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.
Figure - MPLS Layer 2 VPN Topology
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)
