Improving Network Speed and Reliability

Although improving a server's fault tolerance is critical to the reliability of a network, you should not overlook the design of the network itself or the hardware you use to build the network.

If your network carries different types of traffic to two or more servers, you should divide the network into separate segments to better manage traffic flow. However, a segmented network can also be used to provide fault tolerance if you make sure that one segment can take over for another segment in case of failure.

The following sections discuss segmentation and the selection of switches and routers needed to create a faster and more fault-tolerant network.

Segmentation

Segmentation alleviates network bottlenecks by splitting network traffic loads to different network segments. You can use segmentation to lower the total network traffic that is passed on a network by splitting up the traffic as shown in Figure 13.3. If one set of systems need to communicate with each other and not the rest of the network (for example, different departments), you can segment them into different networks. You can also use network segmentation if you have multiple locations that all need to access the same server but no other resources on the other segment (for example, if multiple locations all share the same email or file server).

Figure 13.3. Using network segmentation.

Segmentation can be used to improve a network's fault tolerance. By configuring a server's NICs to connect to different network segments (dual-homing) and using a router or Layer 3 switch to connect the segments, you enable the router to maintain a connection to all network segments, even if a direct connection to one segment fails.

Switches and Routers

You use switches and routers to create the network topology you prefer. As you have learned, the network topology can also help create a network with enhanced levels of fault tolerance. To achieve the desired level of fault tolerance at the network level, you need to understand how switches and routers can be used in a fault-tolerant installation.

Switches

If your network uses hubs to connect network nodes, there are several good reasons to upgrade to switches. Compared to hubs, switches don't generate unnecessary network traffic. Instead of broadcasting data frames to all ports, a switch transmits the packets directly to the destination port. Hubs subdivide total network bandwidth among connected devices (for example, a 10-port 100Mbps Fast Ethernet hub provides only 10Mbps bandwidth to each port), whereas switches provide full bandwidth to each connected device. Also, most recent switches, even on small networks, support full-duplex operation. Thus, a nominally 100Mbps Fast Ethernet connection becomes a 200Mbps Fast Ethernet connection if all devices in the connection (network adapters and switches) support full-duplex operation. Many switches now support Gigabit Ethernet (1000Mbps) connections for servers as well as 10/100Mbps connections for client PCs and other devices.

Intelligent network switches can also be configured to improve network reliability, using a protocol called spanning tree that provides fault tolerance for the switch. A spanning tree reroutes traffic through the secondary switch if it detects a failed network connection or a bad NIC. In this configuration, you use two switches connected to two separate NICs in your server to provide fault tolerance and redundancy for your server's NICs and your network traffic.

Routers

Routers are specialized pieces of hardware that connect at least two separate networks, sending all of the network traffic on the Internet or any other WAN connection to its correct destination, choosing among thousands of available pathways. Routers use routing tables to determine the best pathway to the destination and then send the information to the next hop in the very long data train. The next hop is usually another router that looks up the data's destination point in its routing table. The router then determines the best path to the destination and sends the packet on to the next hop. This continues to happen from router to router until the final router sends the data to the correct server or network device. The data may traverse thousands of geographical miles, but the process takes but a few seconds to complete.

Because routers are a vital component in connecting networks in different locations to each other (and to the Internet, the world's largest network), keeping routers working is essential if your network depends on those connections.

You can use routers to help provide a fault-tolerant connection in several ways. One method is to set up separate routers to connect your network to the Internet via different network segments. By using dual-homed network adapters in your server, you can maintain a connection to the other network or the Internet, even if one connection fails. The major expense in this type of redundancy is the cost of an additional leased line or ISP service.

If you have a reliable connection but are concerned about router failure, you can use routers from Cisco, Nortel, or others that support VRRP; you can also use Cisco routers that use Hot-Swap Router Protocol (HSRP) or similar methods for setting up multiple routers to appear as a single router to the network.

Tip

Although you can buy routers from SOHO vendors such as Linksys, D-Link, and others, most vendors that sell into the SOHO space do not provide routers with support for VRRP, HSRP, or similar standards.