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petehummon
Beginner

Question about Q-in-Q Configuration

Hi Cisco Folks,


I’m a L3 guy who is struggling to understand a L2 configuration. I need a sanity check to make sure I understand how my devices are configured.

 

I have two Cisco switches (running IOS XR 6.6.12) connected by a Bundle-Ethernet interface identically configured on both sides. When I look at a switch’s configuration, I see this:

interface Bundle-Ether3
 description "Bundle to remote switch"
 mtu 9192
!
interface Bundle-Ether3.1234 l2transport
 description vlan 1234 to remote switch
 encapsulation dot1q 1234 second-dot1q 500
!

So after doing a little research and posting on this forum, I believe that Bundle-Ethernet3.1234 is configured with Q-in-Q VLAN traffic.


Let me see if I have this correct: As configured, both switches accept my customer’s traffic on VLAN 1234. But to forward traffic across the bundle, we have to double-tag, perhaps because my customer might someday want to include VLANs 1235, 1236, 1237, etc. So my two switches double-tag, using VLAN 500 as the inner VLAN.


VLAN 500 exists only between the two switches on either end of the bundle. When traffic on VLAN 1234 arrives at Switch A, the switch adds the double tag of VLAN 500 before forwarding the frame across the bundle to Switch B. Switch B removes the VLAN 500 double tag and forwards the frame on VLAN 1234. All other switches in the network are oblivious to VLAN 500’s existence.

 

…do I have this right? Thank you.

1 ACCEPTED SOLUTION

Accepted Solutions

Hello Peter,

your understanding is correct. And I agree with the headers that you have showed.

 

Vlan 1234 and B-E3.1234  are likely part of a bridge domain as these look like to be Cisco IOS XR routers in my opinion.

 

I am not able to give a reason for having used dual VLAN tag over the bundle-ethernet, but it works in the way you have described.

 

Hope to help

Giuseppe

 

View solution in original post

4 REPLIES 4
Giuseppe Larosa
Hall of Fame Master

Hello Peter,

there is a double VLAN stack on the bundle-ethernet.

However, this looks like a sort of so called VLAN normalization rather then Qin Q.

 

Let me explain:

with 802.1Q tunneling a service provider VLAN is added externally to the customer VLAN .

Here you have the reverse VLAN 1234 frames are added an internal VLAN tag of 500.

 

If it would be vlan 500 as first vlan with 1234 as second-dot1q it would be 802.1Q tunneling

 

Hope to help

Giuseppe

 

Thanks so much for writing, Giuseppe,
So if I’m understanding you correctly, my customer traffic arrives at one of my two switches, tagged with VLAN 1234: (Forgive the over-simplified ASCII art):

+-------------+-----------+--------------+
| SA/DA/Etype | TAG: 1234 | Data / FCS   |
+-------------+-----------+--------------+


My switch configuration adds the second-dot1q tag of VLAN 500, to send the frame over the bundle link:

+-------------+-----------+----------+-------------+
| SA/DA/Etype | TAG: 1234 | TAG: 500 | Data / FCS  |
+-------------+-----------+----------+-------------+


Once the frame reaches the other side, the VLAN 500 tag is removed, and the frame is forwarded on as it was before:

+-------------+-----------+--------------+
| SA/DA/Etype | TAG: 1234 | Data / FCS   |
+-------------+-----------+--------------+


Is that essentially what is happening? I see VLAN 1234 configured everywhere on my customer’s network, but I don’t see a reference to VLAN 500, except in the configuration for interface Bundle-Ethernet 3.1234:

interface Bundle-Ether3.1234 l2transport
 description vlan 1234 to remote switch
 encapsulation dot1q 1234 second-dot1q 500
!


Thank you so much!

Hello Peter,

your understanding is correct. And I agree with the headers that you have showed.

 

Vlan 1234 and B-E3.1234  are likely part of a bridge domain as these look like to be Cisco IOS XR routers in my opinion.

 

I am not able to give a reason for having used dual VLAN tag over the bundle-ethernet, but it works in the way you have described.

 

Hope to help

Giuseppe

 

View solution in original post

Excellent, thank you!  THis is exactly what I needed to know.  You've made my day.