Module 7: Ethernet Switching

Question 1

What is the purpose of the organizationally unique identifier (OUI) in an Ethernet MAC address?

Answer 1

The OUI is a unique 24-bit or 3-byte code obtained by vendors from the IEEE to ensure that all Ethernet MAC addresses on their devices/interfaces are globally unique. It is used to identify the manufacturer of the device/interface.

Question 2

What happens when a NIC receives an Ethernet frame?

Answer 2

When a NIC receives an Ethernet frame, it examines the destination MAC address to see if it matches the physical MAC address that is stored in RAM. If there is no match, the device discards the frame. If there is a match, it passes the frame up the OSI layers, where the de-encapsulation process takes place. The Ethernet header includes a Source MAC address and a Destination MAC address, which are used to ensure that the frame is delivered to the correct device on the network.

Question 3

What information is included in the Ethernet header when forwarding a message to an Ethernet network?

Answer 3

The Ethernet header includes a Source MAC address and a Destination MAC address.

Question 4

What happens when a NIC receives an Ethernet frame?

Answer 4

The NIC examines the destination MAC address to see if it matches the physical MAC address that is stored in RAM. If there is no match, the device discards the frame. If there is a match, it passes the frame up the OSI layers, where the de-encapsulation process takes place. Additionally, Ethernet NICs will also accept frames if the Destination MAC address is a broadcast or a multicast group of which the host is a member.

Question 5

What is the purpose of Address Resolution Protocol (ARP) and Neighbor Discovery (ND)?

Answer 5

ARP is used by a source host to determine the MAC address associated with an IPv4 address, while ND is used by a source host to determine the MAC address associated with an IPv6 address. Both protocols are used to resolve network layer addresses (IPv4 and IPv6) to data link layer addresses (MAC addresses) for communication on the local network.

Question 6

What are the features of an Ethernet broadcast in terms of its destination MAC address and how it is flooded across a network?

Answer 6

An Ethernet broadcast has a destination MAC address of FF-FF-FF-FF-FF-FF in hexadecimal, which is equivalent to 48 ones in binary. When an Ethernet broadcast is sent, it is flooded out of all Ethernet switch ports except the incoming port. However, it is not forwarded by a router, which means it is confined to the local network segment.

Question 7

What is the difference between an Ethernet multicast frame and an IPv4/IPv6 multicast packet in terms of their destination MAC addresses?

Answer 7

An Ethernet multicast frame is received and processed by a group of devices that belong to the same multicast group, and its destination MAC address varies depending on the multicast group address. On the other hand, both IPv4 and IPv6 multicast packets have specific destination MAC addresses, with the encapsulated data destination MAC address for IPv4 being 01-00-5E and for IPv6 being 33-33.

Question 8

What happens when an Ethernet multicast frame is received by a switch or a router?

Answer 8

When an Ethernet multicast frame is received by a switch, it is flooded out all Ethernet switch ports except the incoming port, unless the switch is configured for multicast snooping. On the other hand, when an Ethernet multicast frame is received by a router, it is not forwarded, unless the router is configured to route multicast packets.

Question 9

What does a Layer 2 Ethernet Switch use to make its forwarding decisions?

Answer 9

A Layer 2 Ethernet Switch makes its forwarding decisions based solely on the Layer 2 Ethernet MAC addresses

Question 10

How does an Ethernet switch examine its MAC address table to make a forwarding decision for each frame?

Answer 10

An Ethernet switch examines its MAC address table to determine the outgoing port for each frame based on the destination MAC address of the frame. If the MAC address is not in the table, the switch floods the frame out all ports except the incoming port.

Question 11

What is a MAC address table in a Layer 2 Ethernet switch and how does it learn new information?

Answer 11

The MAC address table in a Layer 2 Ethernet switch is a Content Addressable Memory (CAM) table. The switch learns new information by examining the source MAC address and the port number of each frame that enters the switch. If the source MAC address does not exist in the table, it is added along with the incoming port number. If the source MAC address does exist, the switch updates the refresh timer for that entry. By default, most Ethernet switches keep an entry in the table for 5 minutes.

Question 12

What does an Ethernet switch do if it cannot find a match between the destination MAC address of a frame and an entry in its MAC address table?

Answer 12

If an Ethernet switch cannot find a match between the destination MAC address of a frame and an entry in its MAC address table, it will forward the frame out all ports except the incoming port. This is known as an unknown unicast. If the destination MAC address is a broadcast or a multicast, the frame is flooded out all ports except the incoming port

Question 13

What are the two forwarding methods that switches use for switching data between network ports?

