Swithed Ethernet

Tuesday, May 22, 2012

As Ethernet networks continued to grow in size, administrators noticed that their networks significantly slowed down. In an Ethernet 10BaseT network, hubs are used as the central distribution points in the network. The hub, also known as a multiport repeater, simply sends all packets out on all ports. This may work well on a small network without with little requirements for speed and throughput. However, as the number of network nodes increase with multiple conversations taking place at the same time, your network speed is significantly decreased. The first issue with these older network hubs is that your total network bandwidth is limited to 10 Mbps for all ports. Since only one Ethernet node can communicate at one given time on the network, the more nodes you have, the less access time each node has on the network. If you have ten nodes on a hub, you basically provide each node with only 1 Mbps of bandwidth. If you have 100 nodes on your local area network, well, you do not have to do the math to figure out that your network is going to be really slow.

Network Switches

While an Ethernet switch may physically look like a hub, it operates with additional intelligence. Hubs operate at the Physical (layer 1) layer of the OSI model, while Switches operate at the Data Link (layer 2) layer. Switches maintain MAC tables to figure out what ports the network computers are connected on. The switch does this by inspecting the network traffic (at layer 2) that passes through its port and records the MAC address information in its MAC tables. At first, when you power on a switch, it acts exactly as if it were a hub. The reason is because it has no information yet about any MAC addresses. So, the switch will pass all incoming frames right back out to all of the other ports. However, it does not take too long before the switch is aware of the nodes that are connected to it. As the initial frames are passed back and forth they are recorded in the MAC tables. Once the switch knows where the nodes are, it can create virtual connections between the source and target system. This allows other nodes on the same switch to communicate at the same time. This is a huge break-through over standard Ethernet.

network switch

Since the creates a point-to-point connection between any two nodes, it eliminates the need for CSMA/CD. The concept of collisions and collision domains are no longer applicable because collisions do not occur in a switched Ethernet network. However, all the nodes on the same segment are still part of the same broadcast domain because the layer 2 destination address for a broadcast frame is FF-FF-FF-FF-FF-FF. Therefore when a switch receives a broadcast packet on any port, it will send the packet out on all other ports.

Spanning Tree Protocol

With the introduction of hubs and switches we also introduced the concept of a loop. A loop is where you have multiple paths between at least two switches. This creates a huge problem for the network.

network loop

A loop creates an unwanted path sending packets in an endless loop and preventing the network from working properly. To prevent this, the Ethernet standards committee adopted the Spanning Tree Protocol (STP). STP adds protection to the network by allowing switches that detect loops to automatically shut down one of the ports causing the loop before the network is brought to a screeching halt.

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Recommended Books & Training Resources

CCNA Cisco Certified Network Associate Deluxe Study Guide CompTIA Network+ Certification Kit: Exam: N10-004