Troubleshoot static and dynamic 802.1Q trunking

3.1 Layer 2

📘CCNP Encore (350-401-ENCORE-v1.1)

In a Layer 2 network, VLANs (Virtual LANs) are used to separate broadcast domains. When multiple VLANs need to communicate across switches, you need trunk links. Trunks allow multiple VLANs to traverse a single physical connection using 802.1Q encapsulation.

Sometimes, trunks don’t work as expected. Troubleshooting is key for passing the CCNP exam.

1. What is 802.1Q trunking?

802.1Q is a VLAN tagging standard.
On a trunk link, 802.1Q adds a VLAN tag to the Ethernet frame so the receiving switch knows which VLAN it belongs to.
Each trunk can carry multiple VLANs, unlike access ports that carry only one VLAN.

Key terms:

Native VLAN: The VLAN that is not tagged on the trunk. Usually VLAN 1 by default.
Tagged VLAN: VLANs that get the 802.1Q tag on the trunk.

2. Static vs. Dynamic Trunks

A. Static Trunking

Configured manually using: switchport mode trunk switchport trunk allowed vlan 10,20,30 switchport trunk native vlan 99
Advantages: Predictable, more control.
Disadvantages: Manual configuration required on each switch.

B. Dynamic Trunking

Uses Dynamic Trunking Protocol (DTP).
Two main DTP modes: ModeBehaviorautoWill become trunk if neighbor requests it (passive).desirableActively tries to form a trunk.
If both ends match (one desirable and one auto, or both desirable), trunk forms automatically.

Note: Dynamic trunks can fail if DTP is disabled on either end.

3. Common Trunking Issues

A. Mismatched VLANs

If VLANs allowed on the trunk don’t match, some VLAN traffic will be dropped.
Check allowed VLANs: show interfaces trunk

B. Native VLAN mismatch

If the native VLAN differs on both ends, untagged traffic may be misinterpreted.
Symptoms: VLAN traffic leaks, connectivity issues.
Verify: show running-config show interfaces trunk

C. Trunk negotiation failure

DTP can fail due to:
- One side set to access mode.
- DTP disabled.
Use static trunking to avoid DTP issues.

D. Encapsulation mismatch

Switches may support dot1q or isl (older Cisco proprietary).
Both ends must match. Usually use 802.1Q.

switchport trunk encapsulation dot1q
switchport mode trunk

E. Port issues

Trunk may fail if interface is administratively down: show interfaces status
Or if speed/duplex mismatch exists.

4. Troubleshooting Commands

Key commands to verify trunking:

Check trunk status

show interfaces trunk

Shows which VLANs are allowed, active, and native VLAN.

Check interface details

show running-config

Verify switchport mode, trunk encapsulation, and allowed VLANs.

Check DTP negotiation

show dtp interface [interface-id]

Shows DTP status and neighbor info.

Check VLANs

show vlan brief

Confirm VLANs exist on both ends of the trunk.

Check interface operational status

show interfaces [interface-id] status

Ensure interface is up/up.

5. Troubleshooting Steps

When a trunk is not working:

Check physical connection
- Interface is up.
- Correct cables used.
Verify trunk mode
- switchport mode trunk (static)
- Or correct DTP negotiation (dynamic)
Verify encapsulation
- Ensure both ends use 802.1Q.
Check allowed VLANs
- show interfaces trunk
- Add missing VLANs if required.
Check native VLAN
- Ensure native VLAN is consistent.
Check for errors
- Look for misconfigurations or mismatched settings.

6. Exam Tips

Remember: Static trunking is more reliable for exam scenarios.
Watch for mismatches:
- Native VLAN
- Allowed VLANs
- Encapsulation
- DTP negotiation
Commands to memorize:
- show interfaces trunk
- show vlan brief
- show running-config
- show dtp interface [interface-id]
- show interfaces [interface-id] status
Common DTP question: “Trunk not forming even though both sides are connected.”
- Check DTP mode (auto vs. desirable) and try static trunk if unsure.

✅ Summary

802.1Q trunks carry multiple VLANs.
Static trunking is manual, reliable. Dynamic trunking uses DTP but can fail.
Common issues: mismatched VLANs, native VLAN, encapsulation, port issues, DTP failures.
Use show commands to verify trunk status and troubleshoot effectively.