Module 7: The Access Layer

Question 1

___ is technology commonly used in local area networks.

Answer 1

Ethernet

Question 2

Devices access the Ethernet LAN using an ___.

Answer 2

Ethernet Network Interface Card (NIC)

Question 3

Each Ethernet NIC has a unique address permanently embedded on the card known as a ___.

Answer 3

Media Access Control (MAC) address

The MAC address for both the source and destination are fields in an Ethernet frame.

Question 4

7.1.1 Video - The Fields of the Ethernet Frame

Fields of an Ethernet Frame

Ethernet is a Layer 2 protocol that facilitates communication between Network Interface Cards (NICs) on the same network. Each Ethernet frame consists of multiple fields, each with a specific size and function.

1. Preamble (7 bytes)
- Used to synchronize the receiving NIC with the incoming bits.
- Ensures the receiver is ready for data transmission.

2. Start Frame Delimiter (SFD) (1 byte)
- Signals to the receiving NIC that the actual Ethernet frame starts after this field.

3. Destination MAC Address (6 bytes)
- Identifies the MAC address of the recipient NIC on the network.
- Specifies where the Ethernet frame is being sent.

4. Source MAC Address (6 bytes)
- Contains the MAC address of the device that originated the Ethernet frame.

5. Length/Type (2 bytes)
- Can serve two purposes:
- Length: Specifies the size of the data (payload) in the frame.
- Type: Identifies the type of protocol being carried (e.g., IPv4, IPv6).

6. Data (Payload) (46–1500 bytes)
- Contains the actual encapsulated data, which may include:
- IP packets (IPv4/IPv6)
- Transport layer headers (TCP, UDP, etc.)
- Application layer protocols (HTTP, FTP, etc.)
- Ethernet itself does not interpret this data; it only ensures delivery from NIC to NIC.

7. Frame Check Sequence (FCS) (4 bytes)
- Used for error detection by the receiving device.
- Ensures the frame was transmitted correctly without corruption.

Conclusion
Each Ethernet frame follows a structured format to ensure efficient data transmission, error detection, and proper addressing within a network. The combination of synchronization, addressing, payload transmission, and error-checking makes Ethernet the backbone of wired networking.

Answer 4

Noted

Question 5

Fields of an Ethernet Frame

Answer 5

1. Preamble (7 bytes)
2. Start Frame Delimiter (SFD) (1 byte)
3. Destination MAC Address (6 bytes)
4. Source MAC Address (6 bytes)
5. Length/Type (2 bytes)
6. Data (Payload) (46–1500 bytes)
7. Frame Check Sequence (FCS) (4 bytes)

Question 6

Ethernet is a ___ that facilitates communication between ___ on the same network. Each Ethernet frame consists of multiple fields, each with a specific size and function.

Answer 6

Layer 2 protocol,
Network Interface Cards (NICs)

Question 7

Fields of Ethernet Frame

• Used to synchronize the receiving NIC with the incoming bits.
• Ensures the receiver is ready for data transmission.

Answer 7

Preamble (7 bytes)

Question 8

Fields of Ethernet Frame

Signals to the receiving NIC that the actual Ethernet frame starts after this field.

Answer 8

Start Frame Delimiter (SFD) (1 byte)

Question 9

Fields of Ethernet Frame

• Identifies the MAC address of the recipient NIC on the network.
• Specifies where the Ethernet frame is being sent.

Answer 9

Destination MAC Address (6 bytes)

Question 10

Fields of Ethernet Frame

Contains the MAC address of the device that originated the Ethernet frame.

Answer 10

Source MAC Address (6 bytes)

Question 11

Fields of Ethernet Frame

Can serve two purposes:
- ___: Specifies the size of the data (payload) in the frame.
- ___: Identifies the type of protocol being carried (e.g., IPv4, IPv6).

Answer 11

Length/Type (2 bytes)

Question 12

Fields of Ethernet Frame

Contains the actual encapsulated data, which may include:
- IP packets (IPv4/IPv6)
- Transport layer headers (TCP, UDP, etc.)
- Application layer protocols (HTTP, FTP, etc.)

Answer 12

Data (Payload) (46–1500 bytes)

Ethernet itself does not interpret this data; it only ensures delivery from NIC to NIC.

Question 13

Fields of Ethernet Frame

___ itself does not interpret this data; it only ensures delivery from NIC to NIC.

Answer 13

Ethernet

Question 14

Fields of Ethernet Frame

• Used for error detection by the receiving device.
• Ensures the frame was transmitted correctly without corruption.

