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What is Attachment Circuit in MPLS VPN ?

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What is attachment circuit in MPLS VPN ? Definitions are important in networking, if there are alternative usages of the definition, better to know them all for effective communication.


MPLS Layer 2 vpn

Figure – MPLS Layer 2 VPN Topology



In the above topology, I share, MPLS Layer 2 VPN Topology. There are many terminology but let’s focus on Attachment Circuit in this post.


If you look at the link between PE and CE devices, written as ‘ AC ‘

AC stands for Attachment Circuit.


By the way, I recommend you to read ‘ what does PE- CE mean ‘ post for the MPLS Layer 3 VPN definition.


Attachment Circuit as a term is used in MPLS Layer 2 VPN specifically.


It is used to describe the PE (Provider Edge) and the CE (Customer Edge) physical or logical circuit. Logical circuit can be a tunnel , sub-interface , etc.


In MPLS Layer 2 VPN, Attachment circuit is connected to PW in Ethernet over MPLS (Point to Point) and VPLS (Multi point to multipoint) or associated with the BGP in MPLS Layer 2 EVPN.


On top of Attachment Circuit, spanning tree can run , or any other layer 2 control plane, such as VPLS , Ethernet over MPLS , Link Aggregation Group etc.


So, if there are two links between customer equipment and the provider edge device, two of those attachment circuits can be placed in a bundle which is known as Link Aggregation Group (Ether-channel, Port-Channel are the Cisco terms for LAG)


Conclusion :


AC , Attachment Circuit , is a physical or logical circuit between PE and CE devices and used in MPLS Layer 2 VPN. In MPLS Layer 3 VPN, that link is mostly called as just PE-CE link and since in MPLS Layer 3 VPN, Routing Protocol runs between PE and CE link, a routing protocol in that case is called as PE-CE Routing Protocol or Customer Routing Protocol.

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