Wide Area Networks

Sommaire

Introduction

• People in the regional or branch offices of an organization need to be able to communicate and share data.
• Organizations often want to share information with other organizations across large distances.
• Employees who travel on company business frequently need to access information that resides on their corporate networks.

• WANs generally connect devices that are separated by a broader geographic area than a LAN can serve.
• WANs use the services of carriers such as telcos, cable companies, satellite systems, and network providers.
• WANs use serial connections of various types to provide access to bandwidth over large geographic areas.

WAN Support

Cisco supports many layer 2 WAN encapsulations on its serial interfaces, including HDLC, PPP, and Frame Relay, which map to the Cisco exam objectives. You can view them via the encapsulation ? command from any serial interface, but understand that the output you’ll get can vary based upon the specific IOS version you’re running:

I also want to point out that if I had other types of interfaces on my router, I would have a different set of encapsulation options. And never forget that you can’t configure an Ethernet encapsulation on a serial interface or vice versa! Next, I’m going to define the most prominently known WAN protocols used in the latest Cisco exam objectives:

Just so you know, the only WAN protocols you’ll usually find configured on a serial interface are HDLC, PPP, and Frame Relay, but who said you’re stuck with using only serial interfaces for wide area connections? Actually, we’re beginning to see fewer and fewer serial connections because they’re not as scalable or cost effective as a Fast Ethernet connection to your ISP.

Frame Relay A packet-switched technology that made its debut in the early 1990s, Frame Relay is a high-performance Data Link and Physical layer specification. It’s pretty much a successor to X.25, except that much of the technology in X.25 that was used to compensate for physical errors like noisy lines has been eliminated. An upside to Frame Relay is that it can be more cost effective than point-to-point links, plus it typically runs at speeds of 64 Kbps up to 45 Mbps (T3). Another Frame Relay benefit is that it provides features for dynamic bandwidth allocation and congestion control.

Integrated Services Digital Network (ISDN) is a set of digital services that transmit voice and data over existing phone lines. ISDN offers a cost-effective solution for remote users who need a higher-speed connection than analog POTS dial-up links can give them, and it’s also a good choice to use as a backup link for other types of links, such as Frame Relay or T1 connections.

High-Level Data-Link Control (HDLC) was derived from Synchronous Data Link Control (SDLC), which was created by IBM as a Data Link connection protocol. HDLC works at the Data Link layer and creates very little overhead compared to Link Access Procedure, Balanced (LAPB). Generic HDLC wasn’t intended to encapsulate multiple Network layer protocols across the same link—the HDLC header doesn’t contain any identification about the type of protocol being carried inside the HDLC encapsulation. Because of this, each vendor that uses HDLC has its own way of identifying the Network layer protocol, meaning each vendor’s HDLC is proprietary with regard to its specific equipment.

Point-to-Point Protocol (PPP) is a pretty famous, industry-standard protocol. Because all multiprotocol versions of HDLC are proprietary, PPP can be used to create point-to-point links between different vendors’ equipment. It uses a Network Control Protocol field in the Data Link header to identify the Network layer protocol being carried and allows authentication and multilink connections to be run over asynchronous and synchronous links.

Point-to-Point Protocol over Ethernet (PPPoE) encapsulates PPP frames in Ethernet frames and is usually used in conjunction with xDSL services. It gives you a lot of the familiar PPP features like authentication, encryption, and compression, but there’s a downside—it has a lower maximum transmission unit (MTU) than standard Ethernet does. If your firewall isn’t solidly configured, this little factor can really give you some grief! Still somewhat popular in the United States, PPPoE’s main feature is that it adds a direct connection to Ethernet interfaces while also providing DSL support. It’s often used by many hosts on a shared Ethernet interface for opening PPP sessions to various destinations via at least one bridging modem.

Cable In a modern hybrid fiber-coaxial (HFC) network, typically 500 to 2,000 active data subscribers are connected to a certain cable network segment, all sharing the upstream and downstream bandwidth. HFC is a telecommunications industry term for a network that incorporates both optical fiber and coaxial cables to create a broadband network. The actual bandwidth for Internet service over a cable TV (CATV) line can be up to about 27 Mbps on the download path to the subscriber, with about 2.5 Mbps of bandwidth on the upload path. Typically users get an access speed from 256 Kbps to 6 Mbps. This data rate varies greatly throughout the United States and can be much, much higher today.

