VLANs and Layer 2 Technologies
What you must understand
A VLAN is a broadcast domain; a tagged (trunk) port carries multiple VLANs with 802.1Q tags, and an untagged (access) port carries one VLAN to end devices. On ArubaOS-CX, VLAN membership is configured under the interface (vlan trunk allowed, vlan access); on AOS-S it is configured under the VLAN (tagged/untagged per port). The native/untagged VLAN on a trunk is the one that travels without a tag - mismatched native VLANs are a classic troubleshooting scenario.
Also know: GVRP/MVRP for dynamic VLAN registration, voice VLAN (LLDP-MED or voice on AOS-S), private VLANs (primary/isolated/community), jumbo frames (MTU per VLAN/port), and MAC address tables (aging, static MACs, show mac-address-table).
Traps
- Direction of configuration: CX = interface-centric; AOS-S = VLAN-centric. Questions reverse these.
- Only one untagged/native VLAN per trunk port.
- A port must be a member of a VLAN and the VLAN must exist - creating one without the other is a common distractor.
- 802.1Q tag is 4 bytes; max frame with tag is 1522 bytes.
Worked example
You must pass VLANs 10 and 20 to a downstream switch on CX port 1/1/5, with VLAN 10 untagged to a PC. On CX: interface 1/1/5 → no routing → vlan trunk native 10 → vlan trunk allowed 10,20. On AOS-S: vlan 10 untagged 5 and vlan 20 tagged 5. Same result, opposite syntax - the exam tests exactly this.