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Router Interface

Management Ports

Routers have physical connectors that are used to manage the router. These connectors are known as management ports. Unlike Ethernet and serial interfaces, management ports are not used for packet forwarding. The most common management port is the console port. The console port is used to connect a terminal, or most often a PC running terminal emulator software, to configure the router without the need for network access to that router. The console port must be used during initial configuration of the router.

Another management port is the auxiliary port. Not all routers have auxiliary ports. At times the auxiliary port can be used in ways similar to a console port. It can also be used to attach a modem. Auxiliary ports will not be used in this curriculum.

The figure shows the console and AUX ports on the router.

Router Interfaces

The term interface on Cisco routers refers to a physical connector on the router whose main purpose is to receive and forward packets. Routers have multiple interfaces that are used to connect to multiple networks. Typically, the interfaces connect to various types of networks, which means that different types of media and connectors are required. Often a router will need to have different types of interfaces. For example, a router usually has FastEthernet interfaces for connections to different LANs and various types of WAN interfaces to connect a variety of serial links including T1, DSL and ISDN. The figure shows the FastEthernet and serial interfaces on the router.

Like interfaces on a PC, the ports and interfaces on a router are located on the outside of the router. Their external location allows for convenient attachment to the appropriate network cables and connectors.

Note: A single interface on a router can be used to connect to multiple networks; however, this is beyond the scope of this course and is discussed in a later course.

Like most networking devices, Cisco routers use LED indicators to provide status information. An interface LED indicates the activity of the corresponding interface. If an LED is off when the interface is active and the interface is correctly connected, this may be an indication of a problem with that interface. If an interface is extremely busy, its LED will always be on. Depending on the type of router, there may be other LEDs as well. For more information on LED displays on the 1841, see the link below.


"Troubleshooting Cisco 1800 Series Routers (Modular),"
Interfaces Belong to Different Networks

As shown in the figure, every interface on the router is a member or host on a different IP network. Each interface must be configured with an IP address and subnet mask of a different network. Cisco IOS will not allow two active interfaces on the same router to belong to the same network.

Router interfaces can be divided into two major groups:
LAN interfaces - such as Ethernet and FastEthernet
WAN interfaces - such as serial, ISDN, and Frame Relay

LAN Interfaces

As the name indicates, LAN interfaces are used to connect the router to the LAN, similar to how a PC Ethernet NIC is used to connect the PC to the Ethernet LAN. Like a PC Ethernet NIC, a router Ethernet interface also has a Layer 2 MAC address and participates in the Ethernet LAN in the same way as any other hosts on that LAN. For example, a router Ethernet interface participates in the ARP process for that LAN. The router maintains an ARP cache for that interface, sends ARP requests when needed, and responds with ARP replies when required.

A router Ethernet interface usually uses an RJ-45 jack that supports unshielded twisted-pair (UTP) cabling. When a router is connected to a switch, a straight-through cable is used. When two routers are connected directly through the Ethernet interfaces, or when a PC NIC is connected directly to a router Ethernet interface, a crossover cable is used.

Use the Packet Tracer Activity later in this section to test your cabling skills.

WAN Interfaces

WAN interfaces are used to connect routers to external networks, usually over a larger geographical distance. The Layer 2 encapsulation can be of different types, such as PPP, Frame Relay, and HDLC (High-Level Data Link Control). Similar to LAN interfaces, each WAN interface has its own IP address and subnet mask, which identifies it as a member of a specific network.

Note: MAC addresses are used on LAN interfaces, such as Ethernet, and are not used on WAN interfaces. However, WAN interfaces use their own Layer 2 addresses depending on the technology. Layer 2 WAN encapsulation types and addresses are covered in a later course.

Router Interfaces

The router in the figure has four interfaces. Each interface has a Layer 3 IP address and subnet mask that configures it for a different network. The Ethernet interfaces also have Layer 2 Ethernet MAC addresses.

The WAN interfaces are using different Layer 2 encapsulations. Serial 0/0/0 is using HDLC and Serial 0/0/1 is using PPP. Both of these serial point-to-point protocols use a broadcast address for the Layer 2 destination address when encapsulating the IP packet into a data link frame.

In the lab environment, you are restricted as to how many LAN and WAN interfaces you can use to configure hands-on labs. With Packet Tracer, however, you have the flexibility to create more complex network designs.
Related Topic Router

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