Exploring the Functions of Routing

Routing is the process that forwards data packets between networks or subnetworks using a Layer 3 device: a router or gateway. The routing process uses network routing tables, protocols, and algorithms to determine the most efficient path for forwarding the IP packet. Routers greatly expand the scalability of networks by terminating Layer 2 collisions and broadcast domains. Understanding how routers function will help you understand the broader topic of how networks are connected and how data is transmitted over networks. The following sections describe the operation of routers.

Routers

A router or gateway is a network device that determines the optimal path for transmitting data from one network to another. A router is a specialized computing device running programs and algorithms that aid in the optimal delivery of network traffic. Certain characteristics are common to all routers. Figure 4-1 shows a picture of various Cisco 2800 series routers.

Figure 4-1 Cisco 28000 Series Routers

Figure 4-1 Cisco 28000 Series Routers

Routers are essential components of large networks that use TCP/IP, because routers provide the ability to accommodate growth across wide geographical areas. The following characteristics are common to all routers:

■ Routers have these components, which are also found in computers and switches: — CPU

— Motherboard

■ Routers have network adapters to which IP addresses are assigned.

■ Routers can have these types of ports:

— Console port: The router uses a console port for the attachment of a terminal used for management, configuration, and control. A console port might not be found on all routers.

— Network port: The router has a number of network ports, including different LAN or WAN media ports.

Routers have the following two key functions:

■ Path determination: Routers must maintain their routing tables and ensure that other routers know about changes in the network. Routers do this by using a routing protocol to communicate the network information to other routers from a routing table on the router. It is possible to statically populate the routing tables, but statically populating does not scale and leads to problems when the network topology changes, either by design or as a result of outages.

■ Packet forwarding: Routers use the routing table to determine where to forward packets. Routers forward packets through a network interface toward the destination network identified by the destination IP address in the packet.

Path Determination

During the path determination part of transmitting data over a network, routers evaluate the available paths to remote destinations. This section describes how routers determine the most efficient path for forwarding packets.

There can be many paths to get from one network to another. These paths, also known as routes, can have many different characteristics, such as speed, latency, and media type. The purpose of a router is to communicate with other routers to learn and select the best path, as shown in Figure 4-2.

Figure 4-2 Routers Perform Path Selection

Figure 4-2 Routers Perform Path Selection

These routes are held in routing tables within the software of the router and are then used to determine where to send a packet based on the destination addressing. The following three types of entries in routing tables can be used to select the best path to a remote destination:

■ Static routing: This type of routing requires that you manually enter route information into a routing table.

■ Dynamic routing: This type of routing builds a routing table dynamically, using routing information that is obtained from routing protocols.

■ Default routing: This type of routing replaces the need to hold an explicit route to every network. The default route entry can be either statically configured or learned from a dynamic routing protocol.

The routing table holds only one entry per network. If there is more than one source of information regarding paths to a particular destination network, the routing process needs to be able to select which source of information should be used in the routing table. Multiple sources come from having multiple dynamic routing protocols running, and from static and default information being available. The routing protocols use different metrics to measure the distance and desirability of a path to a destination network. Because it is not possible to directly select from information provided by different routing protocols, the Cisco routing process assigns a weight, known as the administrative distance, to each source of information. The best, most trusted source has the lowest number.

Routing Tables

As part of the path determination process, the routing process builds a routing table that identifies known networks and knows how to reach them. Figure 4-3 shows how a routing table maintains network locations.

Figure 4-3 Routing Tables a

Network

Continue reading here: Constructing a Network Addressing Scheme

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