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confussed between the vlan mode and switch mode AToMPLS


Could someone explain to me in simple terms the difference between the 2. I am very confussed about how the VC concepts works.

So i Dot1q tagged packet comes into the switch, the switch applies 2 lables to it, outer and inner, and then it gets to the egress switch where the switches checks the dot1q tag again to decide what to do with the packet?

in vlan rewrite example have have difference vlans on both sides (ingress and egress) how does the switch or MPLS know that the packet is going from one vlan to the other?



Re: confussed between the vlan mode and switch mode AToMPLS

The PID in the ATM header will give the information about the vlan details and the MPLS header details. ATM AAL5 over MPLS encapsulates ATM AAL5 SDUs in MPLS packets and forwards them across the MPLS network. Each ATM AAL5 SDU is transported as a single packet

Re: confussed between the vlan mode and switch mode AToMPLS


in AToM - as you described - there will be two labels involved, very much like in MPLS L3VPN. The top label will allow the transportation to the proper PE and the VC-label will identify the interface/port/VLAN.

With EoMPLS in VLAN mode the PEs will be able to transport dot1Q tagged frames from a CPE on a per-VLAN basis. The configuration includes the VLAN tags at both end of the MPLS VC. So the dot1Q header is stripped and transported and a new dot1Q header is created by the egress PE. The VLAN tags to be used are directly configured at each PE in the form of a subinterface with encapsulation dot1Q.

The decision where to forward the frame in the egress PE is solely determined from the VC label. Therefore one could even have the same VLAN for many customers. This is similar to IP address overlap in the MPLS L3VPN case.

I am not familar with the term "switch mode" you use above. The other option with EoMPLS is called "port mode" afaik. See f.e.

"Ethernet over MPLS

Ethernet over MPLS works by encapsulating Ethernet PDUs in MPLS packets and forwarding them across the MPLS network. Each PDU is transported as a single packet. There are various ways to configure Ethernet over MPLS:

•VLAN mode—transports Ethernet traffic from a source 802.1Q VLAN to a destination 802.1Q VLAN through a single VC over an MPLS network.

•Port mode—allows all traffic on a port to share a single VC across an MPLS network.


In port mode there will be no subinterfaces and VLAN tags defined at the PE routers. Instead all incoming ethernet frames will be "copied" from one port to another. I personally refer to this as an MPLS cloud playing the role of a "Cat5 cable simulator". The only thing a PE does besides transportation of ethernet frames is error checking (and dropping if the FCS is not correct).

Hope this helps! Please rate all posts.

Regards, Martin