Issues connecting switches (with different vlans) to one hub (causing NATIVE_VLAN_MISMATCH)

rafik_rez · ‎09-25-2016

Hello people !

I need some help concerning connecting Cisco switches with defined vlans to a hub, which is causing some vlan mismatch warnings, in addition to some vlan unusual behaviour.

In order to provide the same data to 2 different PCs ClientA (vlan 1) and ClientB (vlan 11) from PCSource, we did the following:

- Connected PCSource to a hub,

- Connectected port 13 of G-SW1 to the hub.

interface FastEthernet0/13
switchport mode access

- Connected port 13 of G-SW2 to the hub and assigned this port to vlan11

interface FastEthernet0/13
switchport access vlan 11
switchport mode access

- G-SW1 connects via a trunk to switch R-SW1.

- G-SW2 connects via a trunk to switch R-SW2.

- ClientA is connected to port 1 of the R-SW1

interface FastEthernet0/1
switchport mode access
spanning-tree portfast

-Similarly ClientB is connected to port 1 of the R-SW2, but assigned to vlan11:

interface FastEthernet0/1
switchport access vlan 11
switchport mode access
spanning-tree portfast

I found two problems with this architecture:

- I have repetitive NATIVE_VLAN_MISMATCH in switch G-SW1 such as:


%CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on FastEthernet0/13 (1), with GBF-BVS7-SW2 FastEthernet0/13 (11).

- I can ping from ClientB to ClientA, where I thought that using vlan11 on the SW2 side will isolate the traffic between PCSource and ClientA from the traffic between PCSource and ClientB.

I tried to know also what are the recommandations concerning configuring Cisco switches ports when they are connected to a hub, but the only indication that I found was to not use spanning-tree portfast.

I would be grateful if you can help me with some advices.

Leo Laohoo · ‎09-25-2016

I think this is for school work.

Kindly move this to the Cisco Learning Network.

rafik_rez · ‎09-25-2016

Dear Lea,

Thanks for taking the time to answer. This problem is happening on a production system, not a laboratory system. Do you think I should move it Cisco Leaning Network?

Leo Laohoo · ‎09-26-2016

I don't believe this is happening in a "production system" and still believe this is school work.

Yes, kindly move this line of questioning to the Cisco Learning Network.

rafik_rez · ‎09-26-2016

Dear Leo,

I didn't know how to move the topic to Cisco Learning Network, so I created the same topic there:

https://learningnetwork.cisco.com/thread/101328

If you don't believe it is happening in a production system, it is your problem.