Answer 14

Frame Check Sequence (FCS) (4 bytes)

Question 15

7.1.2 Encapsulation
When sending a letter, the letter writer uses an accepted format to ensure that the letter is delivered and understood by the recipient. In the same way, a message that is sent over a computer network follows specific format rules in order for it to be delivered and processed.

The process of placing one message format (the letter) inside another message format (the envelope) is called encapsulation. De-encapsulation occurs when the process is reversed by the recipient and the letter is removed from the envelope. Just as a letter is encapsulated in an envelope for delivery, so computer messages are encapsulated.

Each computer message is encapsulated in a specific format, called a frame, before it is sent over the network. A frame acts like an envelope; it provides the address of the intended destination and the address of the source host. The format and contents of a frame are determined by the type of message being sent and the channel over which it is communicated. Messages that are not correctly formatted are not successfully delivered to or processed by the destination host.

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A common example of requiring the correct format in human communications is when sending a letter.

An envelope has the address of the sender and receiver, each located at the proper place on the envelope. If the destination address and formatting are not correct, the letter is not delivered.

The process of placing one message format (the letter) inside another message format (the envelope) is called encapsulation. De-encapsulation occurs when the process is reversed by the recipient and the letter is removed from the envelope.

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Similar to sending a letter, a message that is sent over a computer network follows specific format rules for it to be delivered and processed.

Internet Protocol (IP) is a protocol with a similar function to the envelope example. In the figure, the fields of the Internet Protocol version 6 (IPv6) packet identify the source of the packet and its destination. IP is responsible for sending a message from the message source to destination over one or more networks.

Answer 15

Noted

Question 16

The process of placing one message format inside another message format.

Answer 16

Encapsulation

Question 17

___ occurs when the process is reversed by the recipient and the letter (message format) is removed from the envelope (another message format).

Answer 17

De-encapsulation

Question 18

Each computer message is encapsulated in a specific format, called a ___, before it is sent over the network.

Answer 18

frame

Question 19

A ___ acts like an envelope; it provides the address of the intended destination and the address of the source host.

Answer 19

frame

Question 20

The format and contents of a frame are determined by the ___ being sent and the channel over which it is communicated.

Answer 20

message type

Messages that are not correctly formatted are not successfully delivered to or processed by the destination host.

Question 21

___ is a protocol that is responsible for sending a message from the message source to destination over one or more networks.

Answer 21

Internet Protocol (IP)

Question 22

7.1.3 Check Your Understanding - Encapsulation and the Ethernet Frame

The process of prepending protocol information with information from another protocol is called

◯ packetizing

◯ framing

◯ encoding

◯ encapsulation

Answer 22

encapsulation

Encapsulation is the process of prepending protocol information with information from another protocol.

Question 23

7.1.3 Check Your Understanding - Encapsulation and the Ethernet Frame

When an Ethernet frame is sent out an interface, the destination MAC address indicates:

◯ The MAC address of the router.

◯ The MAC address of the NIC card that sent the Ethernet frame.

◯ The MAC address of the device, which is on this network, that will receive the Ethernet frame.

◯ The MAC address of the NIC card of a device, which is on this network or another network, that will receive the Ethernet frame.

Answer 23

The MAC address of the device, which is on this network, that will receive the Ethernet frame.

When an Ethernet frame is sent out an interface, the Destination MAC address indicates the MAC address of the device, which is on this network, that will receive the Ethernet frame.

Question 24

7.1.3 Check Your Understanding - Encapsulation and the Ethernet Frame

Which Ethernet frame field indicates the beginning of an Ethernet frame?

◯ Destination MAC Address

◯ FCS

◯ Preamble and SFD

◯ Type/Length

Answer 24

Preamble and SFD

The Preamble and Start Frame Delimiter (SFD) indicate the beginning of an Ethernet frame.

Question 25

# 7.1.3 Check Your Understanding - Encapsulation and the Ethernet Frame **The Ethernet protocol is at what layer of the OSI Model?** ◯ Layer 2 Data Link ◯ Layer 3 Network ◯ Layer 4 Transport ◯ Layer 1 Physical

Answer 25

Layer 2 Data Link ## Footnote Ethernet operates at layer 2, the data link layer, of the OSI model.

