Upgrading and Repairing Networks (5th Edition)

Routers are inherently slower than bridges when it comes to forwarding network packets. This is because a router must read further into each network frame to get Network layer addressing information, whereas a bridge merely looks at a fixed location for the MAC address. Hubs, bridges, and switches can be set up in a short amount of time and usually require little or no configuration.

Routers require that the network administrator configure networking information for each port that's used. The command set available to configure a router is quite large because it's a very flexible device and can be confusing for a novice. The kinds of information you need to configure a new router are

  • A list of the network protocols for which you'll be using the router. For example, TCP/IP or IPX/SPX.

  • The routing protocol that you'll use for each network protocol. For example, RIP.

  • Whether or not you'll need to set filters to block certain addresses or IP or UDP ports ”a technique used to create a simple firewall.

  • Information about the address space used on each segment the router will connect.

For additional information about routing protocols, refer to Chapter 37, "Routing Protocols."

Network Protocol Issues

In many networks, more than one network protocol is used on the same medium. To do their job, routers need configuration information about each protocol for each port. For example, because each port on the router connects to a different network segment, each port must have a unique network address that it can use to communicate on the segment. If you plan to restrict some segments for security or other reasons, you'll need to create a set of access control lists (ACLs) for each port, which indicate which frames are allowed through, in both directions.

Using routers to restrict network traffic is often referred to as packet filtering . For more information about this technology, refer to Chapter 49.

When using a router to connect to a larger WAN, you'll probably be faced with having to configure a port on the router that uses a WAN protocol, such as Frame Relay, in addition to protocols you're already familiar with on your network. With a WAN connection, you'll have to coordinate your activities with other system administrators to ensure that the router is configured with the correct information for the larger network.

Network Addressing Issues

Because the router makes decisions based on a higher-level networking protocol, such as IP, you'll have to take into consideration your current address space when you decide to introduce a router into the network. If you're adding new segments to the LAN and have the freedom to choose a new network address, this can be an easy task. If you're going to take an existing LAN and use a router to separate it into more manageable segments, you have two possible choices. You can use your original network address for one segment and create new networks on the remaining segments or you can use subnetting.

Note

To connect to the Internet, you'll have to obtain an IP address that is valid on the Internet. Due to the rapid growth of the Internet, the addresses supported by IP version 4 (IPv4) are slowly becoming exhausted. When IPv6 becomes widely implemented, the fields that store IP addresses will increase from 32 bits to 128 bits. This single factor will increase the available addresses from 2 32 (about 4.29 billion) to 2 128 (about 340 undecillion ”and for reference, 1 undecillion is a 1 followed by 36 zeros!) so that just about every grain of sand on the Earth can have its own IP address. Until then, it's becoming more common to use a set of reserved IP addresses that are valid only on your local network, and to use fewer IP addresses to access the Internet. The technique called network address translation (NAT) makes this possible. This method also helps to keep outsiders from gaining knowledge about the addresses of your clients , and that helps increase security at your site. To understand how NAT works and the addresses you can use on the local network, refer to Chapter 49.

Regardless, you'll have to then reconfigure each client with new addressing information. If you're using DHCP, the process is made simpler because you can make the changes at a central location and have clients request the new information after the changes have been made. DHCP is the most prevalent method used today to configure workstations and other non-server devices on a network.

If you're going to use a router to connect your LAN to a larger corporate network, you might not have to make any addressing changes on your network, depending on the company's overall network plan. You'll still have to configure the ports, however. If you're going to connect the LAN to the Internet, using a router configured as a firewall might be something to consider.

Other Router Management Issues

Routers are very much like smart PCs that have been customized to perform the routing function efficiently . They have CPUs, memory, and I/O ports just like an ordinary PC. They also have an operating system, which is subject to periodic updates by the manufacturer. So, in addition to learning how to configure the router, you'll also need to become familiar with the commands used for such functions as saving a copy of the system image to a server for backup purposes and performing troubleshooting and testing.

Managing a network that uses routers can seem a difficult task at first. However, by enabling you to organize your network according to the hierarchical network address spaces used by upper-level network protocols, the initial configuration problems will be worth the effort.

Tip

The best way to learn how to configure your router is to read the documentation. The most popular routers today for LANs are made by Cisco Systems. Its documentation site, which you can reach online at http://www.cisco.com/univercd/home/home.htm, provides documentation for almost all of Cisco's products. On this page, select Cisco IOS Software and you'll find a wealth of information about router setup and configuration. Because of the in-depth content, you can use this site to learn not just about configuring Cisco routers, but also the concepts behind protocols and other information that can be helpful no matter what kind of router is used on your network. For students, this is an excellent source of information.