Answer 13

The two forwarding methods that switches use for switching data between network ports are:
store-and-forward switching and
cut-through switchin

Question 14

What is the difference between Store-and-forward switching and Cut-through switching in Ethernet networks?

Answer 14

Store-and-forward switching is a method in which the switch receives the entire frame, computes the CRC, and looks up the destination address before forwarding the frame out of the correct port.
Cut-through switching, on the other hand, forwards the frame before it is entirely received. This means that the switch only needs to read the destination MAC address before forwarding the frame out of the correct port, making it faster than store-and-forward switching. However, cut-through switching can result in forwarding frames with errors, as the switch has not yet verified the CRC.

Question 15

What are the two variants of cut-through switching?

Answer 15

The two variants of cut-through switching are fast-forward switching and fragment-free switching.

Question 16

What is port-based memory buffering in Ethernet switching?

Answer 16

Port-based memory buffering is a technique used by Ethernet switches to store frames in queues that are linked to specific incoming and outgoing ports. A frame is only transmitted to the outgoing port when all the frames ahead in the queue have been successfully transmitted.

Question 17

What is shared memory buffering in Ethernet switching?

Answer 17

Shared memory buffering is a technique used by Ethernet switches to deposit all frames into a common memory buffer shared by all switch ports, with the amount of buffer memory required by a port dynamically allocated. The frames in the buffer are dynamically linked to the destination port, enabling a packet to be received on one port and then transmitted on another port without moving it to a different queue.

Question 18

What is the difference between port-based and shared memory buffering in Ethernet switching?

Answer 18

Port-based memory buffering stores frames in queues linked to specific incoming and outgoing ports, while shared memory buffering deposits all frames into a common memory buffer shared by all switch ports. In port-based buffering, a frame is only transmitted to the outgoing port when all frames ahead in the queue have been successfully transmitted, while in shared memory buffering, frames are dynamically linked to the destination port, enabling packets to be received on one port and then transmitted on another port without moving them to a different queue.

Question 19

What are the advantages of shared memory buffering in Ethernet switching?

Answer 19

Shared memory buffering allows larger frames to be transmitted with fewer dropped frames. This is important in asymmetric switching, which allows for different data rates on different ports, enabling more bandwidth to be dedicated to certain ports, such as a server port. Additionally, shared memory buffering enables packets to be received on one port and then transmitted on another port without moving them to a different queue, which can improve network performance.

Question 20

What is the difference between symmetric and asymmetric switching?

Answer 20

In symmetric switching, all ports on a switch have the same data rate, meaning they can transmit and receive at the same speed. In contrast, asymmetric switching allows for different data rates on different ports, enabling more bandwidth to be dedicated to certain ports, such as those connected to servers or other high-bandwidth devices. This can result in better performance and fewer dropped frames for those devices.

Question 21

What is the difference between full-duplex and half-duplex settings on a switch?

Answer 21

Full-duplex and half-duplex are two different types of duplex settings used for communication on an Ethernet network. In full-duplex mode, both ends of the connection can send and receive data simultaneously, while in half-duplex mode, only one end of the connection can send data at a time. Full-duplex mode offers faster and more efficient communication, as it eliminates collisions and allows for a more consistent flow of data. Half-duplex mode, on the other hand, is simpler and more cost-effective, but can lead to slower communication and network congestion due to collisions.

Question 22

What is autonegotiation and what is its purpose?

Answer 22

Autonegotiation is a function found on most Ethernet switches and network interface cards (NICs) that enables two devices to automatically negotiate the best speed and duplex capabilities. The purpose of autonegotiation is to ensure that both devices on a connection operate at the highest possible speed and in the correct duplex mode. By automatically determining the highest speed and correct duplex setting, autonegotiation simplifies the task of setting up a connection and ensures that the connection operates at optimal performance levels.

Question 23

What are the causes and best practices for avoiding duplex mismatch on Ethernet links?

Answer 23

Duplex mismatch can occur when:

One or both ports on a link are reset, and the autonegotiation process does not result in both link partners having the same configuration
Users reconfigure one side of a link and forget to reconfigure the other
To avoid duplex mismatch, it is recommended to:
Configure both Ethernet switch ports as full-duplex
Have both sides of the link set to either autonegotiation on or off
Use the same speed and duplex settings on both sides of the link
Ensure that all devices on the network are using the same duplex settings