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5-4 The Router

The router is the most powerful networking device used today to interconnect LANs. The router is a layer 3 device in the OSI model, which means the router uses the network address (layer 3 addressing) to make routing decisions regarding forwarding data packets. Remember from Chapter 1, section 3, that the OSI model separates network responsibilities into different layers. In the OSI model, the layer 3 or network layer responsibilities include handling of the network address. The network address is also called a logical address, rather than being a physical address such as the MAC address. The physical address is the hardware or MAC address embedded into the network interface card. The logical address describes the IP address location of the network and the address location of the host in the network.

Essentially, the router is configured to know how to route data packets entering or exiting the LAN. This differs from the bridge and the layer 2 switch, which use the Ethernet address for making decisions regarding forwarding data packets and only know how to forward data to hosts physically connected to their ports.

Routers are used to interconnect LANs in a campus network. Routers can be used to interconnect networks that use the same protocol (for example, Ethernet), or they can be used to interconnect LANs that are using different layer 2 technologies such as an Ethernet and token ring. Routers also make it possible to interconnect to LANs around the country and the world and interconnect to many different networking protocols.

Routers have multiple port connections for connecting to the LANs, and by definition a router must have a minimum of three ports. The common symbol used to represent a router in a networking drawing is provided in Figure 5-14. The arrows pointing in and out indicate that data enters and exits the routers through multiple ports. The router ports are bidirectional, meaning that data can enter and exit the same router port. Often the router ports are called the router interface, the physical connection where the router connects to the network.


FIGURE 5-14 The network symbol for a router.

The Router Interface: Cisco 2800 Series

Figure 5-15 shows the rear panel view (interface side) of a Cisco 2800 series router.


FIGURE 5-15 The rear panel view of a Cisco 2800 series router.

The following describes the function of each interface:

  • USB Interface: The USB ports are used for storage and security support.
  • FastEthernet Ports: FE0/0: Fast Ethernet (10/100Mbps) and FE0/1: Fast Ethernet (10/100Mbps).
  • Console Input: This input provides an RS-232 serial communications link into the router for initial router configuration. A special cable, called a console cable, is used to connect the console input to the serial port on a computer. The console cable can have RJ-45 plugs on each end and requires the use of an RJ-45 to DB9 adapter for connecting to the computer’s COM1 or COM2 serial port. The console cable can also have an RJ-45 connector on one end and an integrated DB9 connector on the other end.
  • Auxiliary Input: This input is used to connect a dial-in modem into the router. The auxiliary port provides an alternative way to remotely log in to the router if the network is down. This port also uses an RJ-45 connection.
  • Serial Interface: CTRLR T1 1 and CTRLR T1 0.

    This is a serial connection, and it has a built-in CSU/DSU. This interface is used to provide a T1 connection to the communications carrier. (Note: The CSU/DSU function is presented in Chapter 8, “Introduction to Switch Configuration.”) This type of connection (RJ-45) replaces the older cabling using V.35 cable (shown later in Figure 5-18). There are three LEDs on this interface:

    • AL—alarm
    • LP—loop
    • CD—Carrier Detect
  • Voice Interface Card (VIC2-4FXO): This interface shows four phone line connections. This router can be programmed as a small Private Branch Exchange (PBX) for use in a small office. The PBX function is presented in Chapter 10, “Internet Technologies: Out to the Internet.”
  • WAN Interface Card (WIC2AM): This interface has two RJ-11 jacks and two V.90 analog internal modems. These modems can be used to handle both incoming and outgoing modem calls. This interface is listed as modem in Figure 5-15.
  • VIC-4FXS/DID: This interface is a four-port FXS and DID voice/fax interface card. FXS is a Foreign Exchange Interface that connects directly to a standard telephone. DID is Direct Inward Dialing and is a feature that enables callers to directly call an extension on a PBX. This interface is listed as FXS/DID in Figure 5-15.

The Router Interface—Cisco 2600 Series

Figure 5-16 shows the rear panel view (interface side) of a Cisco 2600 series router.


FIGURE 5-16 The rear panel view of a Cisco 2600 series router.

The following describes the function of each interface to the network:

  • Power On/Off: Turns on/off electrical power to the router.
  • Auxiliary Input: Used to connect a dial-in modem into the router. The auxiliary port provides an alternative way to remotely log in to the router if the network is down. This port also uses an RJ-45 connection.
  • Console Input: Provides an RS-232 serial communications link into the router for initial router configuration. A special cable, called a console cable, is used to connect the console input to the serial port on a computer. The console cable uses RJ-45 plugs on each end and requires the use of an RJ-45 to DB9 adapter for connecting to the COM1 or COM2 serial port.
  • Serial Ports: Provides a serial data communication link into and out of the router, using V.35 serial interface cables.
  • DSU Port: This T1 controller port connection is used to make the serial connection to Telco. This module has a built-in CSU/DSU module. There are five LEDs next to the RJ-45 jack. These LEDs are for the following:

    • TD—Transmit Data
    • P—Loop
    • D—Receive Data
    • D—Carrier Detect
    • L—Alarm
  • Ethernet Port: This connection provides a 10/100Mbps Ethernet data link.
  • Analog Modem Ports: This router has a 16-port analog network module.

A media converter is used to convert the 15-pin AUI port to the 8-pin RJ-45 connector. Figure 5-17 shows an example of an AUI to RJ-45 media converter. Media converters are commonly used in computer networks to adapt layer 1 or physical layer technologies from one technology to another. For example:

  • AUI to twisted pair (RJ-45) AUI to fiber
  • RJ-45 to fiber

FIGURE 5-17 A CentreCom 210TS AUI to RJ-45 media converter.

Figure 5-18 shows a Cisco 7200 series router, which provides adaptable interfaces for connecting to many physical layer technologies such as FastEthernet, gigabit Ethernet, ATM, and FDDI.


FIGURE 5-18 A Cisco 7200 series router (courtesy of Cisco Systems).

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