CCNA Review 04

You can see a plethora of different hardware in a typical network, but the most important two devices are the Switches and the Routers.

Switches were not like that from the start. In the past, everything was connected by network hubs. A hub is basically a repeater with multiple ports. Being a repeater, every device on the network actually shares the bandwidth available through it. Therefore, only one device can transmit at the same time. If more than one device transmits, then it is said that there is a collision.

To combat collisions, the CSMA/CD (Carrier Sense Multiple Access / Collision Detection) protocol is created which detects collisions whenever one happens. When two devices transmits at the same time, a collision is resulted and both devices would transmit a jam signal. The devices would back off for a random amount of time before transmitting again.

There are three types of RX/TX modes:
-Simplex is where the transmitter only transmits, and the receiver only receives. It is a one-way traffic. An example of simplex communication is radio broadcasting.
-Half-Duplex is when only one device can transmit across the wire at the same time. When one talks, all others sharing the medium must listen. An example of half-duplex communication is WiFi.
-Full-Duplex allows all devices to transmit at the same time. Modern switched networks and router interfaces are full-duplex.

The biggest problem with the hub is that it is a shared medium, and therefore it is half-duplex. If 5 devices are transmitting at the same time across a 50Mbps hub, then each of them have less than 10Mbps for transmission. In this case, an entire hub is a collision domain. The repeater and hub is a layer 1 device.

A new device came and allowed collision domains to be segmented. This is called the bridge. The bridge is an intelligent layer 2 device which allows learning of MAC addresses on each interface, much like a switch. Hubs can be connected to the bridges, allowing larger networks to be created. Each interface on the bridge is a collision domain if a hub is connected to it. If a computer is connected directly to a bridge, it has full duplex connectivity. However, the biggest problem with bridges is that they're software based, so they introduce really high latencies and limited bandwidth.

Switches were then created, which is much like a bridge, but moves frames around via ASICs. Every port on the switch is a full-duplex wire. If all ports are full-duplex, there can be no collision occuring on the switch. However, each switchport is still considered a collision domain by definition. Modern switches can support multiple speeds per interface and it is managed and intelligent. It is managed because you can modify settings, and it has a large feature-set and intelligence due to its IOS firmware.

Switches typically have one or two high-speed links for daisy chaining switches together. Switches need to be connected with crossover cables but modern switches have auto-sensing ports that allow connection through straight-through as well. Modern switches also have SFP (Small-Form Factor Pluggable Transceiver) modules which allow chaining together via Fibre Optics.

When switches are first booted up, its CAM (Content Addressable Memory) table is empty. CAM tables are used to store MAC address associations with interfaces. Once STP (Spanning Tree Protocol, covered in a later article) is completed and stable, the switches will start learning MAC addresses from its interfaces.

Suppose that two devices, A and B, connected to the switch wants to communicate. A would first create an ARP request to look for B. At this time, A's MAC will be recorded, while the ARP request is forwarded out of all ports. B would receive the ARP request and generate an ARP reply, which allows the switch to record the location of B's MAC. Now that the switch has both device's MACs, further communication from now would be switched at wire-speed.

By default, CAM entries has a timeout of 5 minutes. If a device stops communicating for 5 minutes, the entry is dropped. If A stops communicating for 5 minutes and his MAC gets dropped, further communication to A will be forwarded out of all open ports like a broadcast until his MAC is learned again.