Native VLAN mismatch discovered on switch

[whiteford]

I keep getting these 2 syslog errors:

4767: 004805: * Native VLAN mismatch discovered on FastEthernet0/2 (2), with switchcore5 FastEthernet0/21 (1).

And then the opposite direction

4768: 004806: * Native VLAN mismatch discovered on FastEthernet0/21 (1), with switchcore5 FastEthernet0/2 (2).

It's a Cisco Cataylst 2950 and I have updated the IOS to the lastest, everything seems to be working ok.

The 2 ports are below:

interface FastEthernet0/2

description Link - Core Switch to LAN

switchport access vlan 2

switchport mode access

speed 100

duplex full

interface FastEthernet0/21

description VLAN 1 for LAN IP

[ankbhasi]

Hi Andy,

This message indicates that the native vlan on trunk ports or vlans on an access ports does not match on the 2 sides.

If you check your config then on int fa0/2 the access vlan is 2 but on fa0/21 I believe you have configured it for vlan 1 although this is not the complete config which you have posted from int fa0/21 but description says it is for vlan 1. Now in this case vlans are different on both the ends and both the ports are connected to each other and CDP will report this message.

If it is intentionally designed for some purpose then you can disable CDP and you will not see this message or else you can configure interface fa0/21 as an access port and assign it to vlan2 something like this

interface fa0/21

switchport mode access

switchport access vlan 2

HTH

Ankur

*Pls rate all helpfull post

[whiteford]

Hi, that is the complete config for port fe0/21. Both ports go into the LAN switches. I think if it's not in a VLAN it goes into VLAN 1? all I need VLAN 1 on FE 0/21 is so I can manage the switch via an IP address.

Let me know if you want the full config or if there is a better way to do this. fe0/2 I need to stay where it is.

VLAN 2 I have our and firewall inside port only and this link to the LAN.

[ankbhasi]

Hi Andy,

Your fa0/21 is not connected to fa0/2? If yes the config on both the sides should be same? Your fa0/2 is in dynamic desirable mode and also in vlan 1 and your fa0/21 is in access mode and in vlan2.

Now as you know any port default in in vlan 1. If these ports are connected to each other then you can configure them also a trunk port so that it can allow both the vlans?

HTH

Ankur

*Pls rate all helpfull post

[whiteford]

I'm not sure I understand. fa0/2 is in Vlan2 and fa0/21 is in Vlan1, how is fa0/2 in vlan1 too?

I would like to keep vlan2 as it is. The Vlans just both go into the LAN for our office but they are different switches.

All I want to use fa0/21 for is to telent to the switch and be able to ping it. Am I doing it the right way?

[ankbhasi]

Hi Andy,

Sorry I typed wrong port numbers in my last post. Anyways what you are doing is correct as per your description. My only concern is you are getting "Native Vlan Mismatch" message and I will like to know these ports are connected to which devices?

Fa0/1 and fa0/21 are connected to each other or some other devices?

HTH

Ankur

[whiteford]

All I can tell you as I'm not at work is that both ports connect to a huge core switch. They each go into a different vlan each. I have only got these errors since upgrading the ios, as it was 3 years old. What actually causes this native vlan error?

[whiteford]

Is this any help? Should negotiation of trunking be on or off?

switch#show interfaces fastEthernet 0/2 trunk

Port Mode Encapsulation Status Native vlan

Fa0/2 off 802.1q not-trunking 1

Port Vlans allowed on trunk

Fa0/2 2

Port Vlans allowed and active in management domain

Fa0/2 2

Port Vlans in spanning tree forwarding state and not prune

Fa0/2 2

switch#show interfaces fastEthernet 0/21 trunk

Port Mode Encapsulation Status Native vlan

Fa0/21 desirable 802.1q not-trunking 1

Port Vlans allowed on trunk

Fa0/21 1

Port Vlans allowed and active in management domain

Fa0/21 1

Port Vlans in spanning tree forwarding state and not prune

Fa0/21 1

switch#

and

switch#sho int fa0/2 swi

Name: Fa0/2

Switchport: Enabled

Administrative Mode: static access

Operational Mode: static access

Administrative Trunking Encapsulation: dot1q

Operational Trunking Encapsulation: native

Negotiation of Trunking: Off

Access Mode VLAN: 2 (fwinside)

Trunking Native Mode VLAN: 1 (default)

Voice VLAN: none

Administrative private-vlan host-association: none

Administrative private-vlan mapping: none

Administrative private-vlan trunk native VLAN: none

Administrative private-vlan trunk encapsulation: dot1q

Administrative private-vlan trunk normal VLANs: none

Administrative private-vlan trunk private VLANs: none

Operational private-vlan: none

Trunking VLANs Enabled: ALL

Pruning VLANs Enabled: 2-1001

Capture Mode Disabled

Capture VLANs Allowed: ALL

Protected: false

Unknown unicast blocked: disabled

Unknown multicast blocked: disabled

Appliance trust: none

switch#sho int fa0/21 swi

Name: Fa0/21

Switchport: Enabled

Administrative Mode: dynamic desirable

Operational Mode: static access

Administrative Trunking Encapsulation: dot1q

Operational Trunking Encapsulation: native

Negotiation of Trunking: On

Access Mode VLAN: 1 (default)

Trunking Native Mode VLAN: 1 (default)

Voice VLAN: none

Administrative private-vlan host-association: none

Administrative private-vlan mapping: none

Administrative private-vlan trunk native VLAN: none

Administrative private-vlan trunk encapsulation: dot1q

Administrative private-vlan trunk normal VLANs: none

Administrative private-vlan trunk private VLANs: none

Operational private-vlan: none

Trunking VLANs Enabled: ALL

Pruning VLANs Enabled: 2-1001

Capture Mode Disabled

Capture VLANs Allowed: ALL

Protected: false

Unknown unicast blocked: disabled

Unknown multicast blocked: disabled

Appliance trust: none

switch#