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Link State Routing Protocols

Link state routing protocols factor in additional metrics for determining the best route, including bandwidth. A link state protocol would see multiple routes from Office A to Office B in Fig. 5.21, including the direct T1 link, and the two-hop T3 route via the data center. The additional bandwidth (45 via 1.5 megabits) would make the two-hop T3 route the winner.

OSPF

Open Shortest Path First (OSPF) is an open link state routing protocol. OSPF routers learn the entire network topology for their “area” (the portion of the network they maintain routes for, usually the entire network for small networks). OSPF routers send event-driven updates. If a network is converged for a week, the OSPF routers will send no updates. OSPF has far faster convergence than distance vector protocols such as RIP. In Fig. 5.21, OSPF would choose the two-hop T3 route from Office A to B, over the single-hop T1 route.

Note: The exam strongly prefers open over proprietary standards, which is why proprietary routing protocols like Cisco’s EIGRP are not discussed here.

BGP

BGP is the Border Gateway Protocol, the routing protocol used on the Internet. BGP routes between autonomous systems, which are networks with multiple Internet connections. BGP has some distance vector properties but is formally considered a path vector routing protocol.

Modem

A Modem is a Modulator/Demodulator. It takes binary data and modulates it into analog sound that can be carried on phone networks designed to carry the human voice. The receiving modem then demodulates the analog sound back into binary data. Modems are asynchronous devices: they do not operate with a clock signal.

DTE/DCE and CSU/DSU

A DTE (Data Terminal Equipment) is a network “terminal,” meaning any type of network-connected user machine, such as a desktop, server, or actual terminal. A DCE (Data Circuit-Terminating Equipment, or sometimes called Data Communications Equipment) is a device that networks DTEs, such as a router. The most common use of these terms is DTE/DCE, and the meaning of each is more specific: the DCE marks the end of an ISP’s network. It connects to Data Terminal Equipment (DTE), which is the responsibility of the customer. The point where the DCE meets the DTE is called the demarc: the demarcation point, where the ISP’s responsibility ends and the customer’s begins.

The circuit carried via DCE/DTE is synchronous (it uses a clock signal). Both sides must synchronize to a clock signal, provided by the DCE. The DCE device is a modem or a CSU/DSU (Channel Service Unit/Data Service Unit).