Solved: Re: Native vlan mismatch question

mahesh18 · ‎10-12-2010

Hi all,

i have 2950t switch with fa0/13 access port connection to 3550B switch.

here is 2950 log

.Oct 12 15:15:47.708 MST: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on FastEthernet0/13 (10), with 3550SMIB FastEthernet0/13 (20).

2950T# sh int fa0/13 switchport
Name: Fa0/13
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: 10 (VLAN0010)
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

port fa0/13 on 2950 is on vlan 10 my question is why log shows that Native VLAN mismatch discovered on FastEthernet0/13 (10) , even though native vlan on 2950 port fa0/13 is 1 by default.

i know to fix this issue either i can config the ports as trunk ports or i can move the port fa0/13 on 2950 from vlan 10 to 20.

2950 is layer 2 switch with vlan 10 only.

other question is that if i make layer 2 vlan 20 on 2950 and i move port fa0/13 on that vlan then is it good practice to do that??

here is port into from 2950t

950T#                                   sh ip int brief
Interface                  IP-Address      OK? Method Status                Protocol
Vlan1                      unassigned      YES NVRAM administratively down down
Vlan10                     192.168.10.5    YES NVRAM up                    up
FastEthernet0/1            unassigned      YES unset administratively down down
FastEthernet0/2            unassigned      YES unset administratively down down
FastEthernet0/3            unassigned      YES unset administratively down down
FastEthernet0/4            unassigned      YES unset administratively down down
FastEthernet0/5            unassigned      YES unset down                  down
FastEthernet0/6            unassigned      YES unset administratively down down
FastEthernet0/7            unassigned      YES unset administratively down down
FastEthernet0/8            unassigned      YES unset up                    up
FastEthernet0/9            unassigned      YES unset administratively down down
FastEthernet0/10           unassigned      YES unset administratively down down
FastEthernet0/11           unassigned      YES unset administratively down down
FastEthernet0/12           unassigned      YES unset administratively down down
FastEthernet0/13           unassigned      YES unset up                    up
FastEthernet0/14           unassigned      YES unset administratively down down
FastEthernet0/15           unassigned      YES unset administratively down down
FastEthernet0/16           unassigned      YES unset administratively down down
FastEthernet0/17           unassigned      YES unset administratively down down
FastEthernet0/18           unassigned      YES unset administratively down down
FastEthernet0/19           unassigned      YES unset down                  down
FastEthernet0/20           unassigned      YES unset administratively down down
FastEthernet0/21           unassigned      YES unset down                  down
FastEthernet0/22           unassigned      YES unset administratively down down
FastEthernet0/23           unassigned      YES unset administratively down down
FastEthernet0/24           unassigned      YES unset administratively down down
GigabitEthernet0/1         unassigned      YES unset administratively down down
GigabitEthernet0/2         unassigned      YES unset administratively down down

many thanks

mahesh

Peter Paluch · ‎10-12-2010

Hello Mahesh,

So you mean to say that when the switch port  is config as access port
in this vlan 10 then the native vlan is also considered as vlan 10 
right?

Absolutely correct. An access port is in a single VLAN - the access VLAN, and this VLAN is untagged, so it can be also considered as the native VLAN. The line in the output:

Trunking Native Mode VLAN: 1 (default)

tells us which VLAN is going to be the native VLAN if and only if the port is operating as a trunk.

it will always show me Trunking Native Mode VLAN: 1 (default) unless i change the native vlan on both sides of the link by command

Switchport trunk native vlan .

Yes, correct.

Best regards,

Peter

View solution in original post

Peter Paluch · ‎10-12-2010

Hi Mahesh,

port fa0/13 on 2950  is on vlan 10 my question is why log shows that 
Native VLAN mismatch discovered on FastEthernet0/13 (10) ,  even though 
native vlan on 2950 port fa0/13 is 1 by default.

It is because the Native VLAN 1 as configured on your Fa0/13 interface is valid only if the interface is operating in trunk mode. However, currently it is configured as a static access port, thus its only VLAN (the VLAN 10) can also be considered as the native VLAN (because currently it does not use any 802.1Q tags, just as a real native VLAN on a trunk).

other question is that if i make layer 2 vlan 20 on 2950 and i move port
 fa0/13  on that vlan then is it good practice to do that??

