Module 7: The Access Layer Flashcards
Explain how communication occurs on Ethernet networks.
___ is technology commonly used in local area networks.
Ethernet
Devices access the Ethernet LAN using an ___.
Ethernet Network Interface Card (NIC)
Each Ethernet NIC has a unique address permanently embedded on the card known as a ___.
Media Access Control (MAC) address
The MAC address for both the source and destination are fields in an Ethernet frame.
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.
Noted
Fields of an Ethernet Frame
- Preamble (7 bytes)
- Start Frame Delimiter (SFD) (1 byte)
- Destination MAC Address (6 bytes)
- Source MAC Address (6 bytes)
- Length/Type (2 bytes)
- Data (Payload) (46–1500 bytes)
- Frame Check Sequence (FCS) (4 bytes)
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.
Layer 2 protocol,
Network Interface Cards (NICs)
Fields of Ethernet Frame
- Used to synchronize the receiving NIC with the incoming bits.
- Ensures the receiver is ready for data transmission.
Preamble (7 bytes)
Fields of Ethernet Frame
Signals to the receiving NIC that the actual Ethernet frame starts after this field.
Start Frame Delimiter (SFD) (1 byte)
Fields of Ethernet Frame
- Identifies the MAC address of the recipient NIC on the network.
- Specifies where the Ethernet frame is being sent.
Destination MAC Address (6 bytes)
Fields of Ethernet Frame
Contains the MAC address of the device that originated the Ethernet frame.
Source MAC Address (6 bytes)
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).
Length/Type (2 bytes)
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.)
Data (Payload) (46–1500 bytes)
Ethernet itself does not interpret this data; it only ensures delivery from NIC to NIC.
Fields of Ethernet Frame
___ itself does not interpret this data; it only ensures delivery from NIC to NIC.
Ethernet
Fields of Ethernet Frame
- Used for error detection by the receiving device.
- Ensures the frame was transmitted correctly without corruption.
Frame Check Sequence (FCS) (4 bytes)
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.
///
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.
///
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.
Noted
The process of placing one message format inside another message format.
Encapsulation
___ occurs when the process is reversed by the recipient and the letter (message format) is removed from the envelope (another message format).
De-encapsulation
Each computer message is encapsulated in a specific format, called a ___, before it is sent over the network.
frame
A ___ acts like an envelope; it provides the address of the intended destination and the address of the source host.
frame
The format and contents of a frame are determined by the ___ being sent and the channel over which it is communicated.
message type
Messages that are not correctly formatted are not successfully delivered to or processed by the destination host.
___ is a protocol that is responsible for sending a message from the message source to destination over one or more networks.
Internet Protocol (IP)
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
encapsulation
Encapsulation is the process of prepending protocol information with information from another protocol.
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.
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.
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
Preamble and SFD
The Preamble and Start Frame Delimiter (SFD) indicate the beginning of an Ethernet frame.
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
Layer 2 Data Link
Ethernet operates at layer 2, the data link layer, of the OSI model.
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.
Noted
Ethernet switches operate at ___ of the OSI model, making forwarding decisions based on MAC (Media Access Control) addresses found in the Ethernet frame header.
Layer 2 (Data Link Layer)
Ethernet switches operate at Layer 2 (Data Link Layer) of the OSI model, making forwarding decisions based on ___ found in the Ethernet frame header.
MAC (Media Access Control) addresses
Ethernet switches maintain a ___ that maps MAC addresses to specific switch ports.
MAC address table
The switch learns the MAC addresses of devices by inspecting the ___ of incoming frames and associating them with the corresponding switch port.
source MAC address
If the switch knows the destination MAC address, it forwards the frame only to the specific port towards the ___, reducing unnecessary network traffic.
destination MAC address
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 ___.
unknown unicast
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:
-
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
-
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:
-
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.
-
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.
Noted
A switch builds its MAC address table by recording the ___ and its corresponding ___.
source MAC address, port
MAC address entries are not permanent – they have an aging timer (usually ___ minutes by default).
5
If the receiving host detects a wrong frame, it will ___ it; otherwise, it will ___ the rest of the frame.
discard, read
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)
Destination MAC address
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
source MAC address
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.
Forward the frame out all ports except in the incoming port.
7.2.3 Check Your Understanding - The Access Layer
Ethernet hubs are considered:
◯ a wireless device
◯ a security device
◯ cutting-edge
◯ obsolete
obsolete
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.
Noted
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.
Noted
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.
It forwards the frame out of all ports except for the port at which the frame was received.
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
switch
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
the source Layer 2 address of incoming frames
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
to determine if errors occurred in the transmission and reception
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
determines which interface is used to forward a frame based on the destination MAC address
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
the source MAC address and the incoming port
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
encapsulation
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.
The switch will forward the frame to all ports except port 4.
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
source physical address
frame check sequence
destination physical address
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.
It will discard the frame.
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.
Frame forwarding decisions are based on MAC address and port mappings in the MAC Address table.
