CCNA- Basic Switching Interview Question & Answers

What is Switch?

A Network Switch is a device which is used to connect multiple computers inside Local Area Network (LAN).

Network Switches operate at Layer Two (Data Link Layer) of the OSI model.

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What is LAN?

Local Area Network (LAN) is a network, which is limited to a small office, single building, multiple buildings, college campus etc.

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What is the basic function of a Switch?

The basic function of a Switch is to forward Layer 2 packets (Ethernet frames) from source device to destination device.

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What are the features of a Switch?

ü Switches are used to connect network devices together in a Local Area Network (LAN).

ü Switches can learn Layer 2 (MAC) addresses and forward Layer 2 packets (Ethernet frames) to exact destination.

ü Switches can control of who has access to various parts of the network.

ü Switches have provision to monitor network usage.

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What are the differences between a Switch and Bridge?

switch and bridgeClick on the Image

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What is a Switching method?

A Switching Method determines how a switch receives, processes, and forwards a Layer 2 Ethernet frame.

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What are the diff Switching Methods?

Important Switching Methods are

  1. Store & Forward
  2. Cut-Through
  3. Fragment-Free

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What is Store and Forward Switching?

  • Ø In Store and Forward Switching, Switch copies each complete Ethernet Frame into the switch memory and computes a Cyclic Redundancy Check (CRC) for errors.
  • Ø If a Cyclic Redundancy Check (CRC) error is found, the Ethernet Frame is dropped and if there is no Cyclic Redundancy Check (CRC) error, the switch forwards the Ethernet frame to the destination device.

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What is the disadvantage of Store and Forward Switching?

Store and Forward Switching can cause delay in switching since Cyclic Redundancy Check (CRC) is calculated for each Ethernet frame.

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What is Cut-Through Switching?

  • Ø In Cut-Through switching, the switch copies into its memory only the destination MAC address (first 6 bytes of the frame) of the frame before making a switching decision.
  • Ø A switch operating in cut-through switching mode reduces delay because the switch starts to forward the Ethernet frame as soon as it reads the destination MAC address and determines the outgoing switch port.

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What is the disadvantage of Cut-Through Switching?

Problem related with Cut-Through switching is that the switch may forward bad frames.

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What are Ethernet runt frames?

Ethernet frames smaller than 64 bytes are called Ethernet runt frames.

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What is Fragment-free (runt-less switching) switching?

  • Ø Fragment-free (runt-less switching) switching is an advanced form of cut-through switching. The switches operating in cut-through switching read only up to the destination MAC address field in the Ethernet frame before making a switching decision.
  • Ø The switches operating in fragment-free switching read at least 64 bytes of the Ethernet frame before switching it to avoid forwarding Ethernet runt frames (Ethernet frames smaller than 64 bytes).

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What is Half-Duplex?

  • Ø In a half-duplex Local Area Network (LAN) a device can only, either send or receive frames at a time but not both at the same time.
  • Ø In a half-duplex mode, one device sends the frame and all other devices in the collision domain listen for the frame. Collision is a problem in half duplex networks.
  • Ø Half-duplex Local Area Networks (LANs) are networks of the past.

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What is Full-Duplex?

  • Ø In a Local Area Network (LAN) operating in full duplex mode, a device can send and receive frames simultaneously at the same time.
  • Ø In a full-duplex mode, there are separate channels to send and receive frames. There is no collision detection in devices operating in full duplex mode, since collision detection is not required.
  • Ø Almost all current networks operate in full-duplex mode.

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What are the Basic functions of a Switch?

Three basic functions of a switch are

1. Learning; 2. Forwarding;  3. Preventing Layer2 Loops.

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What is the Learning function of a Switch?

  • Ø Learning is the process of obtaining the MAC address of connected devices.
  • Ø When a frame reaches into the port of a switch, the switch reads the MAC address of the source device from Ethernet frame and compares it to its MAC address table (also known as CAM (Content Addressable Memory) table). If the switch cannot find a corresponding entry in MAC address table, the switch will add the address to the table with the port number via the Ethernet frame arrived.
  • Ø If the MAC address is already available in the MAC address table, the switch compares the incoming port with the port already available in the MAC table. If the port numbers are different, the switch updates the MAC address table new port number. This will normally happen when network administrators remove the cable from one port and attach it to another port.

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What is the Forwarding function of a Switch?

  • Ø Forwarding is the process of passing network traffic of a device connected to one port of a Switch to another device connected to another port on the switch.
  • Ø When a Layer2 Ethernet frame reaches a port on the Switch, it not only reads the source MAC address of the Ethernet frame as a part of learning function, but also reads the destination MAC address as a part of forwarding function. The destination MAC address is important to determine the port number which the destination device is connected.
  • Ø As the destination MAC address is found on the MAC address table, the switch forwards the Ethernet frame via the corresponding port of the MAC address.

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What is Flooding?

  • Ø If the destination MAC address is not found on the MAC address table, the switch forwards the Ethernet frame through all its ports except the source port. This is known as flooding.
  • Ø When the destination device receives the Ethernet frame and sends a reply frame to the source device, the switch reads the MAC address of the destination device and adds it to the MAC address table, which is the function of learning process.

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What is Filtering?

If the source MAC address is same as the destination MAC address, the switch will drop the Ethernet frame. This is known as filtering.

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What is a Redundant Link?

In practical Local Area Networking, Redundant links are created to avoid complete network failure in an event

of failure of one link.

redundant links Click on the Image

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What is Disadvantage of a Redundant Link?

Redundant links can cause Layer2 Switching Loops and Broadcast Storms.

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What is Layer2 Switching Loop?

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What is Broadcast Storm?

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What is Management VLAN?

  • Ø Management VLAN is used for managing the switch from a remote location by using protocols such as telnet, SSH, SNMP, syslog etc.
  • Ø Normally the Management VLAN is VLAN 1, but you can use any VLAN as a management VLAN.
  • Ø Cisco recommends not to use VLAN 1 and not to use any VLAN that carries user data traffic as Management VLAN.
  • Ø You must configure IP address and default gateway for Management VLAN.

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Configuring Management VLAN:-

Switch>enable

Switch#configure terminal

Switch(config)#interface vlan 1

Switch(config-if)#ip address 192.168.100.28 255.255.255.0

Switch(config-if)#no shutdown

Switch(config-if)#exit

Switch(config)#ip default-gateway 192.168.100.1

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