Layer 2 Vpn Architectures [Electronic resources] نسخه متنی

Summary

This chapter reviewed some of the basic properties of several Layer 2 protocols. As mentioned in the chapter introduction, this chapter was not meant to be an exhaustive examination of each of these protocols; instead, it examined the relevant aspects of HDLC, PPP, Frame Relay, and ATM in the context of pseudowire emulation.

Following are several key aspects to take away from this chapter:

HDLC uses a simple framing mechanism to encapsulate its data. Cisco HDLC encapsulation is a modified form of HDLC and adds a Protocol Identifier field to determine the Layer 3 protocol that is stored in the HDLC payload.

PPP utilizes a framing mechanism that is similar to HDLC. Although PPP has a rich set of negotiation protocols such as LCP, different optional authentication methods and various NCPs, this was not discussed because they are transparent to the pseudowire emulation protocols.

Frame Relay adopts an encapsulation format that is similar to HDLC and PPP. A DLCI identifier in the Frame Relay header distinguishes logically distinct Frame Relay circuits.

Frame Relay LMI conveys circuit status information between network devices through periodic status messages.

Frame Relay has the option of performing Frame Relay policing to enforce a traffic contract for traffic that is inbound to the network from the customer. You can either discard noncompliant traffic or mark it with a lower priority by setting the DE bit. Conversely, you can apply Frame Relay shaping on network egress toward the customer. Shaping queues traffic that is nonconforming to meet a long-term average rate.

ATM has a well-defined protocol stack that takes upper layer protocol data and processes it through the AAL, ATM, and Physical layers. ATM also uses VPI/VCI to uniquely represent a logical ATM circuit.

From a fault management perspective, ATM OAM indicates faults within the network and performs end-to-end connectivity checks.

ATM traffic policing offers a set of admission control options to enforce ingress traffic contracts from end customer devices. You can either discard out-of-contract traffic or mark it with a lower priority through the use of the CLP bit. Conversely, ATM traffic shaping enforces an average rate of traffic on egress toward customer devices and can employ queuing for nonconforming traffic.