Chapter D: Acronyms
| 2G | Second-generation mobile network |
| 3G | Third-generation mobile network |
| 6PE | A scheme to allow IPv6 traffic to be carried over an IPv4 MPLS network |
| AAL | ATM Adaptation Layer |
| ABR | Area Border Router. A router used to connect two OSPF areas |
| AC | Attachment Circuit. In the context of Layer 2 VPNs, the physical or logical circuit used to connect a CE to a PE |
| ACH | Associated Channel. In the context of VCCV, the ACH is the header that is added to the packet to trigger its processing by the control plane |
| AF | Assured Forwarding DiffServ class |
| AFI | Address Family Identifier. In BGP, the identity of the network layer protocol associated with the network layer reachability information being advertised |
| AIS | Alarm Indication Signal. In SONET/SDH networks and ATM networks, a means of signaling in the downstream direction the existence of a fault |
| AN | Access Node. Used especially in the context of Seamless MPLS, an AN is a generic term for a customer-facing device at the outer edge of the network, for example a DSLAM |
| APS | Automatic Protection Switching. A method for providing protection at the SONET/SDH layer by moving the traffic to a standby link |
| ARP | Address Resolution Protocol |
| AS | Autonomous System. A collection of routers belonging to the same administrative entity and having a common external routing policy |
| ASBR | Autonomous System Border Router. A router used to connect two ASs |
| ATM | Asynchronous Transfer Mode |
| ATM PVC | ATM Permanent Virtual Channel |
