Pseudowire Termination: Explicit Notifications for Pseudowire Down Status
As the demand for MPLS-based Layer 2 services grows, new challenges arise for service providers to be able to interoperate Layer 2 with Layer 3 and give their customers value-added services. MPLS in the access networks is already used by applications like mobile or DSL backhaul to achieve a more cost-efficient solution, better service reliability, and quality of service. Most of the traditional access network infrastructure is built over TDM circuits such as DS3 for higher speeds, ATM, or Frame Relay as access trails in a Layer 3 service. For higher bandwidth requirements and more flexibility, service providers use Ethernet as access technology for a wide range of network services. Although Ethernet provides a convenient link topology for access networks, it is not well suited for Layer 2 switching and for aggregating traffic from the access network to the core. MPLS is already used in the core and now its presence in the access network enables use of a single technology across the network. When MPLS is deployed in the access network, Ethernet is used as a link-layer encapsulation technology only, and MPLS switches perform traffic forwarding and provide other Layer 2 services. There is an increase in demand for using pseudowires as access circuits in the service delivery points in the network. These pseudowires terminate on a service node on which the service provider applies Layer 3 or Layer 2 services to the customer data.
The following is a generic topology for understanding termination for pseudowire into a Layer 2 or Layer 3 instance and the notifications for both cases.
The following terminologies are used for the network elements:
Access node (AN): An access node is typically a customer edge device that processes the packets entering or exiting the network at Layer 2. This includes devices such as DSLAMs and MSANs.
Transport node(TN): A transport node acts like a P router as it does not have any customer or service state. It is either used for connecting the access node to the service node or to two service nodes.
Service node (SN): A service node is a PE router that applies services to the customer packets. It includes Layer 2 PE, Layer 3 PE, peering routers, video servers, base station controllers, and media gateways.
The following example shows a linear L2-L3 interconnection set up with the absence of pseudowire redundancy. Here, the access circuit pseudowire is configured between the access PE (SN1) and service node (SN2), which defines the boundary of the L2 domain. The Layer 3 VPN is configured between SN2 and SN3, which constitute the L3 domain. Layer 2 circuit pseudowire terminates in the VRF of the device interconnecting the L2-L3 domains (SN2); that is, the service node performs stitching between the Layer 2 circuit and the Layer 3 VPN.
