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Exploring Interfaces

Routers support two types of physical interfaces that are used for exchanging packets with neighboring devices. Depending on the type and purpose of the network they are connecting to one or another will be a better fit:

  • Ethernet interfaces: These are the most common type of interface on routers and are used to connect to wired networks using Ethernet cables. Ethernet interfaces typically have a specific speed, which defines the ethernet type, and may support Power over Ethernet (PoE) to power connected devices. Ethernet ports supports speed up to 10 Mbps, Fast Ethernet ports up to 100 Mbps, while Gigabit Ethernet ports up to 1000 Mbps. Some Cisco devices also support interfaces capable of operating up to 10, 25, 40, and even 100 Gbps of speed.
  • Serial interfaces: These interfaces are used to connect routers to other devices using serial communication protocols. Serial interfaces are typically used for connecting to legacy equipment or industrial control systems. When using serial interfaces, you need to define which data link layer protocol (encapsulation) you want to use, such as High-Level Data Link Control (HDLC), Point-to-point (PPP) or Frame Relay.

To configured certain parameters, you need to enter the interface configuration mode by using the “interface type interface-identifier” command. Routers use interface identifiers to distinguish between interfaces of the same type. Depending on the model of the router the interface-identifier may be:

  • An interface number, such as “interface FastEthernet 1”
  • A slot/interface number, such as “interface FastEthernet 0/1”
  • A module/slot/interface number, such as “interface FastEthernet 1/0/1”

Loopback Interface

A loopback interface is a virtual (logical) interface that can be configured on a router. The loopback interfaces are primarily used to testing connectivity and performing diagnostics in the networks.

Just like any other physical interface, the loopback interface can be assigned an IP address, but it does not connect to any physical network devices. Instead, it is always virtually available and provides a stable IP address for management services running on the router, such as Telnet, SSH, and SNMP.

The loopback interfaces are very important for performing verification when using routing protocols. Also, by using a loopback interface, a router can simulate a network connection without needing an actual physical connection.