Digital subscriber line (DSL) is a technology used by traditional telephone companies to deliver advanced services such as high-speed data and sometimes video over twisted-pair copper telephone wires. It typically has lower data-carrying capacity than HFC networks, and data speeds can be limited in range by line lengths and quality. Digital subscriber line is not a complete end-to-end solution but rather a Physical layer transmission technology like dialup, cable, or wireless. DSL connections are deployed in the last mile of a local telephone network—the local loop. The connection is set up between a pair of DSL modems on either end of a copper wire located between the customer premises equipment (CPE) and the Digital Subscriber Line Access Multiplexer (DSLAM). A DSLAM is the device that is located at the provider’s central office (CO) and concentrates connections from multiple DSL subscribers.

MultiProtocol Label Switching (MPLS) is a data-carrying mechanism that emulates some properties of a circuit-switched network over a packet-switched network. MPLS is a switching mechanism that imposes labels (numbers) to packets and then uses them to forward the packets. The labels are assigned on the edge of the MPLS network, and forwarding inside the MPLS network is carried out solely based on the labels. The labels usually correspond to a path to layer 3 destination addresses, which is on par with IP destination-based routing. MPLS was designed to support the forwarding of protocols other than TCP/IP. Because of this, label switching within the network is achieved the same way irrespective of the layer 3 protocol. In larger networks, the result of MPLS labeling is that only the edge routers perform a routing lookup. All the core routers forward packets based on the labels, which makes forwarding the packets through the service provider network faster. This is a big reason most companies have replaced their Frame Relay networks with MPLS service today. Last, you can use Ethernet with MPLS to connect a WAN, and this is called Ethernet over MPLS, or EoMPLS.

Asynchronous Transfer Mode (ATM) was created for time-sensitive traffic, providing simultaneous transmission of voice, video, and data. ATM uses cells that are a fixed 53 bytes long instead of packets. It also can use isochronous clocking (external clocking) to help the data move faster. Typically, if you’re running Frame Relay today, you will be running Frame Relay over ATM.

Cellular 3G/4G Having a wireless hot spot in your pocket is pretty normal these days. If you have a pretty current cellular phone, then you can probably can gain access through your phone to the Internet. You can even get a 3G/4G card for an ISR router that’s useful for a small remote office that’s in the coverage area.

Very Small Aperture Terminal (VSAT) can be used if you have many locations geographically spread out in a large area. VSAT uses a two-way satellite ground station with dishes available through many companies like Dish Network or Hughes and connects to satellites in geosynchronous orbit. A good example of where VSATs are a useful, costeffective solution would be companies that use satellite communications to VSATs, like gasoline stations that have hundreds or thousands of locations spread out over the entire country. How could you connect them otherwise? Using leased lines would be cost prohibitive and dial-ups would be way too slow and hard to manage. Instead, the signal from the satellite connects to many remote locations at once, which is much more cost effective and efficient! It’s a lot faster than a modem (about 10x faster), but the upload speeds only come in at about 10 percent of their download speeds.

Metro Ethernet Metropolitan-area Ethernet is a metropolitan area network (MAN) that’s based on Ethernet standards and can connect a customer to a larger network and the Internet. If available, businesses can use Metro Ethernet to connect their own offices together, which is another very cost-effective connection option. MPLS-based Metro Ethernet networks use MPLS in the ISP by providing an Ethernet or fiber cable to the customer as a connection. From the customer, it leaves the Ethernet cable, jumps onto MPLS, and then Ethernet again on the remote side. This is a smart and thrifty solution that’s very popular if you can get it in your area.

Cisco Intelligent WAN (IWAN)

Virtual Private Network (VPN)

A VPN, or virtual private network, is a secure tunnel between two or more devices. VPNs are used to protect private web traffic from snooping, interference, and censorship.

Generic Routing Encapsulation (GRE)

Generic Routing Encapsulation (GRE) is a tunneling protocol developed by Cisco Systems that can encapsulate a wide variety of network layer protocols inside virtual point-to-point links or point-to-multipoint links over an Internet Protocol network.

GRE TUNNELS

Generic Routing Encapsulation (GRE) is a tunneling protocol that can encapsulate many protocols inside IP tunnels. Some examples would be routing protocols such as EIGRP and OSPF and the routed protocol IPv6.