3.1 Layer 2

What this section tests

Section 3.1 covers the three ways Layer 2 fails on the exam:

1. A VLAN does not cross a link (trunking).
2. Several links do not act like one link (EtherChannel).
3. Spanning tree blocks, protects, or converges in an unexpected way.

For each topic, name the control point and the proof command:

• Trunking: the control point is the trunk state on both ends of the link. Proof: show interfaces trunk.
• EtherChannel: the control point is the port-channel and its member state. Proof: show etherchannel summary.
• Spanning tree: the control point is the root bridge election and the port roles. Proof: show spanning-tree.

Study order

1. 802.1Q trunking
2. EtherChannels
3. RSTP, MST, root guard, and BPDU guard

Pass check

You are ready for section 3.1 when you can do these things:

• Find a trunk mismatch from show interfaces trunk output.
• Explain why a port-channel member is suspended or standalone.
• Name the root bridge and the role of each port in a small topology.