Trunks & 802.1Q Tagging

Mental model

An access port is a single-purpose wire — it belongs to exactly one VLAN and frames go through naked (untagged). A trunk is a shared wire — it carries many VLANs by stamping each frame with an “I belong to VLAN N” sticker called the 802.1Q tag.

That stamp is 4 bytes inserted into the Ethernet header. Without it, the receiving switch would have no way to know which VLAN a given frame belongs to.

The 802.1Q tag, byte by byte

+-------------+-------------+-------------+-------------+
|   TPID (2)  |  PCP | DEI  |     VID (12 bits)         |
|   0x8100    |  3b  |  1b  |     1–4094                |
+-------------+-------------+-------------+-------------+

• TPID — fixed value 0x8100, tells the receiver “this is a tagged frame”
• PCP — Priority Code Point, used by QoS (CoS values 0–7)
• DEI — Drop Eligible Indicator (rarely used)
• VID — the VLAN ID itself, 12 bits = values 1 through 4094 (0 and 4095 reserved)

You don’t memorize this byte-by-byte for the exam, but you should remember that the tag is 4 bytes — that’s why an Ethernet frame on a trunk can be up to 1522 bytes instead of the usual 1518.

Commands

Configure a trunk port (both ends)

SW1(config)# interface GigabitEthernet0/24
SW1(config-if)# switchport trunk encapsulation dot1q   ! on older switches only
SW1(config-if)# switchport mode trunk
SW1(config-if)# switchport trunk native vlan 999
SW1(config-if)# switchport trunk allowed vlan 10,20,30

Mirror exactly on the other end.

Restrict the allowed list (adding/removing VLANs)

! Add VLAN 40 to an existing allowed list
SW1(config-if)# switchport trunk allowed vlan add 40

! Remove VLAN 20
SW1(config-if)# switchport trunk allowed vlan remove 20

! Replace the entire list
SW1(config-if)# switchport trunk allowed vlan 10,30,40

! Allow everything (default)
SW1(config-if)# switchport trunk allowed vlan all

Gotcha: plain switchport trunk allowed vlan 40 REPLACES the list — it does not add. Always use add / remove when modifying an existing trunk.

Verification

SW1# show interfaces trunk
SW1# show interfaces GigabitEthernet0/24 switchport
SW1# show vlan brief

show interfaces trunk is the most useful single command: it confirms which interfaces are trunks, which VLANs they carry, and what the native VLAN is.

Common mistakes

1. Native VLAN mismatch. The biggest CCNA exam trap. If SW1’s native is VLAN 1 and SW2’s native is VLAN 99, CDP/LLDP raises a %CDP-4-NATIVE_VLAN_MISMATCH and STP behaviour gets weird. Set both sides to the same unused VLAN ID.

2. switchport trunk allowed vlan 40 instead of ... add 40. This silently replaces the whole allowed list. Suddenly VLANs 10, 20, 30 stop crossing the trunk.

3. Forgetting to set switchport mode trunk and relying on DTP. Dynamic Trunking Protocol can auto-negotiate trunks, but it’s a security risk (VLAN-hopping attacks). Always hard-code mode trunk + switchport nonegotiate.

4. Mismatched encapsulation. On older switches that support both ISL and 802.1Q, you must explicitly set encapsulation to dot1q. ISL is legacy, never use it in 2026.

5. Putting management traffic on the native VLAN. A misconfigured trunk could leak management frames into a user VLAN. Keep the management VLAN separate from the native VLAN.

Lab to try tonight

1. Two switches, three VLANs (10/20/30), three PCs per switch (one per VLAN).
2. Configure the inter-switch link as a trunk on both ends. Set native VLAN to 999. Allow VLANs 10, 20, 30 only.
3. Confirm PC1-VLAN10 can ping PC4-VLAN10 (same VLAN, across the trunk).
4. Verify with show interfaces trunk that the allowed list is exactly 10,20,30.
5. Run switchport trunk allowed vlan 40 on one side. Check what happens to existing inter-VLAN traffic. (Spoiler: it dies.)
6. Recover with switchport trunk allowed vlan add 10,20,30 to add the lost VLANs back.

Cheat strip

Concept	Plain English
Trunk	A port that carries multiple VLANs by tagging frames with 802.1Q
802.1Q tag	4 bytes inserted into the Ethernet header; contains the VLAN ID (1–4094)
Native VLAN	The one VLAN whose frames travel untagged on a trunk. Set explicitly.
Allowed list	Which VLANs may cross this trunk. add/remove to modify safely.
DTP	Cisco’s trunk-negotiation protocol. Turn it off in production.
Frame size	Tagged frame = 1522 bytes max (1518 + 4 tag). Some old gear chokes on >1518.