Question 26

# 7.2.1 Video - Ethernet Switches **How Ethernet Switches Work** Ethernet switches operate at **Layer 2 (Data Link Layer)** of the **OSI model**, making forwarding decisions based on **MAC (Media Access Control) addresses** found in the Ethernet frame header. **MAC Address Table and Forwarding Process** **MAC Address Table** - Ethernet switches maintain a **MAC address table** (also known as a **CAM table**) that maps MAC addresses to specific switch ports. - This table helps the switch **efficiently forward** frames to the correct destination without unnecessary broadcasts. **Frame Transmission Example** Consider a network with **four hosts (H1, H2, H3, H4)**, each with a unique **MAC address**: - **H1 (AA-AA)** - **H2 (BB-BB)** - **H3 (CC-CC)** - **H4 (DD-DD)** Suppose **H1 wants to send an Ethernet frame to H4**: - H1 constructs an **Ethernet frame** with: - **Source MAC Address**: AA-AA (H1’s NIC MAC) - **Destination MAC Address**: DD-DD (H4’s NIC MAC) - The frame is **sent from H1** and received by the switch on **Fast Ethernet 0/1 (Fa0/1)**. **Switch Forwarding Decision** - The switch **examines the destination MAC address (DD-DD)** in the Ethernet frame. - It looks up **DD-DD** in its **MAC address table** and finds it mapped to **Fast Ethernet 0/4 (Fa0/4)**. - The switch **forwards the frame only through Fa0/4**, ensuring **efficient, direct communication**. **Key Features of Ethernet Switching** - **MAC Learning:** The switch learns the MAC addresses of devices by inspecting the **source MAC address** of incoming frames and associating them with the corresponding switch port. - **Forwarding & Filtering:** If the switch **knows the destination MAC address**, it **forwards** the frame only to the specific port, reducing unnecessary network traffic. - **Flooding (Unknown MAC Addresses):** If the destination MAC is **not in the table**, the switch **floods** the frame to all ports **except the source port**. - **Loop Prevention:** **Switches use Spanning Tree Protocol (STP)** to prevent loops in the network. **Conclusion** Ethernet switches improve network efficiency by **intelligently forwarding frames** based on **MAC addresses**, reducing collisions and unnecessary traffic. This **Layer 2 forwarding mechanism** makes switches an essential component of modern networking.

Answer 26

Noted

Question 27

Ethernet switches operate at ___ of the OSI model, making forwarding decisions based on MAC (Media Access Control) addresses found in the Ethernet frame header.

Answer 27

Layer 2 (Data Link Layer)

Question 28

Ethernet switches operate at Layer 2 (Data Link Layer) of the OSI model, making forwarding decisions based on ___ found in the Ethernet frame header.

Answer 28

MAC (Media Access Control) addresses

Question 29

Ethernet switches maintain a ___ that maps MAC addresses to specific switch ports.

Answer 29

MAC address table

Question 30

The switch learns the MAC addresses of devices by inspecting the ___ of incoming frames and associating them with the corresponding switch port.

Answer 30

source MAC address

Question 31

If the switch knows the destination MAC address, it forwards the frame only to the specific port towards the ___, reducing unnecessary network traffic.

Answer 31

destination MAC address

Question 32

If the destination MAC is not in the table, the switch floods the frame to all ports except the source port, which is also known as ___.

Answer 32

unknown unicast

Question 33

# 7.2.2 Video - MAC Address Tables **How a Switch Builds Its MAC Address Table** Ethernet switches use a **MAC address table** (also called a **CAM table**) to keep track of which MAC addresses are associated with specific switch ports. The switch **dynamically learns** MAC addresses through the process of **MAC learning**. --- **1. Initial State – Empty MAC Address Table** - When a switch is first powered on, its **MAC address table is empty**. - It does not know which devices are connected to which ports. --- **2. Learning Process – Source MAC Address Learning** Each time a switch **receives an Ethernet frame**, it performs the following steps: 1. **Extracts the Source MAC Address** - The switch reads the **source MAC address** from the incoming Ethernet frame. - Example: If a device with MAC address **AA-AA** sends a frame through **Fa0/1**, the switch notes: - **AA-AA → Fa0/1** 2. **Updates the MAC Address Table** - The switch **records the MAC address and its corresponding port** in the MAC address table. - This way, the switch learns that **AA-AA is reachable via Fa0/1**. --- **3. Forwarding Process – Destination MAC Lookup** When a switch receives a frame, it checks the **destination MAC address** and follows these steps: 1. **If the Destination MAC is in the Table:** - The switch **forwards** the frame **only to the specific port** associated with the MAC address. - This prevents unnecessary network congestion. 2. **If the Destination MAC is NOT in the Table (Unknown MAC):** - The switch **floods** the frame to **all ports except the source port** (unknown unicast). - The intended recipient responds, allowing the switch to learn its MAC address. --- **4. Aging and Table Maintenance** - **MAC address entries are not permanent** – they have an **aging timer** (usually **5 minutes** by default). - If no traffic is received from a MAC address within this time, the switch **removes** it from the table. - This prevents the table from becoming too large with outdated addresses. --- **5. Handling Broadcast and Multicast Traffic** - **Broadcast frames** (e.g., ARP requests) are always sent to **all switch ports** because they do not have a specific destination MAC address. - **Multicast traffic** is forwarded to **specific groups of devices** based on IGMP (Internet Group Management Protocol). --- **Summary of the MAC Learning Process** 1. **Receives an Ethernet frame** on a port. 2. **Reads the source MAC address** and records it in the MAC address table. 3. **Checks the destination MAC address**: - **If known**, forwards it to the correct port. - **If unknown**, floods the frame to all ports except the source port. 4. **Removes inactive MAC addresses** after a timeout period. By dynamically learning and managing MAC addresses, switches optimize network efficiency and **reduce unnecessary traffic**, ensuring fast and reliable communication between devices.

