Non VLAN Tagged Traffic Not Going Through Trunk Between Cisco Switches

Question

Good day all.

I am running through a packet tracer exercise and am stuck on where to go next. Forgive me if I am missing something obvious, I have not touched any cisco networking or networking in about 5 years.

Scenario

2 hosts on vlan 79

• PC 1: 192.168.79.10
• PC 2: 192.168.79.20

2 hosts not on vlan

• PC 1: 192.168.80.10
• PC 2: 192.168.80.20

I have the 2 PCs on vlan 79, the 192.168.79.0 subnet, which can successfully ping each other. However, I cannot get the two PCs that are on the 192.168.80.0 subnet and not on a vlan to communicate with each other

My Configuration Steps

I am using the default native vlan 1. I know that in a production environment this would be changed to a different vlan.

Switch 0

enable
conf t
vlan 79
name test
exit
interface vlan 79
no shutdown
interface vlan 1
no shutdown
exit
interface gigEthernet 0/1
switchport mode trunk
switchport trunk native vlan 1
switchport trunk allowed vlan 79
exit
interface fa0/1
switchport mode access
switchport access vlan 79
no shutdown

Switch 1

enable
conf t
vlan 79
name test
exit
interface vlan 79
no shutdown
interface vlan 1
no shutdown
exit
interface gigEthernet 0/1
switchport mode trunk
switchport trunk native vlan 1
switchport trunk allowed vlan 79
exit
interface fa0/1
switchport mode access
switchport access vlan 79
no shutdown

Configuration Outputs

Switch 0

Switch 1

I have been trying to debug this for a couple of days but cannot figure out why I can't get the two PC's that are not on a vlan to communicate with one another.

I apologize in advance if I have missed something obvious, but I have watched more than a dozen videos on the matter, numerous google searches, and even resorted to chat GPT just to see if it could spot my error.

Thank you

Answer 1

You set the interface native (untagged) VLAN to VLAN ID 1 and then only allowed VLAN 79 on the Trunk interface so untagged packets (VLAN 1) cannot pass over it. Add VLAN 1 to the allowed VLAN list on the required interfaces and it should pass just fine.

Trunk interfaces pass only tagged frames and the native VLAN so if you apply a VLAN allow list, that will exclude any unlisted VLAN even if it is the native or untagged VLAN.