Using a Router to Segment the Network

Like bridges, routers can be used to isolate traffic between network segments. Unlike bridges, routers further reduce network bandwidth use because they do not pass broadcast messages from one segment to another unless programmed to do so. A router also does not have to take time to learn which nodes are connected to each segment. The information it needs is configured in advance ”the administrator assigns protocols and addresses to each port. Routing protocols also use various methods to update each other about network topology as it changes.

One very important reason why routers are used to help organize a network into segments is that routers enable you to connect many network segments. Whereas bridges are limited to a few thousand nodes, depending on the topology used, routers can enable the LAN to be connected to an infinitely larger WAN, such as the Internet.

The internal processing that routers must perform make them slower than bridges (although that might not be the case with most high-end routers being manufactured today), which need to examine only a small amount of data in the packet header. Although this performance difference will not be noticed on network segments with only moderate traffic use, you might find that you need to place routers at only strategic locations throughout the network, retaining switches for connecting other computers or network segments. Remember that you can connect individual computers to a switch port or use the port to connect to other switches.

The method you use will depend on the usage patterns that can be monitored for each segment and the cost of the links used to connect different segments. Another thing to consider is that many of the high-end routers available today operate at what is called "wire speed." That means they can route packets at virtually the same speed as the network medium, with just the very slightest delay for processing time.

Connecting to a Larger WAN or the Internet

When connecting the LAN to a WAN, a router is required. When connecting to the Internet, for example, you cannot use a bridge or a repeater. The Internet is composed of a hierarchical IP address space and a router is needed to participate in this hierarchy. Or you might plan to use a dedicated line of some sort to connect to a larger corporate network. In that case, placing a router between your LAN and the WAN hardware, such as an ATM switch connection, will help reduce the traffic that crosses the expensive dedicated connection by keeping local traffic confined to the local network segments.

Note

There are two situations in which a router is not needed to make an Internet connection. The first is if you have a modem-based dial-up connection. Although it's possible to set up routing tables in operating systems such as Windows and Unix/Linux, this isn't really a practical method for connecting a small office LAN to the Internet due to the very limited speed.

The other situation is when you use a broadband connection, such as a cable or DSL modem. In this case, you can connect the high-bandwidth modem to a single computer and then set up routing tables so that other computers can send and receive traffic through the computer, which operates as a router. However, there's a better idea in a Small Office/Home Office (SOHO) environment or a home environment where everyone from the parents to the kids have their own computers: Purchase an inexpensive 4 “6 port router (for less than $100 in most cases) that you can connect to the cable/DSL modem. These types of routers require very little knowledge about computers and can usually be set up in less than a half- hour . For more information about configuring a SOHO environment, see Chapter 17, "Digital Subscriber Lines (DSL) Technology," Chapter 18, "Using a Cable Modem," and Chapter 54, "Troubleshooting Small Office and Home Office (SOHO) Networks."

Although you'll certainly have to configure the ports that connect the local LAN and the WAN interface, you might have to reconfigure addressing information on clients. For example, if you're already using a valid TCP/IP network address, possibly a subnet of the corporate network address space, you'll need to configure only routers.

If your business has just been acquired by a larger concern, however, you might find that your LAN has been assigned a new subnet by the larger corporation. In such a case, you'll probably have to plan on downtime for end users in order to make changes to important servers, such as Domain Name System (DNS) servers. DHCP servers (which workstations can find themselves ) are used to translate between user -friendly names (such as http://www.twoinc.com ) and IP addresses. Although DHCP can dynamically assign configuration information to workstations, important servers, such as DNS servers or gateways to other networks (usually routers), must have a static (unchanging) address. This is because part of the configuration information that DHCP supplies to clients is those addresses! If the address of a DNS server changed with every reboot of the server, you would have to reconfigure the information on each workstation client ”a tedious effort even in a small network!

By using DHCP, you can overcome client configuration headaches such as this. Just reconfigure the DHCP server with the address range for the new subnet, add in the DNS servers and default gateway, and reboot your client computers. This is a simple explanation of the information supplied by DHCP servers. Indeed, you can use DHCP to provide configuration information for many other network parameters.

For more information about using DHCP and how it works, refer to Chapter 29, "BOOTP and Dynamic Host Configuration Protocol (DHCP)."

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