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Segmentation

A network's congestion problems cannot always be corrected simply by increasing bandwidth. When troubleshooting congestion problems, one thing to look into is the source of all the traffic. Examining broadcasts traversing the network using a network sniffer can yield important insight into the problem of network congestion. A network sniffer is software that uses a promiscuous NIC card to collect data frames and packets from the network to view and diagnose what's happening in the network. Sometimes bad NICs can send out tons of broadcasts onto your network, without your knowledge. These faulty NICs are sometimes referred to as jabbering NICs. Servers also send broadcasts. The sniffer can uncover such unnecessary broadcasts. Once you have isolated the issue of network congestion, you can implement devices that are designed to alleviate unnecessary traffic, increase the bandwidth, and correct network bottlenecks.

One way of solving congestion problems and increasing performance on a LAN is to divide a single Ethernet segment into multiple network segments—a process called segmentation. Network segmentation reduces the size of the collision domain. This helps maximize available bandwidth by reducing the number of users in the collision domain or broadcast domain. Fortunately, you have at your disposal a few tried-and-true devices to segment a LAN and help relieve Ethernet congestion: bridges, routers, and LAN switches. The following sections discuss these devices in detail.

Segmenting a Network with Bridges

A bridge can connect several LANs together and provide more bandwidth to the user. Bridges also reduce collisions by making smaller collision domains. Bridges operate on the Data Link layer (or Layer 2), and they dynamically build a forwarding table, which is a table of information that contains the learned MAC addresses and their corresponding port information based on the source address the device learned from incoming data frames. A bridge uses the destination address in a frame to make a forwarding decision. One disadvantage of using a bridge is its inability to filter broadcasts; as a result, it sends the broadcast on all the segments. Occasionally, broadcasts use up all or a majority of the available bandwidth, a situation referred to as a broadcast storm.

Segmentation Using LAN Switches

Switches are Layer 2 devices, similar to bridges in that they learn the topology dynamically and forward, as well as filter, data traffic. Switches read the destination Layer 2 MAC address and often begin to forward the frames before the entire packet is received. This ability results in lower latency. Switches also give you a higher port density than bridges do, and they cost less than a bridge. In some ways, a switch is really a multiport bridge, because the switch can filter to the port via MAC addresses and each port on the switch is in its own collision domain.

Using a LAN switch offers some advantages over using a bridge, including full-duplex communication, media rate adaptation, and easy migration. Also, implementing LAN switches usually doesn't require many changes to the network. You can use the existing wiring, and simply replace hubs with switches or slowly migrate by replacing one hub at a time.

Segmenting with a Router

Routers can connect networks that use different media types. Switches and bridges filter and route data at the Data Link layer by MAC addresses. Routers operate at the Network layer and filter by logical addresses. A logical address is a Layer 3 address assigned by a protocol such as IP or IPX addresses.

Routers keep a table that records where devices are located on the network. The difference between a router and a bridge, however, is that a bridge keeps track of the hosts or addresses on the network, whereas the router keeps track of networks, not hosts. Using the routing tables, the router can make an informed routing decision on where to send incoming data.

NOTE

By default, routers won't forward broadcasts to the rest of the network, which alleviates unnecessary traffic on the network caused by broadcasts.

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