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The migration of customers from the ATM-based network onto the new IP/MPLS network will occur over a transition period. During the transition period, a number of customers will have existing sites attached to the ATM network and new sites attached to the new IP/MPLS network. In order to provide connectivity between the TLS network and the VPLS network, a number of ATM VCs are configured at the boundary between the two networks. Each VC connects user ports on the Ethernet switch in the ATM network to a VPLS instance on the gateway provider edge (PE) router dedicated to this user.  Multiple VPLS instances are supported to provide isolation between multiple users.  Given that there is already some level of meshing in the ATM network, there is no need to configure an ATM VC for each Ethernet switch.

The Ethernet frames are sent and received on an ATM VC using RFC 2684 bridged encapsulation. The gateway PE, i.e., either PE 1 or PE 3 in Figure 1, reassembles the AAL5 protocol data unit (PDU) and processes the Ethernet frame. From that point, all other datapath operation is similar to the case of a frame received on a regular Ethernet interface terminating on a VPLS instance.

Because the ATM TLS network provides full or partial mesh with the ATM VCs, it is important to be able to configure some of the ATM VCs terminating in the PE gateway into split horizon groups to avoid loops. The VPLS instance in the gateway PE will disable forwarding between VCs that are part of the same split horizon group.

The provisioning steps are the same as those required for configuring a regular Ethernet interface on a VPLS instance. Let us assume that a VPLS instance is already configured in the gateway PE – PE 1 – with at least a network IP/MPLS tunnel and one Ethernet interface as a service interface. Next, the user specifies an ATM VC as a second ingress VPLS service interface and configures the slot/port/VPI/VCI identifying it.

Next, an ATM traffic descriptor should be configured and applied to the ATM VC. The user specifies the ATM traffic parameters, the ATM service category, and enables such functions as ATM policing and shaping.

Once an AAL5 PDU is reassembled by the ATM segmentation and reassembly sublayer (SAR) at the ingress ATM port in PE 1, the contained Ethernet frame is extracted. Ingress classification of the Ethernet frames received on an ATM VC allows the user to filter the received frames based on both MAC and IP criteria. An example of a MAC match criterion is the match on the 802.1 p field or a match on the source or destination MAC address field. An example of an IP match criterion is a match on the differentiated services code point (DSCP) field or a match on the source IP address field in the header of an IP packet inside the Ethernet frame. Frames that match a specified criterion can be directed to a separate forwarding class queue dedicated to this ATM VC as configured in the ingress QoS policy. Frames that did not match any of the configured criteria will be queued in the default forwarding class queue assigned to this VC as specified in the ingress QoS policy.