Native VLAN Trunk and Access Ports

[christopher.wood]

I just need to make sure I have this concept correct.

I have two switches trunked to each other.
The configuration looks like this on both ends of the trunk:
interface GigabitEthernet0/1
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 900
 switchport trunk allowed vlan none
 switchport mode trunk

The rest of the ports on switches are configured as such:
interface GigabitEthernet0/3
 switchport mode access

Now, the native vlan for that G0/3 with switchport mode access would be VLAN 1, correct? Which is untagged traffic.
The trunks can really only send untagged traffic between each other over native vlan 900, since no vlans are being trunked, which means no tagged traffic is going across that trunk link.

Should the untagged traffic from Gig0/3 be able to talk to the untagged traffic coming in on the trunk port that has native vlan 900???

Thanks.

[Rajeev Sharma]

Hey Chris,

Regarding your questions:

1. The native vlan for that G0/3 with switchport mode access would be VLAN 1, correct? - Yes

2. Should the untagged traffic from Gig0/3 be able to talk to the untagged traffic coming in on the trunk port that has native vlan 900??? - Nope

HTH.

Regards,

RS.

[christopher.wood]

How would I get traffic that's untagged on vlan 900 to be able to communicate with untagged traffic on a switch that exists using just switchport access on its interfaces?

Could I do switch those switches to trunk 900 and then map from vlan 900 to vlan 1 with a vlan mapping? Would that allow the traffic to communicate with devices on the switchport access interfaces?

[paul driver]

Hello

Now, the native vlan for that G0/3 with switchport mode access would be VLAN 1, correct? Which is untagged traffic. - No in your configuration,the Native vlan will be 900 and switchports will still be in vlan 1
 

The trunks can really only send untagged traffic between each other over native vlan 900, since no vlans are being trunked, which means no tagged traffic is going across that trunk link.- The trunks between to devices should have the same native vlan defined and you should allow the vlans which to you want to cross the trunks

Example::

interface GigabitEthernet0/1
 switchport trunk allowed vlan add 1, 900

res

Paul

[fogemarttt]

So on your GigabitEthernet0/1 

the native vlan is vlan 900. You must allow it on the other switch even if you would have 

vlan mismatch. 

By default all ports belong to vlan 1 which is not the native vlan now because you have changed it to vlan 900. 

You cannot have two native vlan on the same switch.

Rate if helpful

[InayathUlla Sharieff]

I would totally agree with Fogemartt,

nterface GigabitEthernet0/1
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 900

Based  on the above output now you have configured native vlan 900 hence make a note that the other side needs to be the same valn as native else you will end up in trouble of vlan leaking.

Hope you are aware of that.

[paul driver]

Hello

@forgematt

"You cannot have two native vlan on the same switch." -  This is incorrect you can have multiple native vlans define on one switch.

The native vlan only defines untagged traffic crossing the trunk interconnects between device's so you can have multiple trunks with different native vlans on each trunk if you wish.

However the native vlan DOES need to be same on either side of the interconnect otherwise you could experience STP loops.

res

Paul

[christopher.wood]

I think you misunderstood.

have two switches trunked to each other.
The configuration looks like this on both ends of the trunk:
interface GigabitEthernet0/1
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 900
 switchport trunk allowed vlan none
 switchport mode trunk

Gig0/3 is just an access port.
 

Here's what's going on. We have a Cisco UCS environment with 1kv for handing the vethernets to the cluster.
We need to get some VMs moved over from our old cluster to the new UCS environment. The VMs we are moving require access to our DMZ, which is basically just a switch with access ports, no vlans.
So UCS basically need to tag everything, we were told to fake it with a native vlan and set the interfaces towards our DMZ as native interfaces. I don't really like the idea of using vlan 1 as that native vlan on UCS with our old network core still attached to it. It's a disaster zone with vlan 1 flying everywhere.
So I was trying to use 900 as the native vlan... I thought any untagged traffic would talk to other untagged traffic regardless what the native vlan was.

[paul driver]

Why don't you trunk the UCS port  instead then?

res

Paul

[christopher.wood]

I would love to do that, but I need to be able to get the data from the trunked vlan to talk to the existing access switch ports, which are all untagged.

And that's why I was wondering if I did vlan mapping from let's say vlan 900 to vlan 1 on the DMZ switch, would that allow the traffic coming from the truck to talk to the access ports since they are native vlan 1?






 

[Suresh Vinayagam Subramanian]

Hello Chris ,

By default , Native vlan (Vlan 1)traffic would be send untagged thr' trunk But you have changed native vlan to vlan 900 . So your vlan 900 traffic would be send untaggged and vlan 1 traffic would be tagged .

Now, the native vlan for that G0/3 with switchport mode access would be VLAN 1, correct? - Yes .By default all ports would be in vlan 1 if we are not assigning to specific vlan

Which is untagged traffic.? ----> but vlan1 traffic would be tagged 

Should the untagged traffic from Gig0/3 be able to talk to the untagged traffic coming in on the trunk port that has native vlan 900??? --- > No 

HTH
Regards,
VS.Suresh.
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