Configure a Cisco Router as a PPPoA Client

To clear up a rather widespread misconception, PPPoA is defined in RFC 2364 as PPP over AAL5. However, it is commonly referred to simply as PPP over ATM. Chapter 5, "Configuring DSL Access with PPPoE," covered the configuration of PPPoE on a home router for DSL connectivity in some detail. The relative technology behind PPPoA is identical in nature to PPPoE. However, there are some significant differences that exist on the provider-facing side of the configuration, primarily:

■ The handling of the ATM interface

■ The configuration of the ATM permanent virtual circuit (PVC) or switched virtual circuit (SVC)

DSL is a Layer 1 access methodology. The Layer 1 connection exists across the local loop between the CPE and the DSL access multiplexer (DSLAM). Layer 2 connectivity is provided by ATM from the CPE to the DSLAM and beyond. Layer 3 connectivity is established between the CPE and an aggregation router located somewhere beyond the DSLAM. For purposes of review and to provide a point of reference for discussion topics in this chapter, Figure 6-1 provides a topological view. It shows the connectivity between the CPE and the Internet. The data traffic must traverse the local loop to the DSLAM and then go across the ATM network to the aggregation router.

Although similar to PPPoE, PPPoA is its own technology. It does present several configuration differences (due to the needs of an ATM interface versus an Ethernet interface) when compared to PPPoE. The principal difference in PPPoA is that the CPE router is now using RFC 1483/2684 encapsulation to transport PPP frames across the local loop inside of ATM cells. In other words, it actually gets to be a router this time, rather than a bridge.

Like PPPoE, a logical interface is used for managing the PPP connection. This interface is known as a virtual access interface. It will be associated with the ATM PVCs configured on the ATM interface. This configuration encapsulates each PPP connection into a separate PVC or SVC to allow each session to appear as if it is being terminated on a traditional PPP serial interface. To facilitate these connections, a virtual interface template is created to provide configuration details when the virtual circuit is created.

Configure a Cisco Router as a PPPoA Client 131

Figure 6-1 PPPoA DSL Topology

Figure 6-1 PPPoA DSL Topology

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