Inside Routing

Routers are Computers

A router is a computer, just like any other computer including a PC. The very first router, used for the Advanced Research Projects Agency Network (ARPANET), was the Interface Message Processor (IMP). The IMP was a Honeywell 316 minicomputer; this computer brought the ARPANET to life on August 30, 1969.

Note: The ARPANET was developed by Advanced Research Projects Agency (ARPA) of the United States Department of Defense. The ARPANET was the world's first operational packet switching network and the predecessor of today's Internet.

Routers have many of the same hardware and software components that are found in other computers including:

• CPU
• RAM
• ROM
• Operating System

Typical users may be unaware of the presence of numerous routers in their own network or in the Internet. Users expect to be able to access web pages, send e-mails, and download music - whether the server they are accessing is on their own network or on another network half-way around the world. However, networking professionals know it is the router that is responsible for forwarding packets from network-to-network, from the original source to the final destination.

A router connects multiple networks. This means that it has multiple interfaces that each belong to a different IP network. When a router receives an IP packet on one interface, it determines which interface to use to forward the packet onto its destination. The interface that the router uses to forward the packet may be the network of the final destination of the packet (the network with the destination IP address of this packet), or it may be a network connected to another router that is used to reach the destination network.

Each network that a router connects to typically requires a separate interface. These interfaces are used to connect a combination of both Local Area Networks (LANs) and Wide Area Networks (WANs). LANs are commonly Ethernet networks that contain devices such as PCs, printers, and servers. WANs are used to connect networks over a large geographical area. For example, a WAN connection is commonly used to connect a LAN to the Internet Service Provider (ISP) network.

In the figure, we see that routers R1 and R2 are responsible for receiving the packet on one network and forwarding the packet out another network toward the destination network.
The primary responsibility of a router is to direct packets destined for local and remote networks by:
Determining the best path to send packets
Forwarding packets toward their destination

The router uses its routing table to determine the best path to forward the packet. When the router receives a packet, it examines its destination IP address and searches for the best match with a network address in the router's routing table. The routing table also includes the interface to be used to forward the packet. Once a match is found, the router encapsulates the IP packet into the data link frame of the outgoing or exit interface, and the packet is then forwarded toward its destination.

It is very likely that a router will receive a packet that is encapsulated in one type of data link frame, such as an Ethernet frame and when forwarding the packet, the router will encapsulate it in a different type of data link frame, such as Point-to-Point Protocol (PPP). The data link encapsulation depends on the type of interface on the router and the type of medium it connects to. The different data link technologies that a router connects to can include LAN technologies, such as Ethernet, and WAN serial connections, such as T1 connection using PPP, Frame Relay, and Asynchronous Transfer Mode (ATM).

In the figure, we can follow a packet from the source PC to the destination PC. Notice that it is the responsibility of the router to find the destination network in its routing table and forward the packet on toward its destination. In this example, router R1 receives the packet encapsulated in an Ethernet frame. After decapsulating the packet, R1 uses the destination IP address of the packet to search its routing table for a matching network address. After a destination network address is found in the routing table, R1 encapsulates the packet inside a PPP frame and forwards the packet to R2. A similar process is performed by R2.

Static routes and dynamic routing protocols are used by routers to learn about remote networks and build their routing tables. These routes and protocols are the primary focus of the course and will be discussed in detail in later chapters along with the process that routers use in searching their routing tables and forwarding the packets.