Ethernet Switching

Question 1

Define Ethernet

Answer 1

Ethernet operates in the data link layer and the physical layer. It is a family of networking technologies defined in the IEEE 802.2 and 802.3 standards. It supports data bandwidths of the following:

10 Mbps
100 Mbps
1 Gbps
10 Gbps
40 Gbps
100 Gbps Ethernet standards define both the Layer 2 protocols and the Layer 1 technologies.

Question 2

What is the MAC sub-layer responsible for?

Answer 2

Data encapsulation and accessing the media.

Question 3

Talk about Data encapsulation

Answer 3

IEEE 802.3 data encapsulation includes the following:
1) Ethernet frame
2) Ethernet Addressing
3) Ethernet Error detection

Question 4

Talk about Ethernet frame

Answer 4

the internal structure of the Ethernet frame

Question 5

Talk about Ethernet adressing

Answer 5

The Ethernet frame includes both a source and destination MAC address to deliver the Ethernet frame from Ethernet NIC to Ethernet NIC on the same LAN.

Question 6

Talk about Ethernet error detection

Answer 6

The Ethernet frame includes a frame check sequence (FCS) trailer used for error detection.

Question 7

Talk about accessing the media

Answer 7

IEEE 802.3 MAC sublayer includes the specifications for different Ethernet communications standards over various types of media including copper and fiber.

Question 8

Talk about the size of the Ethernet frame fields

Answer 8

The minimum Ethernet frame size is 64 bytes and the expected maximum is 1518 bytes.
Any frame less than 64 bytes in length is considered a “collision fragment” or “runt frame”.
Frames with more than 1500 bytes of data are considered “jumbo” or “baby giant frames”.
If the size of a transmitted frame is less than the minimum, or greater than the maximum, the receiving device drops the frame. Dropped frames are likely to be the result of collisions or other unwanted signals. They are considered invalid. Jumbo frames are usually supported by most Fast Ethernet and Gigabit Ethernet switches and NICs.

Question 9

What are the Ethernet frame fields?

Answer 9

1) Preamble and Start Frame Delimiter Fields
2) Destination MAC Address Field
3) Source MAC Address Field
4) Type / Length
5) Data Field
6) Frame Check Sequence Field

Question 10

Talk about Preamble and Start Frame Delimiter Fields

Answer 10

The Preamble (7 bytes) and Start Frame Delimiter (SFD), also called the Start of Frame (1 byte), fields are used for synchronization between the sending and receiving devices. These first eight bytes of the frame are used to get the attention of the receiving nodes. Essentially, the first few bytes tell the receivers to get ready to receive a new frame.

Question 11

Talk about Destination MAC Address Field

Answer 11

This 6-byte field is the identifier for the intended recipient. This address is used by Layer 2 to assist devices in determining if a frame is addressed to them. The address in the frame is compared to the MAC address in the device. If there is a match, the device accepts the frame. Can be a unicast, multicast, or broadcast address.

Question 12

Talk about Source MAC Address Field

Answer 12

This 6-byte field identifies the originating NIC or interface of the frame.

Question 13

Talk about Type/Length

Answer 13

This 2-byte field identifies the upper layer protocol encapsulated in the Ethernet frame.

Question 14

Talk about Data Field

Answer 14

This field (46 - 1500 bytes) contains the encapsulated data from a higher layer, which is an IPv4 packet. All frames must be at least 64 bytes long. If a small packet is encapsulated, additional bits called a pad are used to increase the size of the frame to this minimum size.

Question 15

Talk about Frame Check Sequence Field

Answer 15

(4 bytes) is used to detect errors in a frame. It uses a cyclic redundancy check (CRC). The sending device includes the results of a CRC in the FCS field of the frame. The receiving device receives the frame and generates a CRC to look for errors. If the calculations match, no error occurred. Calculations that do not match are an indication that the data has changed; therefore, the frame is dropped. A change in the data could be the result of a disruption of the electrical signals that represent the bits.

Question 16

How are IPv4 , IPv6, and Ethernet addresses represented?

Answer 16

IPv4 addresses are represented using the decimal base ten number system and the binary base 2 number system. IPv6 addresses and Ethernet addresses are represented using the hexadecimal base sixteen number system.

Question 17

What does the hexadecimal numbering system use?

Answer 17

The hexadecimal numbering system uses the numbers 0 to 9 and the letters A to F.

Question 18

Why is hexadecimal used to identify an Ethernet address?

Answer 18

A single hexadecimal digit represents four binary bits. Therefore, a 48-bit Ethernet MAC address can be expressed using only 12 hexadecimal values.

Question 19

What does an Ethernet MAC address consist of?

Answer 19

is a 48-bit address expressed using 12 hexadecimal digits. Because a byte equals 8 bits, we can also say that a MAC address is 6 bytes in length.

Question 20

How do you ensure that a MAC address is unique to the Ethernet device or Ethernet interface?

Answer 20

all vendors that sell Ethernet devices must register with the IEEE to obtain a unique 6 hexadecimal (i.e., 24-bit or 3-byte) code called the organizationally unique identifier (OUI).