Answer 33

Noted

Question 34

A switch builds its MAC address table by recording the ___ and its corresponding ___.

Answer 34

source MAC address, port

Question 35

MAC address entries are not permanent – they have an aging timer (usually ___ minutes by default).

Answer 35

5

Question 36

If the receiving host detects a wrong frame, it will ___ it; otherwise, it will ___ the rest of the frame.

Answer 36

discard, read

Question 37

# 7.2.3 Check Your Understanding - The Access Layer **Ethernet switches make their forwarding decision based on what field of the Ethernet frame?** ◯ Type/Length ◯ Source MAC address ◯ FCS ◯ Destination MAC address ◯ SFD (Start Frame Delimiter)

Answer 37

Destination MAC address

Question 38

# 7.2.3 Check Your Understanding - The Access Layer **Ethernet switches add entries to their MAC address table based on what field of the Ethernet frame?** ◯ Type/Length ◯ source MAC address ◯ SFD (Start Frame Delimiter) ◯ FCS ◯ destination MAC address

Answer 38

source MAC address

Question 39

# 7.2.3 Check Your Understanding - The Access Layer **When a switch receives an Ethernet frame and the destination MAC address of that frame is not in its MAC address table, the switch will:** ◯ Add the Source MAC address to the table. ◯ Add the Destination MAC address to the table. ◯ Forward the frame out all ports except in the incoming port. ◯ Drop the frame.

Answer 39

Forward the frame out all ports except in the incoming port.

Question 40

# 7.2.3 Check Your Understanding - The Access Layer **Ethernet hubs are considered:** ◯ a wireless device ◯ a security device ◯ cutting-edge ◯ obsolete

Answer 40

obsolete

Question 41

# 7.3.1 What Did I Learn in this Module? **Encapsulation and the Ethernet Frame** The process of placing one message format inside another message format is called **encapsulation**. **De-encapsulation** occurs when the process is reversed by the recipient and the letter is removed from the envelope. Just as a letter is encapsulated in an envelope for delivery, so computer messages are encapsulated. A message that is sent over a computer network follows specific format rules for it to be delivered and processed. The **Ethernet protocol** standards define many aspects of network communication including frame format, frame size, timing, and encoding. The format for Ethernet frames specifies the location of the destination and source MAC addresses, and additional information including preamble for sequencing and timing, start of frame delimiter, length and type of frame, and frame check sequence to detect transmission errors.

Answer 41

Noted

Question 42

**The Access Layer** The **access layer **is the part of the network in which people gain access to other hosts and to shared files and printers. The access layer provides the first line of networking devices that connect hosts to the wired Ethernet network. Within an Ethernet network, each host can connect directly to an access layer networking device using an **Ethernet cable**. **Ethernet hubs** contain multiple ports that are used to connect hosts to the network. Only **one message** can be sent through an Ethernet hub at a time. Two or more messages sent at the same time will cause a collision. Because excessive retransmissions can clog up the network and slow down network traffic, **hubs** are now considered **obsolete** and have been replaced by **Ethernet switches**. An **Ethernet switch** is a device that is used at **Layer 2**. When a host sends a message to another host connected to the same switched network, the switch accepts and decodes the frames to read the MAC address portion of the message. A table on the switch, called a **MAC address table**, contains a list of all the active ports and the host MAC addresses that are attached to them. When a message is sent between hosts, the switch checks to see if the **destination MAC address** is in the table. If it is, the switch builds a temporary connection, called a **circuit**, between the source and destination ports. Ethernet switches also allow for sending and receiving frames over the same Ethernet cable simultaneously. This improves the performance of the network by eliminating collisions. A switch builds the MAC address table by **examining the source MAC address of each frame that is sent between hosts**. When a new host sends a message or responds to a flooded message, the switch immediately learns its MAC address and the port to which it is connected. The table is dynamically updated each time a new source MAC address is read by the switch.

