Switch Port Mode Access OR Trunk

[Nitin Kapoor]

Dear All,

I recently starting to working on cisco configuration etc on switches. I have one doubt as below. Can seniors please help me to understand this?

1) Currently we did one configuration for dedicated connection between company X(My employer) and Y (Customer).

2)  We configured few things in core switch (3750 model : WS-C3750G-12S)

Vlan

Vlan Interface

giganet interface which assigned to particular Vlan we created.

"Switchport mode" type we configured as ACCESS.

Question : Is that mandatory to use the same Vlan ID between you and your customer with whom you configured dedicated IP connection?  Our customer requested us to use the same VLAN ID, but when i changed it to test purpose at my side i received error

If yes, then why and if not then why not. Currently we configured the same.

I tried to change the Vlan ID, and did the same configuration  with new Vlan ID at my side, but i was getting the error as below.

CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet x/x/x

Thank you in advance.

Regards,

Nitin

[Ryan Huff]

If you are getting a neighbor native mismatch between two access ports it is because both sides of the link are using different VLAN IDs, as you elude to. However, in the Layer 2 world, that doesn't necessarily mean it can't work that way.

With Cisco switches, CDP (Cisco Discovery Protocol) has a mechanism that communicates all sorts of meta information over a link about neighboring devices. In your case, CDP is telling you the both sides of the link are using different VLAN IDs which, in some contexts (intervlan routing), can work just fine.

As long as everything is working, you can just disable CDP on the switchports of the link and the error messages will go away (but this means that CDP neighbor information won't be available for the that link either, if that matters).

Thanks,

Ryan

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[Nitin Kapoor]

Hi Ryan,

Thank you very much for your reply.

However we received this VLAN ID from our customer only, because they wanted to use the same VLAN ID.

Do when both side are using "switchport mode access" is that mandatory to use same vlan id?

OR 

if they are using switchport mode trunk, is that mandatory to use same vlan id?

[Ryan Huff]

Technically, it depends on how your infrastructure is setup; there is no 'black and white' answer I can give you, based on what I know about your network at this point.

Is this a true layer 2 connection (point to point) or is this a layer 3 connection? In other words, how is your switch connected to the customer's device?

Thanks,

Ryan

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[Nitin Kapoor]

Hi Ryan,

Its Layer 2 (point to point) connection.