dot1q Trunking with NM-T3 card...

Unanswered Question
Nov 7th, 2008

Yet another question. I have a test network environment setup and I'm trying to setup trunking between my two switches. My question is, what encapsulation do my serial interfaces on the routers need to be to allow dot1q trunking? Right now the encap is HDLC. I've included a network drawing attachment.


I have this problem too.
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don-sullivan Fri, 11/07/2008 - 14:31

I'm not sure why you'd use dot1q trunking in this example. Will you be doing intervlan routing on switch 1 or switch 2?

Based on the diagram you've provided the encapsulation between the routers is fine. No need to use dot1q, just set the clock rate on one of them.

Jon Marshall Fri, 11/07/2008 - 14:32

802.1q runs on ethernet. You can't make serial interfaces run 802.1q.


CriscoSystems Fri, 11/07/2008 - 15:14

Obviously, but maybe mgar's concern is to allow either of his routers to perform inter-VLAN routing for each respective Ethernet segment, IN ADDITION to routing VLAN traffic across the serial link.

If that's indeed what you're going for mgar, you need to go into interface config mode for your Fa0/0 interfaces on each router and configure the "encapsulation dot1q ____" command (fill in ____ with the ID of the vlan you want the router to know about).

(Please rate helpful posts.)

mgar18777 Fri, 11/07/2008 - 15:20

Yes, that is exactly what I want to do. I'll give that a try (on Monday) and update this post.

Jon Marshall Fri, 11/07/2008 - 15:22


I wasn't trying to be difficult/funny and what may be to obvious to you may not be obvious to someone else.

Maybe Mgar's concern is to allow vlan traffic to be routed across the connection but i didn't interpret the question that way.

I do try to be helpful :-)


CriscoSystems Fri, 11/07/2008 - 15:27

Who said you were being difficult? Who said you were missing the obvious?

All I did was offer _my_ take on mgar's question...

Jon Marshall Fri, 11/07/2008 - 15:32


Looks like a misunderstanding. I wasn't saying i missed the obvious rather the way you said obviously - may have been obvious to you and to me but the OP may not have known this.

Anyway not worth worrying about.


mgar18777 Mon, 11/10/2008 - 08:25

I don't have the option for "encapsulation dot1q ____" on either of my routers. I'm working with two C2650XM's. One is ver 12.3(14)T7 and the other is ver 12.3(5a).

Jon Marshall Mon, 11/10/2008 - 08:53

Where are you trying to configure this ? It needs to be under the interface eg.

int fa0/0

encapsulation dot1q

If is is still not available could you post the full IOS version.


mgar18777 Mon, 11/10/2008 - 09:22

Ok, I had to create sub-if on each router to configure the dot1q encap. So if I want to stretch my VLAN 1 across both my routers to my switches, do I use "encap dot1q 1" on both router interfaces?

Jon Marshall Mon, 11/10/2008 - 09:27


Stretching a L2 vlan across a routed link ie. your serial connections is very different from what you proposed in your diagram.

Your diagram shows vlan 1 on one side and vlan 2 on the other side and the responses you have had were to do with how to allow these 2 vlans to talk to each other.

If you want to stretch one vlan across then you need to look at bridging or L2TPv3 which is something very different.

Could you clarify exactly what you want to do ?


mgar18777 Mon, 11/10/2008 - 09:41

Ah, I apologize for the confusion. I'm, obviously, new to Cisco, so my terminology is a little skewed, to say the least. =) I guess I should have been more clear on my original post, I am trying to stretch my vlans, and I guess I wasn't aware that you can't create dot1q encap on a serial interface. Than again, this is why I'm practicing in a test environment. So, to answer your question, I am trying to stretch my vlans accross my routers, according to the drawings setup. Is this easily doable with my current setup?

Jon Marshall Mon, 11/10/2008 - 09:51


No need to apologize :-)

L2TPv3 will allow you to do this. Not sure how "easily doable" it is - really depends on what you mean by easy !

The above is a link for L2TPV3 and how to set it up. I believe your IOS version should support it but you can check by trying out some of the commands from this example in the same document -



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