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19
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Native VLAN mismatch detected?

John Adams
Level 1
Level 1

Hi -

I've got an ADSL router that has 4 ethernet ports in under the default vlan1.

It looks like this:

interface FastEthernet0

description Uplink

no ip address

!

interface FastEthernet1

no ip address

shutdown

!

interface FastEthernet2

no ip address

shutdown

!

interface FastEthernet3

no ip address

shutdown

!

interface Vlan1

description Switch default vlan1 Network - all Ports

ip address 192.168.0.1 255.255.255.0

ip nat inside

ip virtual-reassembly in

!

The Router works fine. When I plug my laptop into FastEthernet1 it gets a 192.168.0.x IP from the DHCP of the Router and can get on the internet etc. No issues.

The Router needs an uplink to a switch so some access points can use the internet. The switch port is in access mode and has the vlan ID of 50.

When I plug my Router into the switch I get errors on the switch about 'Native VLAN mismatch detected'.

Should my ADSL router be set to use VLAN 50 as well? Would that explain the error?

How do I change my interface vlan1 to be interface vlan 50?

Thanks.

19 Replies 19

In some switches, it automatically creates the VLAN when a port is assigned to it.

However, I don't believe this behavior is present on route-switch modules.

As Best Practice, always create the VLAN manually.

Not sure what type of switch module do you have, but here is an example:

http://www.cisco.com/en/US/products/hw/modules/ps2797/products_configuration_example09186a0080810449.shtml

Hi,

I managed to try this.

I did the following. Create all of this:

interface FastEthernet0

description Uplink

no ip address

switchport mode acesss

switchport access vlan 50

!

interface FastEthernet1

no ip address

shutdown

switchport mode access

switchport access vlan 50

!

interface FastEthernet2

no ip address

shutdown

switchport mode access

switchport access vlan 50

!

interface FastEthernet3

no ip address

shutdown

switchport mode access

switchport access vlan 50

!

interface Vlan50

description Switch default vlan1 Network - all Ports

ip address 192.168.0.1 255.255.255.0

ip nat inside

ip virtual-reassembly in

!

I also set my interface vlan 1 to 'no ip address, no ip nat inside and no ip virtual-reassembly in'. I did not shut it down. Just left it like that.

I then removed my 'dumb' TPLINK adsl router modem that uplinked to my cisco switch.

I plugged the Cisco 887 in.

And then Cisco switch port received a bpdu guard and disabled the port as errdisable status and showed as it had been shut down.

I'd really like to know why that happened.

To resolve I removed the bpdu guard settings from the switch port and then set the port to be active again (and shut, no shut).

Upon doing that everything was perfect.

I'd like to know why the switch port (on the switch) went into an error state though?

Thanks.

Also when I ran the 'switchport mode access and switchport access vlan xx' commands they both applied but in the show run it only shows the swithport access vlan xx  - is this correct? How come the other command does not show in the show run?

Thanks.

John

You have asked 2 questions and here are my answers.

Why did the switch port go into error disabled? The switch port was configured with a feature called bpdu guard. Some switches do this on access ports by default and many switches allow it as an option. The point of bpdu guard is the assumption that an access port would connect to an end device/host and not to a switch. As such it should not receive a Spanning Tree BPDU. If the access port does receive a BPDU then that indicates a problem and the switch puts the interface into error disable. When you connected the 887 its port is a switch port and sends BPDUs which caused the error disable on the switch, until you removed bpdu guard from the port configuration.

Why did the command that you entered not show up in running config? In one of my previous responses I mentioned this, but now let me explain it. If show run did show every command the output would be very extensive. So Cisco has adopted the convention that, for the most part, show run does not include commands that are at their default setting or default value. So since switchport mode access is the default setting for these switch ports then it does not show up in the output of show run.

HTH

Rick

HTH

Rick

Thank you. I consider this issue resolved now.

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