Cisco "show interfaces trunk" Command Explained
show interfaces trunk — shows which ports are trunking, their native VLANs, and exactly which VLANs are allowed and forwarding — the trunk truth-table. Runs in privileged EXEC mode.
Syntax and common variants
| Variant | Purpose |
|---|---|
show interfaces trunk | All trunking ports and their VLAN lists |
Reading the output
| Output field | Meaning |
|---|---|
Mode | on / desirable — how the trunk formed |
Encapsulation | 802.1q |
Native vlan | Untagged VLAN — must match both ends |
Vlans allowed on trunk | What the allowed-list permits |
…allowed and active | Allowed AND existing on this switch |
…in spanning tree forwarding state | What's actually passing — the line that matters |
When to use it
When same-VLAN devices across two switches can't talk: is the link trunking at all, is the VLAN in the allowed list, does it exist on both switches, and is STP forwarding it? This one output answers all four — and empty output means the port isn't trunking, the most common surprise. Browse more in the command reference or practise in the free labs.
Frequently asked questions
The command returns nothing — why?
No port is currently trunking — check switchport mode and DTP negotiation; an access port never appears here.
A VLAN is allowed but not in the forwarding line — why?
Either it doesn't exist on this switch (create it) or STP is blocking it on this port.
What if native VLANs differ between the two ends?
Untagged traffic leaks between VLANs and CDP logs mismatch errors — set both ends' native VLAN identical.
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