3/2/12 3 WAN Protocols \ou should know: HDLC, PPP, and Frame-Rela\

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3 WAN Protocols \ou should know: HDLC, PPP, and Frame-Rela\by David Davis, vExpert, VCP, CCIE 9369 - January 7, 2009 Printer Friendly Version

Your company is connected to the Internet, right? (everyone nod your head yes) So what WANprotocol do you use to connect to the Internet? Chances are, that if you have a T1 leased line tothe Internet or a private network between locations, you use one of these three WAN Protocols:HDLC, PPP, or Frame-relay. Lets 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 mentionedin this article, HDLC is a Layer 2 protocol (see OSI Model for more information on Layers). HDLC is asimple protocol used to connect point to point serial devices. For example, you have point to pointleased line connecting two locations, in two different cities. HDLC would be the protocol with theleast amount of configuration required to connect these two locations. HDLC would be running overthe WAN, between the two locations. Each router would be de-encapsulating HDLC and turningdropping it off on the LAN.

HDLC performs error correction, just like Ethernet. Ciscos version of HDLC is actually proprietarybecause 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-configon a Cisco router, your serial interfaces (by default) wont have any encapsulation. This is becausethey are configured to the default of HDLC. If you do a show interface serial 0/0, youll see thatyou 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 upconnection 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:

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PPP is not proprietary when used on a Cisco router

PPP has several sub-protocols that make it function.

PPP is feature-rich with dial up networking features

Because PPP has so many dial-up networking features, it has become the most popular dial upnetworking protocol in use today. Here are some of the dial-up networking features it offers:

Link quality management monitors the quality of the dial-up link and how many errors havebeen taken. It can bring the link down if the link is receiving too many errors.

Multilink can bring up multiple PPP dialup links and bond them together to function as one.

Authentication is supported with PAP and CHAP. These protocols take your username andpassword to ensure that you are allowed access to the network you are dialing in to.

To change from HDLC to PPP, on a Cisco router, use the encapsulation ppp command, like this:

After changing the encapsulation to ppp, I typed ppp ? to list the PPP options available. There aremany PPP options when compared to HDLC. The list of PPP options in the screenshot is only a partiallist of what is available.

What is Frame-Rela\?

Frame Relay is a Layer 2 protocol and commonly known as a service from carriers. For example,people will say I ordered a frame-relay circuit. Frame relay creates a private network through acarriers network. This is done with permanent virtual circuits (PVC). A PVC is a connection from onesite, to another site, through the carriers network. This is really just a configuration entry that acarrier makes on their frame relay switches.

Obtaining a frame-relay circuit is done by ordering a T1 or fractional T1 from the carrier. On top ofthat, you order a frame-relay port, matching the size of the circuit you ordered. Finally, you order aPVC that connects your frame relay port to another of your ports inside the network.

The benefits to frame-relay are:

Ability to have a single circuit that connects to the frame relay cloud and gain access to allother sites (as long as you have PVCs). As the number of locations grow, you would save moreand more money because you dont need as many circuits as you would if you were trying tofully-mesh your network with point to point leased lines.

Improved disaster recovery because all you have to do is to order a single circuit to the cloudand PVCs to gain access to all remote sites.

By using the PVCs, you can design your WAN however you want. Meaning, you define whatsites have direct connections to other sites and you only pay the small monthly PVC fee foreach connection.

Some other terms you should know, concerning frame relay are:

LMI = local management interface. LMI is the management protocol of frame relay. LMI is sent

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between the frame relay switches and routers to communicate what DLCIs are available and ifthere is congestion in the network.

DLCI = data link connection identifier. This is a number used to identify each PVC in the framerelay network.

CIR = committed information rate. This is the amount bandwidth you pay to guarantee you willreceive, on each PVC. Generally you have much less CIR than you have port speed. You can,of course, burst above your CIR to your port speed but that traffic is marked DE.

DE = discard eligible. Traffic marked DE (that was above your CIR) CAN be discarded by theframe-relay network if there is congestion.

FECN & BECN = forward explicit congestion notification & backward explicit congestionnotification. These are bits set inside LMI packets to alert the frame-relay devices that there iscongestion in the network.

Article Summar\

We talked about three different WAN protocols that every network administrator should be familiarwith. HDLC would be the most common protocol used to connect to the Internet but all three WANprotocols are offered, by carriers, as possible encapsulations to use to connect to the Internet

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