PPP, HDLC 和 Frame Relay.pdf

3 WAN Protocols you should know: HDLC, PPP, and Frame-Relay Your company is connected to the Internet, right? (everyone nod your head yes) So what WAN protocol do you use to connect to the Internet? Chances are, that if you have a T1 leased line to the Internet or a private network between locations, you use one of these three WAN Protocols: HDLC, PPP, or Frame-relay. Let’s explore the differences and similarities of these protocols. What is HDLC? HDLC stands for High-Level Data Link Control protocol. Like the two other WAN protocols mentioned in this article, HDLC is a Layer 2 protocol (see OSI Model for more information on Layers). HDLC is a simple protocol used to connect point to point serial devices. For example, you have point to point leased line connecting two locations, in two different cities. HDLC would be the protocol with the least amount of configuration required to connect these two locations. HDLC would be running over the WAN, between the two locations. Each router would be de-encapsulating HDLC and turning dropping it off on the LAN. HDLC performs error correction, just like Ethernet. Cisco’s version of HDLC is actually proprietary because they added a protocol type field. Thus, Cisco HDLC can only work with other Cisco devices. HDLC is actually the default protocol on all Cisco serial interfaces. If you do a show running-config on a Cisco router, your serial interfaces (by default) won ’t have any encapsulation. This is because they are configured to the default of HDLC. If you do a show interface serial 0/0, you’ll see that you are running HDLC. Here is an example: What is PPP? You may have heard of the Point to Point Protocol (PPP) because it is used for most every dial up connection to the Internet. PPP is documented in RFC 1661. PPP is based on HDLC and is very similar. Both work well to connect point to point leased lines. The differences between PPP and HDLC are: • PPP is not proprietary when used on a Cisco ro