Wednesday, March 26, 2008

From Bridges to Routers

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

  1. A list of the network protocols for which you'll be using the router. For example, TCP/IP or IPX/SPX.
  2. The routing protocol that you'll use for each network protocol. For example, RIP.
  3. Whether or not you'll need to set filters to block certain addresses or IP or UDP ports—a tech• nique used to create a simple firewall.
  4. Information about the address space used on each segment the router will connect.

Network Protocol Issues

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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.

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.

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.

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