Network Access

What this domain actually tests

Switching: VLANs, trunking, inter-VLAN routing, EtherChannel, and spanning tree — plus wireless fundamentals.

Spanning Tree is best learned as a decision procedure you can execute, not a concept you can describe:

1. The switch with the lowest bridge ID (priority first, then MAC address) becomes the root bridge.
2. Every non-root switch selects exactly one root port — the one with the lowest path cost back to the root.
3. Each segment selects one designated port.
4. Everything else blocks.

The exam shows you show spanning-tree output and asks you to identify the root bridge, or explain why a particular port is blocking. Run that four-step procedure and the answer falls out.

The traps

The native VLAN mismatch. On an 802.1Q trunk, frames in the native VLAN are sent untagged. If two ends of a trunk disagree about which VLAN is native, traffic leaks between VLANs — a security problem and a connectivity problem at once.

This is a favourite scenario, and the fix is to make the native VLAN match on both ends (and, as a hardening practice, to use an unused VLAN as the native one).

Trunk versus access ports. An access port carries one VLAN and sends untagged frames to an end device. A trunk carries many and tags them. Read show interfaces trunk and know what "allowed VLANs" means.

EtherChannel needs consistent configuration on every member port — same speed, duplex, VLAN configuration, and mode. A single mismatched port stops the bundle forming. Know the negotiation protocols (LACP is the open standard; PAgP is Cisco proprietary) and that mismatched modes on either end will fail.

Priority is set in increments of 4096. Lowering a switch's priority is how you force it to be root, and questions ask how to make a specific switch the root bridge — the answer is to set a lower priority, not to change its MAC.

Inter-VLAN routing. Router-on-a-stick (subinterfaces on one physical link) versus a Layer 3 switch with SVIs. Know both configurations by sight.

How to study it

Build a three-switch topology in Packet Tracer. Create VLANs. Trunk them. Then break the native VLAN on purpose and read what show interfaces trunk tells you about the mismatch.

Then force a different switch to become root by changing its priority, and watch the port roles recalculate. Generating show spanning-tree output yourself twenty times is what makes reading it effortless on exam day — and staring at it blankly is exactly what happens to candidates who only read about STP.

At 20% this is a fifth of the exam, and it is almost entirely a lab domain. Hands on the CLI is the only way through it.