Describe the purpose and functions of various network devices

Exam: 200-120 - Interconnecting Cisco Networking Devices: Accelerated (CCNAX)


As your network grows, it needs to be broken into smaller networks or network segments to ensure better performance of the network. Too many network hosts can cause network traffic congestion and when too many packets are transmitted on a network, the performance of the network degrades to an extent that no packet is delivered. To avoid network traffic congestion, a large network is usually broken into small network segments. The breaking of a network into smaller network segments is called subnetting.

Network Devices

The subnetting of a network is possible only through network devices such as switches, routers and bridges. Some other network devices such as Hubs, and Access Points are also commonly used in a network. Let us understand each of these network devices in detail:

Hub: A hub a very basic network device that connects all the computers together in a network. It sends all the network data to all the computers connected to it, without using any intelligence of its own. Each port on the hub is on the same network segment. Thus a hub has one segment and one collision domain. The job of a hub is simply sending out anything that comes to it to other computers connected to it.

Every computer connected to hub can see every other computer on the hub. With the increase in the number of ports in a hub network, the possibility of collision also increases. A hub operates on the Network Layer 1 of the OSI model and can only detect basic network errors such as network collisions.

The only benefit of a hub in a network is to connect various network devices together, which may range anything from connecting a computer to a computer, computers to network devices, and a computer network to another computer network. It allows the network resources to be shared among computers.

Bridge: The bridges are just one step above the hubs. They also send all the data to all the computers connected to it but unlike hubs, they have just one source and one destination to deliver data packets. A bridge has two interface devices. It receives data from one interface and delivers it to the other interface, it is connected to. Thus a bridge creates two collision domains. A bridge is also called an intelligent hub because it considers the destination of data before delivering it.

The working of a bridge can be compared to an email, which can only be delivered to the destination email address and nowhere else. If the email address says tom@abc.com then the system would search tom@abc.com n the other side. If the same email address exits then the mail would be delivered else the delivery of the email would fail.

Bridges are usually required to connect parts of a network that do not communicate very often but need to remain connected.

Switch: Switches do same what a hubs do but they do it intelligently. They learn the MAC address of the requester and the port or the location of the device which responded to the request almost instantly. The first time, a request received by a switch is sent to all the computers connected to it. However, as soon as the request is responded by a computer, the switch learns the network location of the port that responded to the request and the Mac address of the source computer to handle the similar subsequent requests.

For example, consider computers called CompA, CompB, and CompC are connected to a switch. When a message destined for CompA is received by the switch, it broadcasts the message to all the computers. Now as soon as CompA responds to the message, the switch learns the path and sends all messages destined for CompA to CompA only.

It does this by remembering the MAC address of the originator of the request and then it sends the response from CompA to the same MAC address (originator of the request) only. Thus all subsequent messages received from the same source for the same destination are delivered to the same source and the destination computers only.

Switches operate on the Network Layer 2 of the OSI model and create many collision domains on the same segment. The switches make each port its own collision domain.

Router: Routers are the most intelligent among all the network devices. They can be programmed to use the most efficient route to transmit the data to the desired computers. They operate on the Network Layer 3 of OSI model and can route data packets from one network to another based on their IP address. Also they don’t forward broadcasts by default.

Routers make each of its port into a separate segment and a separate collision domain. Thus routers have many segments and many collision domains. The routers can separate broadcast domains unlike switches that create separate collision domains but same broadcast domains.

By default, routers break the broadcast domains and keep all devices separate on a segment from other segments. This means if messages are sent on a segment, the devices on that segment will be able to get that broadcast message and not all other segments of the same network. The functions of a router are: Path selection, Packet filtering, Packet switching, and Internetworking. The routers perform packet switching and internetworking using logical addressing and packet filtering using access lists. Routers use routing tables or map of the internetwork to make path selection and send data packets to remote networks.

Access Points: Access points are wireless network devices that are used to provide wireless access to a wired network. The access points are similar to hubs because the RF frequency they use on the wireless side is a shared media. It then further shares the frequency with all the computers accessing it just like a hub. The access points have one segment and one collision domain.

Example Question

Which of the following network device would you use to create separate LANs or internetworks?

  • Switch
  • Router
  • Hub
  • Bridge

Correct Answer: B

Explanation

Switch: Switches do not allow you to forward packets to other networks. They can only switch frames from one port to another on a same network segment.

Router: The routers are used to create internetworks. They use routing tables or map of the internetworks to make path selection and send data packets to other networks.

Hubs: Hubs operate on the same network segment they cannot divide a network.

Bridge: A Bridge has just 2 interface devices. It can connect only two networks that do not communicate very often.

The switches and bridges can create LANs and separate collision domains but they cannot be used to create separate LANs.


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