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connecting switches with different native vlan - help required

Hi,

Can you connect 2 switches with different native vlan? For example port gi1/1 on switch A has native vlan 1 and port gi 0/1 on switch B has native vlan 2. Please refer interface snapshots to below.

The ports connecting switch A and switch B are access ports.

When these 2 switches are connected up, a message on native vlan mismatched is diplayed eventhough the connection works.

Can you advice on this issue?

TIA

PF

Switch A

sh int gi 1/1sw

Name: Gi1/1

Switchport: Enabled

Administrative Mode: static access

Operational Mode: static access (member of bundle Po200)

Administrative Trunking Encapsulation: negotiate

Operational Trunking Encapsulation: native

Negotiation of Trunking: Off

Access Mode VLAN: 10 (servers)

Trunking Native Mode VLAN: 1 (default)

Siwtch B

sh int gi 0/1 sw

Name: Gi0/1

Switchport: Enabled

Administrative Mode: static access

Operational Mode: static access (member of bundle Po1)

Administrative Trunking Encapsulation: dot1q

Operational Trunking Encapsulation: native

Negotiation of Trunking: Off

Access Mode VLAN: 2 (operational)

Trunking Native Mode VLAN: 2 (operational)

15 REPLIES

Re: connecting switches with different native vlan - help requir

Hi Friend,

The connection between the switch is working as the operationl mode of the links are static access.

If you want the link betwen the switches to be a trubk and use dot1q as the encapsulation, the trunk wont cone uo due to the native vlan mismatch.

HTH, rate if it does

Narayan

New Member

Re: connecting switches with different native vlan - help requir

Narayan,

This is not a trunk link (the sh int switchport in the earlier attachment confirmed that) and I have the message native vlan mismatch appearing every min or 2. To get rid of the message, I have gave the vtp domain a name and this stops the message from appearing. However, this does not solve the problem but rather masking the problem. Am I right?

What do you think?

Thanks.

PF

Re: connecting switches with different native vlan - help requir

Hi

As suggested in the other post u can make the link as trunk link with encap dot1q.i think this two vlans are using the same subnet thats why the link is working.

one way to get rid of the vlan mismatch is disable cdp on the interfaces.

Thanks

Mahmood

New Member

Re: connecting switches with different native vlan - help requir

Mahmood,

Thanks for the reply. I do not want to make this a trunk port. I thought the native vlan comes into play only if you are connecting using trunk and not static access ports.

Switch A is 6500 switch with default native vlan 1 and siwtch B is a cisco siwtch in an IBM bladecentre. The default native vlan is vlan 2.

We also have a vlan2 defined on the catalyst 6500 (no relation to the vlan 2 on the switch in the bladecentre). Will this cause us issue? the cisco switch on the bladecentre came configured with vlan 2 as the access vlan and native vlan.

Can you please advice?

TIA

PF

Re: connecting switches with different native vlan - help requir

Hi

As long as they are having different vtp domains this will not cause any problem.

Thanks

mahmood

Silver

Re: connecting switches with different native vlan - help requir

Hello,

If you set your ports to access there is no meaning of native vlan, since native vlan is significant only on trunk links (frames are not tagged on native).

And also VTP information will not pass over access link, just over trunk link, but if you set trunk native vlan should be the same on both device. When the native vlan mismatch message appear what is your VTP settings? Set VTP on the devices to differ (at least differ domain name).

Switch A

sh int gi 1/1sw

Name: Gi1/1

Switchport: Enabled

Administrative Mode: static access

Operational Mode: static access (member of bundle Po200)

Access Mode VLAN: 10 (servers)

And also these ports are member of POrtchannel so all of the member in the channel should have the same settings eg. trunking, access, native vlan, speed, duplex...

bye

FCS

Please rate me if I helped.

New Member

Re: connecting switches with different native vlan - help requir

FCS,

Thanks for the reply. I have attached the sh int switchport on a port on Siwtch A and a port on Switch B. All the channel ports have the same information, ie same native vlan, same speed/duplex... etc on the same switch. The vtp domain name in switch A is swA and there is no domain name on switch B. Whe the native vlan mismatch message occur, I have gave switch B a domain name and this stops the message from occuring.

