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Smart Data Routing

Here is a brief introduction to routers the preferred devices in large networks operating on different technologies

A router is a device that operates at the network layer, or layer 3 of the OSI model. When compared to hubs and switches, routers are much smarter. Routers use a more complete packet "address" to determine which router or workstation should receive the next packet. Routers can use either MAC addresses or administratively assigned logical addresses (such as IP addresses) to handle data routing.

Based on a network road map called a "routing table," routers can help ensure that packets travel the most efficient path to their destinations. If a link between two routers fails, the sending router can determine an alternate route to keep traffic moving.

Routers are designed to link many network technologies, and are frequently used in large campus and enterprise networks. At the network layer of operation, you find a wider range of mechanisms that are designed to deal with the issues that can arise when building large network systems. By operating at the network layer, routers can easily deal with computers attached to everything from slow-serial links to high-speed LAN systems.

In sufficiently large and complex networks, and especially networks with multiple network technologies, you may wish to use a router for the advantages that it can bring. Many vendors provide routers with multi-protocol capabilities, making it possible to deal with a variety of high-level protocols in a single device. Examples of protocols include Internet Protocol (IP), Internet Packet Exchange (IPX), and AppleTalk. While routers are more complex to configure, the advantages tend to offset the added complexity of their operation for many network mangers.

In operation, a router unpacks all Ethernet frames sent to it, and deals with the high-level protocol data carried in the frame. When a router hears an Ethernet broadcast, it does the same thing as all other stations on the channel must do: it reads the frame in and tries to figure out what to do with it. If the broadcast happens to come from a client station attempting to discover a service on a server station on that channel, for example, the router will do what all other machines on the channel other than the server will do: discard the broadcast since it has nothing to do with them.

Therefore, unlike a switching hub, a router does not automatically forward broadcast and multicast frames. Because of this, routers limit the flow of broadcasts and multicasts to the local LAN, thereby creating separate broadcast domains and protecting a large network system from the high multicast and broadcast traffic rates that can occur when you link a large number of stations together with a layer 2 switch. On large networks with many stations, this is a decided advantage both for the reduced traffic levels and for the reduction in problems that can be caused by broadcasts.

Routers vs. Switching Hubs: Which to Use?
Here we briefly describe some of the differences in operation between routers and switching hubs, and the different impact of routers and switching hubs on the operation of your network. Although they are both store and forward switches, and can both be used to extend Ethernet by building larger network systems, routers and layer 2 switching hubs operate in very different ways. It's up to you to decide which device is best suited to your needs, and which set of capabilities is most important for your network design.

Bridging routers, or brouters, offer the best of both worlds. Remember that some protocols are non-routeable, such as NetBEUI. What if your network consisted of NetBEUI traffic and TCP/IP traffic? In this scenario you could use a brouter to route TCP/IP, and bridge the NetBEUI traffic.

Some advantages of routers are:

  • Routers block the flow of broadcasts. This, combined with their ability to structure the flow of traffic through a system based on layer 3 network protocol address allows you to design more complex network topologies while still retaining stability of network operation as your network system grows and evolves.
  • Routers provide the ability to link multiple IP networks without fragmentation problems.
  • Routers use routing protocols that can provide information about paths such as the bandwidth of the path. Using that information routers can provide best-path routing, and can use multiple paths to provide load sharing.

Some disadvantages of routers are:

  • Routers are more complex to configure, and require routing software for each high-level protocol suite that you need to route.
  • Routers cannot provide support for non-routable protocols that were designed to work on a single LAN and that do not provide the information required for routing (e.g., NetBIOS).

These advantages and disadvantages need to be considered when deciding on the appropriate technology for extending your Ethernet system. As always, the network world is in rapid evolution, and things are changing all the time. For example, as fast switching circuits continue to drop in price, vendors are making routers available at lower costs.

The world of switching hubs is rapidly evolving, and switches are being given more router-like capabilities. Switches that can base their forwarding decisions on network layer protocol addresses are sometimes called layer 3 switching hubs. In addition, "multilayer switches" are being developed which combine layer 2 switching and layer 3 routing capabilities in the same box. You need to carefully evaluate these approaches to extending networks and decide how well they will fit into your network system, given the requirements at your site.

Routers strip off the outer layers of Ethernet or Token- Ring data before they send a packet from one LAN to the other, so they reduce the total number of bits going across the inter-LAN communications link. The remote router at the receiving end repackages the data into a packet or frame appropriate for its LAN segment.

Mahesh Rathod can be reached at rathodmp@hotmail.com

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