Usually, it is the best practice to interconnect switches using trunk ports, not using access ports as you have right now. Without trunk ports between switches, you are isolating the switch into a single VLAN (or allowing only a single VLAN to pass from one switch to another), and also, you cannot use the VTP for VLAN management purposes. Using access ports between switches therefore has to be considered whether it is appropriate.

In any case, moving the Fa0/13 port to the VLAN 20 will solve your native VLAN mismatch problem but in such a case, you will probably need to move all other ports on your 2950 switch also to the VLAN 20. Otherwise, they would become cut off from the rest of the network.

You could also move the Fa0/13 port on your 3550 to the VLAN 10.

Best regards,

Peter

mahesh18 · ‎10-12-2010

Hi peter ,

many thanks for reply if you can explain me this is more detail please

It is because the Native VLAN 1 as configured on your Fa0/13 interface is valid only if the interface is operating in trunk mode. However, currently it is configured as a static access port, thus its only VLAN (the VLAN 10) can also be considered as the native VLAN (because currently it does not use any 802.1Q tags, just as a real native VLAN on a trunk).

regards

mahesh

Peter Paluch · ‎10-12-2010

Hello Mahesh,

A native VLAN is simply a VLAN that does not use any tagging, i.e. the frames carried within this VLAN are untagged. With a trunk port that belongs to many VLANs, one of them can be defined as native and thus will not use tagging. Such a VLAN is configured using the switchport trunk native vlan command.

However, if you have an access port then this port belongs to a single VLAN only. Moreover, all frames within this VLAN are transmitted as untagged - an access port does not transmit or receive tagged frames. Thus, the access VLAN of an access port fulfills the formal definition of a native VLAN because it does not use tags, correct?

Therefore, in a broader sense, an access VLAN on an access port is its own native VLAN if the port is operating in the access mode. Some literature indeed considers even the access VLAN of a port to be a native VLAN, and in a sense, it is correct. Usually, however, we talk about the native VLAN only with regard to trunk ports, and I personally try to strictly avoid using the term native VLAN with regards to any other ports than trunks.

What your output of the show interface fa0/13 switchport tells you is the Trunking Native Vlan - a native VLAN setting used only if the port is working as a trunk.

Is this more understandable than my previous reply? Please feel free to ask further.

Best regards,

Peter

mahesh18 · ‎10-12-2010

Hi Peter,

Thanks for the reply.

So you mean to say that when the switch port is config as access port in this vlan 10 then the native vlan is also considered as vlan 10 right?

now i move port fa0/13 to vlan 20 as shown below

2950T# sh int fa0/13 switchport
Name: Fa0/13
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: 20 (VLAN0020)
Trunking Native Mode VLAN: 1 (default)
Voice VLAN: none

so this shows that on access port when we do sh int switchport it will always show trunk native vlan 1 default right ??

also on trunk ports which carry multiple vlans when i do

2950T# sh int fa0/8 switchport
Name: Fa0/8
Switchport: Enabled
Administrative Mode: dynamic desirable
Operational Mode: trunk*********************************
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)****************************
Voice VLAN: none

it will always show me Trunking Native Mode VLAN: 1 (default) unless i change the native vlan on both sides of the link by command

Switchport trunk native vlan .

let me know if i understand right or not

mahesh

Peter Paluch · ‎10-12-2010

Hello Mahesh,

So you mean to say that when the switch port  is config as access port
in this vlan 10 then the native vlan is also considered as vlan 10 
right?

Absolutely correct. An access port is in a single VLAN - the access VLAN, and this VLAN is untagged, so it can be also considered as the native VLAN. The line in the output:

Trunking Native Mode VLAN: 1 (default)

tells us which VLAN is going to be the native VLAN if and only if the port is operating as a trunk.

it will always show me Trunking Native Mode VLAN: 1 (default) unless i change the native vlan on both sides of the link by command

Switchport trunk native vlan .

Yes, correct.

Best regards,

Peter

mahesh18 · ‎10-13-2010

Hi Peter,

Many thanks for good explanation on it.

regards

mahesh

matrix lin · ‎10-12-2010

To see whether both ports are configured to the same duplex and speed.