Identify basic switching concepts and the operation of Cisco switches

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

Switches are the devices that connect different types of packet switched Ethernet segments and create a simultaneous and parallel connection between the segments. Switches are Layer 2 devices and perform on the hardware level. They use Application Specific Integrated Circuits (ASIC) to build and manage filter tables that help them transmit the data packets to their source location. Switches can also be considered as multiport bridges because just like multiport bridges, they also break collision domains, forward layer 2 broadcasts, and make forwarding decisions on the basis of layer 2 addresses. Also, both multiport bridges and switches use MAC address by examining the fames received by them.

Switches are however more efficient than bridges because switches have a higher number of ports as compared to bridges. They have high wire speed, low latency and are less costly. In addition, the switches make no modifications to data packets because they are hardware based.

Switches perform much faster than routers because they do not look into logical address to transmit data. They only read the data packets that appear on their interface and then encapsulate them before forwarding them. The switching is less prone to errors than routing process. Layer 2 switching can be used to create workgroups as well as to create network segments that are created to break the collision domain. Switches cannot be used to break broadcast domains.

Some other benefits of individual Layer 2 switches are:

Some other benefits of layer 2 switches are:

• Dedicated bandwidth: The individual users get the benefit of dedicated bandwidth by using layer 2 switches. The switches provide dedicated bandwidth to individual users by using the microsegmenting technique to allocated bandwidth to each switch port.
• Creation of VLANs: Switches allow you to create virtual LANs by allowing you to group individual ports into logical groups. The VLANs restrict the broadcast domain to the VLAN member ports. VLANs are also known as switched domains and autonomous switching domains.
• Use of ApaRT: Switches allow the use of ApaRT (Automatic Packet Recognition and Translation), which is a Cisco technology to convert different Ethernet protocol formats into industry-standard CDDI/FDDI formats and allow easy migration of 10Base-T LANs to 100-Mbps server access.

Limitations of Layer 2 switches

Layer 2 switches break collision domains but do not break broadcast domains. This may reduce the network performance if the network is big. Also this can become a serious concern when you need to design a hierarchical scalable network that has a good possibility of increasing or growing further.

Functions of Layer 2 switches

Layer two switches perform three important functions. These functions are:

• Address learning:The layer two switches remember the source address of the frames it receives on its interface. The switches create and manage a MAC database called Forward/filter table to store this information and use it to forward the frames. The command show mac address-table displays the forward/filter table used on the LAN switch.

  The address learning is performed in following steps:

  1. The MACforward/filtering table is empty when theswitch is turned on.
  2. Switch receives frames.
  3. Switch puts the frame's source address in MACforward/filtering table to remember the source of the frame.
  4. Switch floods the network with the frame on all the network ports except the source port to find out the destination port.
  5. Switch receives an answer from a destination device.
  6. Switch stores the MAC address of the device in the MACforward/filtering tablethat has responded the frame to remember the destination of the frame.
  7. Switch creates a point to point connection between the source and destination devices.
  8. Switch sends each constitutiveframe from the same source to the corresponding saved destination.

  Each time the switch receives a new frame, it follows the same process of remembering the source and destination address to avoid flooding the network ports.

• Forward/Filter decisions:The switches make the forward decisions on the frames it receives on its interface by checking the MAC database for the for the destination hardware address. If the destination is found, it forwards the frames only to the specified destination port. The process it uses for decision making to forward frame is same as mentioned in address learning.

  As soon as a frame arrives on a switch interface, the switch compares the destination hardware address to the forward/filter MAC database. If the destination is found, the frame is forwarded to the destination through the correct exit interface. Else the frame is broadcasted to all the active interfaces except the interface on which the frame was received.

  As soon as a device answers the flooded frame, the MAC database is updated and all the future frames from the same source are transmitted to the same saved destination.

• Loop Avoidance:Switches use STP (Spanning Tree protocol) to avoid the loops that are created for redundancy purpose. STP stops network loops while still maintaining redundancy.

Exam Questions

1. The switches perform better than routers because

1. Switches perform at hardware level and do not look into logical address to transmit data
2. Switches perform at software level and do not look into logical address to transmit data
3. Switches perform at hardware level and modify the logical address to transmit data
4. Switches perform at software level and modify the logical address to transmit data

Correct Answer A

Switches perform much faster than routers because perform at hardware level and they do not look into logical address to transmit data. They only read the data packets that appear on their interface and then encapsulate them before forwarding them.

2. Which of the following statements are true?

• Switches can neither break broadcast domain nor break collision domains
• Switches break both broadcast and collision domains
• Switches break collision domains but do not break broadcast domains
• Switches break broadcast domains but do not break collision domains

Correct Answer C

Switches break collision domains but do not break broadcast domains. This may reduce the network performance if the network is big.

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