Here's the vtp status from switch A

sh vtp status

VTP Version : 2

Configuration Revision : 12

Maximum VLANs supported locally : 1005

Number of existing VLANs : 51

VTP Operating Mode : Server

VTP Domain Name : swA

VTP Pruning Mode : Disabled

VTP V2 Mode : Disabled

VTP Traps Generation : Enabled

MD5 digest : 0xF9 0x89 0x5B 0xEE 0xAF 0x2C 0x50 0xE4

Here's the VTP status from Switch B

sh vtp status

VTP Version : 2

Configuration Revision : 0

Maximum VLANs supported locally : 250

Number of existing VLANs : 6

VTP Operating Mode : Transparent

VTP Domain Name : swB

VTP Pruning Mode : Disabled

VTP V2 Mode : Disabled

VTP Traps Generation : Disabled

MD5 digest : 0xFB 0x33 0x31 0xEA 0x2A 0x44 0xA3 0x1C

There wasn't a domain name on switch B originally.

I have vlan2 and switch A as an admin vlan for certain servers and vlan 2 on switch B for certain servers (vlan 2 came defined on the IBM bladecenters). Will the vlan 2 traffic on switch A be mixed with the traffic on switch B? I do not want them to be in the same broadcast domain.

What does it exactly mean if the same vlan (for instance vlan 2) is defined on different vtp domain?

Below is the sh int switchport on a port on switch A and a port on switch B.

Switch A

sh int gi 1/1 sw

Name: Gi1/1

Switchport: Enabled

Administrative Mode: static access

Operational Mode: static access (member of bundle Po200)

Administrative Trunking Encapsulation: negotiate

Operational Trunking Encapsulation: native

Negotiation of Trunking: Off

Access Mode VLAN: 10 (servers)

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

Switch B

sh int gi 0/1 sw

Name: Gi0/17

Switchport: Enabled

Administrative Mode: static access

Operational Mode: static access (member of bundle Po1)

Administrative Trunking Encapsulation: dot1q

Operational Trunking Encapsulation: native

Negotiation of Trunking: Off

Access Mode VLAN: 2 (operational)

Trunking Native Mode VLAN: 2 (operational)

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

ture VLANs Allowed: ALL

TIA.

PF

Silver

Re: connecting switches with different native vlan - help requir

Based upon what you sent the native vlan really mismatch:

SwitchA

Name: Gi1/1

Trunking Native Mode VLAN: 1 (default)

Switch B

Name: Gi0/17

Trunking Native Mode VLAN: 2 (operational)

Please check your settings double, also checked the portchannel interface...

bye

FCS

Please rate me if I helped.

New Member

Re: connecting switches with different native vlan - help requir

FCS,

Thanks very much for the reply. I am aware that there is a native vlan mismatch. If the mode is access and NOT trunk for the interfaces on switch A and switch B, trunking native vlan information does not come into play or does it?

The IBM bladecentre switches (Cisco) came configured with vlan 2 as default native vlan as vlan 1 is used for management only.

I can change the native vlan to 1 on switch B to match the native vlan on switch A. I wanted to fully understand the role of the native vlan when connecting 2 switches A and B using access mode and not trunk mode.

TIA

PF

New Member

Re: connecting switches with different native vlan - help requir

Hi PF

I belive a problem come from CDP. please try this

Switch(config)#no cdp advertise-v2

please let me know if it help.

New Member

Re: connecting switches with different native vlan - help requir

Weerapatr,

Thanks for the response. I have not try the command as I have changed the vtp domain name on SwitchB to Sw2 and it stops it the message from appearing. However, I believe the command will work. I will try this command next time with a new bladecentre.

Anyway, as we only use access mode and not trunk mode for the connection, the native vlan will not be used. I will remove the default native vlan 2 from switch B. This will resolve my issue with the mismatch.

Thanks very much everyone, It is indeed very much appreciated.

PF

Silver

Re: connecting switches with different native vlan - help requir

You are right, I didn't check the port mode, so if the port mode is access, native vlan is not an issue.

Bye

FCS

Re: connecting switches with different native vlan - help requir

Hi, The native vlan is not tagged on a trunk link. If you connect 2 switches together with different native Vlans over the trunk link you are "leaking" traffic between the VLAN's. This will work but I would avoid doing it on a live network as it makes troubleshooting difficult.

New Member

Re: connecting switches with different native vlan - help requir

Mark,

Thanks for the response I understand it more now.

New Member

Re: connecting switches with different native vlan - help requir

FCS,

Thanks very much for the reply. I do understand it more now.

PF

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