VLANs and trunking for the CCNA
VLANs and trunks are the core of the Network Access domain, which is 20% of the CCNA 200-301 exam. This guide covers what you must be able to explain and configure.
Why VLANs exist
A VLAN splits one physical switch into several separate Layer 2 broadcast domains. Devices in different VLANs cannot talk directly; traffic between them needs a Layer 3 device. That reduces broadcast traffic and improves security.
VLAN ranges
Normal-range VLANs are 1–1005 and extended-range VLANs are 1006–4094. VLAN 1 is the default VLAN and cannot be deleted.
Create a VLAN and assign an access port
vlan 10 name SALES interface g0/1 switchport mode access switchport access vlan 10
802.1Q trunks
A trunk carries several VLANs between switches. 802.1Q inserts a 4-byte tag into the Ethernet frame holding the VLAN ID. Frames in the native VLAN are sent untagged. The default native VLAN is 1, and both ends of the trunk must agree on it.
interface g0/24 switchport mode trunk switchport trunk allowed vlan 10,20 switchport trunk native vlan 99 switchport nonegotiate
switchport nonegotiate turns off DTP so a port will not negotiate a trunk on its own.
Voice VLAN
An access port can carry one data VLAN and one voice VLAN for an IP phone using switchport voice vlan 150.
Inter-VLAN routing
With router-on-a-stick, one router link is a trunk, and each VLAN has a subinterface:
interface g0/0.10 encapsulation dot1Q 10 ip address 192.168.10.1 255.255.255.0
A multilayer switch can instead route using a switch virtual interface (SVI) with interface vlan 10 and ip routing.
Verification and troubleshooting
show vlan brief show interfaces trunk show interfaces g0/1 switchport show mac address-table
Typical faults are a port in the wrong VLAN, a VLAN missing from the trunk's allowed list, a native VLAN mismatch, and a VLAN that does not exist on the other switch. A port assigned to a VLAN that does not exist shows no traffic until you create the VLAN.
Use the cheat sheet for quick copy-paste commands and the 12-week roadmap to schedule your labs.
Frequently asked questions
What is the native VLAN?
The VLAN whose frames cross an 802.1Q trunk without a tag. The default is VLAN 1, and it should match on both ends of the trunk.
What is router-on-a-stick?
A way to route between VLANs using one router interface split into 802.1Q subinterfaces, one per VLAN.
How do I check which VLANs a trunk carries?
Use show interfaces trunk.
Facts about the exam come from Cisco's CCNA page; always confirm there before booking.