Answer 42

Noted

Question 43

# 7.3.3 The Access Layer Quiz **What will a Layer 2 switch do when the destination MAC address of a received frame is not in the MAC table?** ◯ It initiates an ARP request. ◯ It broadcasts the frame out of all ports on the switch. ◯ It notifies the sending host that the frame cannot be delivered. ◯ It forwards the frame out of all ports except for the port at which the frame was received.

Answer 43

It forwards the frame out of all ports except for the port at which the frame was received.

Question 44

# 7.3.3 The Access Layer Quiz **Which network device has the primary function to send data to a specific destination based on the information found in the MAC address table?** ◯ hub ◯ router ◯ switch ◯ modem

Answer 44

switch

Question 45

# 7.3.3 The Access Layer Quiz **What addressing information is recorded by a switch to build its MAC address table?** ◯ the destination Layer 3 address of incoming packets ◯ the destination Layer 2 address of outgoing frames ◯ the source Layer 3 address of outgoing packets ◯ the source Layer 2 address of incoming frames

Answer 45

the source Layer 2 address of incoming frames

Question 46

# 7.3.3 The Access Layer Quiz **What is the purpose of the FCS field in a frame?** ◯ to obtain the MAC address of the sending node ◯ to verify the logical address of the sending node ◯ to compute the CRC header for the data field ◯ to determine if errors occurred in the transmission and reception

Answer 46

to determine if errors occurred in the transmission and reception

Question 47

# 7.3.3 The Access Layer Quiz **What is one function of a Layer 2 switch?** ◯ forwards data based on logical addressing ◯ duplicates the electrical signal of each frame to every port ◯ learns the port assigned to a host by examining the destination MAC address ◯ determines which interface is used to forward a frame based on the destination MAC address

Answer 47

determines which interface is used to forward a frame based on the destination MAC address

Question 48

# 7.3.3 The Access Layer Quiz **Which information does a switch use to keep the MAC address table information current?** ◯ the destination MAC address and the incoming port ◯ the destination MAC address and the outgoing port ◯ the source and destination MAC addresses and the incoming port ◯ the source and destination MAC addresses and the outgoing port ◯ the source MAC address and the incoming port

Answer 48

the source MAC address and the incoming port

Question 49

# 7.3.3 The Access Layer Quiz **What process is used to place one message inside another message for transfer from the source to the destination?** ◯ access control ◯ decoding ◯ encapsulation ◯ flow control ◯ the source MAC address and the incoming port

Answer 49

encapsulation

Question 50

# 7.3.3 The Access Layer Quiz **Refer to the exhibit. The exhibit shows a small switched network and the contents of the MAC address table of the switch. PC1 has sent a frame addressed to PC3. What will the switch do with the frame?** [Image Description] The image shows a network topology with four computers (PC1, PC2, PC3, and PC4) connected to a switch. The switch's MAC address table only records PC2 (Port 3) and PC4 (Port 1). Note: PC1 (Port 4) & PC3 (Port 2). ◯ The switch will discard the frame. ◯ The switch will forward the frame only to port 2. ◯ The switch will forward the frame to all ports except port 4. ◯ The switch will forward the frame to all ports. ◯ The switch will forward the frame only to ports 1 and 3.

Answer 50

The switch will forward the frame to all ports except port 4.

Question 51

# 7.3.3 The Access Layer Quiz **Which three fields are found in an 802.3 Ethernet frame? (Choose three.)** ▢ source physical address ▢ source logical address ▢ media type identifier ▢ frame check sequence ▢ destination physical address ▢ destination logical address

Answer 51

source physical address frame check sequence destination physical address

Question 52

# 7.3.3 The Access Layer Quiz **What will a host on an Ethernet network do if it receives a frame with a unicast destination MAC address that does not match its own MAC address?** ◯ It will discard the frame. ◯ It will forward the frame to the next host. ◯ It will remove the frame from the media. ◯ It will strip off the data-link frame to check the destination IP address.

Answer 52

It will discard the frame.

Question 53

# 7.3.3 The Access Layer Quiz **Which statement is correct about Ethernet switch frame forwarding decisions?** ◯ Frame forwarding decisions are based on MAC address and port mappings in the MAC Address table. ◯ Frames addressed to unknown MAC addresses are dropped. ◯ Switches build up their MAC Address tables based on the destination MAC address of incoming frames. ◯ Unicast frames are always forwarded regardless of the destination MAC address.

Answer 53

Frame forwarding decisions are based on MAC address and port mappings in the MAC Address table.