Cisco Support Community
Community Member

Vlan on router fast int

Hi All,

Have asked this before and gotten a short answer, but need a little more detail.

If I am to configure a vlan id on a fast int of my router, i understand that I need to create a sub interface. do i also need to create a sub interface for the native vlan as well? I see the commands on the sub int that I have created for the specific vlan. If only int, i only have:

encap dot1q 124

this is stating that this int will carry vlan traffic in 124 correct? do i also need to add

encap dot1q 100 native

and do i also need to have a subint of .100?

I suppose i confused as to what is needed in the way of adding a native vlan to these interfaces.




Re: Vlan on router fast int

actully native vlan means, this vlan is not carring the taging on trunk...

and yes you need to creat the subinterfaces in order to make communication between different vlans...and you can use any arbitary number for sub interface...but you just need to assing the encapsulation for the perticular vlan nothing mroe...

hope this will help you

rate this post if it helps



Re: Vlan on router fast int

Hello R,

take a look on this text, it helped me:

"You can configure 802.1q native vlan under a subinterface or under the physical interface on a router.

If configured under a subinterface, you use the encap dot1q vlan-id native subcommand, with the inclusion of the nateive keyword meaning that the frames exiting this subinterface should not be tagged.

As with other router trunking configs, the associated IP address would be configured on that same interface. Alternately, if not configured on a subint , the router assumes that the native vlan is associated with the physical interface;

the associated IP address, however, would need to be configured under the physical interface."

so I guess you dont actually need a subinterface. but if you do have one, you can also use native sub command, or not (leaving it to the physical int IP to use it).



Re: Vlan on router fast int

A couple more points:

First of all, it is not necessary to make the subinterface number correspond to the VLAN, but it does help make the configuration readable. The subinterface number is purrly arbitrary. An equally valid (but more confusing) configuration would be:

int FastEthernet0/0

  no ip address


int FastEthernet0/0.1

  encapsulation dot1q 42


int FastEthernet0/0.2

  encapsulation dot1q 56

Secondly, as far as I am aware there is no obligation to define a native VLAN on the trunk, just in the same way that a switch can have a native VLAN on a trunk and simply not use it. If you don't define a native VLAN, then any untagged frames will be discarded. I believe this to be true, but look out for bugs concerning protocols that are passed between switches untagged or on VLAN 1, like CDP.

Third (of the "couple" of points ;-), is that the router will discard any incoming frames tagged with a VLAN for which there is no subinterface to attach them to.

Kevin Dorrell


Community Member

Re: Vlan on router fast int

Thanks all for your replies. So this is what I taking away from this, correct me if wrong.

I understand that the number does not have to match the sub int, but i am all about easy! ;)

I may not actually have to tag a native vlan if i believe there to be no untagged traffic. This is for a host mpls connection, in which cdp is disabled, so i am thinking that all traffic will be tagged with my vlan that was given to me by the provider. in this case, i will not need a native vlan tag.

All sound good?

Thanks again for your help.

CreatePlease to create content