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I am trying to figure out the configuration required to vlan tag with vlan 201 all traffic coming from one port and send it out another to the ONT for CenturyLink. I have a Cisco 3750e that I am using, and I am trying to put that infront of a running DDWRT that doesn't support vlan tagging on the wan interface for vlans above 15.

Basically I want to use the 3750e to tag all the traffic so the switch just does the PPPoE authentication for the internet connection and traffic tagging is handled by the 3750e.

Right now, I have two ports setup on vlan 201 with one set as a native trunk (planning to plug that one into the ONT feed from the outside) and the other to go to the modem setup as an access port. I haven't tested this yet, but does this sound right?

  • Using PPP will preclude the use of VLAN tags since PPP does not support VLANs. Also, you cannot send VLAN tags over the Internet. A layer-3 device like a router strips off any layer-2 frame before forwarding a packet. A VLAN tag is part of an ethernet frame. – Ron Maupin Sep 30 '16 at 18:42
  • Well, the connection to the ONT must be tagged 201 and it's PPPoE authenticated fiber to the premises internet. It works with a OpenWRT, but both of the devices I have tried that on have other issues with OpenWRT. I'm trying to switch to DDWRT because it doesn't have the issues that OpenWRT does, but it only supports vlans 0-15. – Allen George Sep 30 '16 at 18:51
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VLAN tags are used to mark which ethernet frames belong to which VLAN on a trunk link, usually between switches. Setting an access port to VLAN 201 does not add VLAN tags to the frames because most end-devices do not understand VLAN tags in an ethernet frame. By setting a native VLAN on a trunk, you are saying that that particular VLAN will not have a VLAN tag.

All you need to do to tag frames is to set the interface to trunk. For any VLAN which you want tagged, do not set that VLAN to native. Something like:

interface GigabitEthernet1/1/1
 description Trunk to CenturyLink
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan <VLAN Range>
 switchport mode trunk
 switchport nonegotiate
 no shutdown
!
| improve this answer | |
  • Thank you for the explanation. At one point I'd covered all the material for the CCNA cert in college, but I'm a Windows/Storage guy now and I've forgotten almost everything I learned. I'll give this a try this evening and see if that works. – Allen George Sep 30 '16 at 18:58
  • If this has answered your question, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. – Ron Maupin Nov 29 '16 at 23:33
  • Done. I couldn't figure out how to mark it as the answer for about 5 minutes. – Allen George Dec 4 '16 at 4:52

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