- 802.1Q Tagging: The standard protocol that injects a 4-byte “tag” into the Ethernet frame header. This tag specifies exactly which VLAN the data belongs to, so the receiving switch knows where to send it.
- Native VLAN: The one specific VLAN on a trunk port that does not get a tag injected into its headers. If a switch receives an untagged frame on a trunk port, it automatically drops it into the Native VLAN.
- Exam Warning: Both sides of a trunk link must have matching Native VLAN configurations. A mismatch will trigger errors and cause traffic to leak between networks.
Specialized Ports, Speed, and Duplex
- Voice VLAN: A separate, prioritized VLAN configured on an access port alongside a data VLAN. It isolates IP phone traffic from regular PC traffic, often pairing with Quality of Service (QoS) to prevent choppy phone calls.
- Link Aggregation (LAG/EtherChannel): Binds multiple physical switch ports into a single logical, high-bandwidth link. If you combine four 1 Gbps cables, you get a 4 Gbps pipe that also provides redundancy if one cable is accidentally unplugged.
- Speed & Duplex:
- Speed: Must match on both ends (e.g., 100 Mbps or 1 Gbps).
- Duplex: Full-Duplex allows devices to send and receive data simultaneously. Half-Duplex forces devices to take turns (like a walkie-talkie). A mismatch here causes severe network slowdowns and packet errors.
3. Spanning Tree Protocol (STP)
If you connect two switches with two separate cables for redundancy, packets will endlessly loop. This creates a Broadcast Storm that will completely crash the local network within seconds.
- What STP does: It dynamically prevents Layer 2 loops. It monitors the topology, elects a master switch called the Root Bridge, and strategically shuts down/blocks backup ports to maintain a single clear, loop-free path. If a live cable is cut, STP automatically opens the blocked backup port to restore connectivity.
4. MTU & Jumbo Frames
- Maximum Transmission Unit (MTU): The maximum size (in bytes) of a single packet or frame that can be sent across a network interface. The standard Ethernet MTU is 1,500 bytes.
- Jumbo Frames: Specialized Ethernet frames that support an MTU up to 9,000 bytes.
- Why use them: They are commonly used in high-performance environments like a Storage Area Network (SAN) or backup networks to move huge chunks of data efficiently while reducing CPU overhead.
- Exam Catch: Every single device along the path (the server, the switch ports, and the storage target) must support and be configured for Jumbo Frames, or the packets will be dropped.
Switching Comparison Matrix for the Exam
| Term | Category | Key Identifying Phrase for the Exam |
| 802.1Q | Trunking | Standard protocol used to tag VLAN traffic across a trunk. |
| SVI | Layer 3 | Virtual interface providing an IP address to an entire VLAN. |
| Native VLAN | Trunking | Handles all untagged traffic crossing a trunk port link. |
| LAG / LACP | Performance | Bundles physical links for higher bandwidth and redundancy. |
| STP | Loop Prevention | Elects a Root Bridge and blocks ports to stop broadcast storms. |
| Jumbo Frames | Performance | 9,000-byte MTU used to optimize heavy storage/SAN traffic. |