Question 21

What should a vendor do when it assigns a MAC address to a device or Ethernet interface?

Answer 21

1) Use its assigned OUI as the first 6 hexadecimal digits
2) Assign a unique value in the last 6 hexadecimal digits.
Therefore, an Ethernet MAC address consists of a 6 hexadecimal vendor OUI code followed by a 6 hexadecimal vendor-assigned value

Question 22

What’s the responsibility of the vendor?

Answer 22

to ensure that none of its devices be assigned the same MAC address
However, it is possible for duplicate MAC addresses to exist because of mistakes made during manufacturing, mistakes made in some virtual machine implementation methods, or modifications made using one of several software tools.

Question 23

What should be done i there’s a duplicate MAC address?

Answer 23

it will be necessary to modify the MAC address with a new NIC or make modifications via software.

Question 24

Talk about frame processing

Answer 24

Sometimes the MAC address is referred to as a burned-in address (BIA) because the address is hard coded into read-only memory (ROM) on the NIC.
When the computer boots up, the NIC copies its MAC address from ROM into RAM. When a device is forwarding a message to an Ethernet network, the Ethernet header includes these:
1) Source MAC address - This is the MAC address of the source device NIC.
2) Destination MAC address - This is the MAC address of the destination device NIC.
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.

Note: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 25

What does a Layer 2 Ethernet switch use?

Answer 25

uses Layer 2 MAC addresses to make forwarding decisions.
An Ethernet switch examines its MAC address table to make a forwarding decision for each frame

Question 26

How does a switch fill the MAC address table?

Answer 26

1) Learn
2) Forward

Question 27

Talk about learning

Answer 27

It does this by examining the source MAC address of the frame and the port number where the frame entered the switch. If the source MAC address does not exist, it is added to the table along with the incoming port number. If the source MAC address does exist, the switch updates the refresh timer for that entry in the table. By default, most Ethernet switches keep an entry in the table for 5 minutes.
Note: If the source MAC address does exist in the table but on a different port, the switch treats this as a new entry. The entry is replaced using the same MAC address but with the more current port number.

Question 28

Talk about Forwarding

Answer 28

If the destination MAC address is a unicast address, the switch will look for a match between the destination MAC address of the frame and an entry in its MAC address table. If the destination MAC address is in the table, it will forward the frame out to the specified port. If the destination MAC address is not in the table, the switch will forward the frame out to all ports except the incoming port. This is called an unknown uni-cast.
If the destination MAC address is a broadcast or a multicast, the frame is also flooded out all ports except the incoming port.

Question 29

What are the frame-forwarding methods on Cisco switches?

Answer 29

1) Store-and-forward switching
2) Cut-through switching

Question 30

Talk about Store-and-forward switching

Answer 30

This frame-forwarding method receives the entire frame and computes the CRC.
CRC uses a mathematical formula, based on the number of bits (1s) in the frame, to determine whether the received frame has an error. If the CRC is valid, the switch looks up the destination address, which determines the outgoing interface. Then the frame is forwarded out of the correct port.

Question 31

What’s the advantage of store-and-forward switching?

Answer 31

it determines if a frame has errors before propagating the frame. When an error is detected in a frame, the switch discards the frame. Discarding frames with errors reduces the amount of bandwidth consumed by corrupt data. Store-and-forward switching is required for quality of service (QoS) analysis on converged networks where frame classification for traffic prioritization is necessary.

Question 32

Talk about Cut-Through Switching

Answer 32

the switch acts upon the data as soon as it is received, even if the transmission is not complete. The switch buffers just enough of the frame to read the destination MAC address so that it can determine to which port it should forward out the data. The switch does not perform any error checking on the frame.

Question 33

What are the two variants of cut-through switching?

Answer 33

1) Fast-forward switching
2) Fragment-free switching

Question 34

Talk about Fast-Forward switching

Answer 34

offers the lowest level of latency. It immediately forwards a packet after reading the destination address. Because it starts forwarding before the entire packet has been received, there may be times when packets are relayed with errors. This occurs infrequently, and the destination NIC discards the faulty packet upon receipt. In fast-forward mode, latency is measured from the first bit received to the first bit transmitted. Fast-forward switching is the typical cut-through method of switching.

Question 35

Talk about Fragment-free switching

Answer 35

The switch stores the first 64 bytes of the frame before forwarding. The reason for that is that most network errors and collisions occur during the first 64 bytes. It tries to enhance fast-forward switching by performing a small error check on the first 64 bytes of the frame to ensure that a collision has not occurred before forwarding the frame. Its a compromise between the high latency and high integrity of store-and-forward switching, and the low latency and reduced integrity of fast-forward switching.

Question 36

Talk about Auto-MDIX

Answer 36

Most switch devices now support the automatic medium-dependent interface crossover (auto-MDIX) feature. When enabled, the switch automatically detects the type of cable attached to the port and configures the interfaces accordingly.
The auto-MDIX feature is enabled by default on switches running Cisco IOS Release 12.2(18)SE or later