EoMPLS port-tunneling and VLAN tunneling understanding issue

Unanswered Question
Apr 3rd, 2009

Hi All,

I was reading L2 VPN architecture book chapter 7 LAN Protocol over MPLS Case studies and I am not able to understand the following paragraph:

In port-tunneling mode, the packet does not have ingress port information. For inclusion of ingress information, the port-tunneled interface is put into the QinQ mode. A hidden VLAN is then created and added onto the packet. A hidden VLAN is a VLAN that is numbered outside the allowed range for VLAN IDs. This is how the NP learns the ingress information. The hidden VLAN concept applies to a switch platform (that is, 6500 and 7600 platforms). In contrast, VLAN-tunneled mode does not require a hidden VLAN. The NP can discern the ingress information from the packet's dot1Q header.

A similar concept applies to routers (VLAN stacking method). It involves the use of subinterfaces.

Another difference between the port-tunneling mode and VLAN-tunneling mode is in the handling of VLAN IDs. In port-tunneling mode, the VLAN ID is transparently passed from the ingress PE to the egress PE over MPLS in a single VLAN. In VLAN-tunneling mode, however, the VLAN ID at each end of the EoMPLS tunnel can be different. To overcome this, the egress side of the tunnel that is mapped to a VLAN rewrites the VLAN ID in outgoing dot1Q packets to the ID of the local VLAN.

It will be great if some one can explain me what it says.


Devang Patel

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Laurent Aubert Fri, 04/03/2009 - 18:05


In port mode, to be able to find the right PW, the router needs to know from which interface the packets was received.

On 6500/7600 platform, a hidden VLAN is associated to the interface where the xconnect is applied. This VLAN is associated to the packet during the forwarding lookup and allows the router to find the right PW to use.



devang_etcom Sat, 04/04/2009 - 11:21


Thanks for your reply. So Router will create the VLAN per interface where you have assigned the xconnect configration. So for router it will be for each subinterfaces, so using VLAN ID which is hidden as well as random it will track the interface from which it has received or transmit the packets. So what will be the role of VC ID on interface?

How will it work on switch if switch has multiple interface in one vlan?


Devang Patel

Laurent Aubert Mon, 04/06/2009 - 06:27

Hi Devang,

the VC-ID is used to de-multiplex several PWs between the same peers when you are receiving the traffic from the core.

For SVI interface, it works as you think it will ;-): received traffic will be bridged based on the VLAN MAC table and if the remote MAC was learned from the PW, then the traffic will be sent to it.

Due to hardware implementation, on 7600, you need a SIP card for your core interface to do that.




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