Data Link Layer Overview

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Questions and Answers

Which topology connects two endpoints and routes all communications through a central device?

Mesh

Point-to-point (correct)

Hybrid

Hub and Spoke

What characteristic is associated with full-duplex communication?

Devices can transmit and receive simultaneously. (correct)

All devices must share a single transmission time.

Devices can only send or receive one at a time.

Shared medium usage is strictly alternated.

Which of the following topologies employs a chain of interconnected end systems terminated on each end?

Bus (correct)

Ring

Mesh

Star

In which access control method do all nodes compete for the use of the medium?

Contention-based access Signup and view all the answers

What is a defining feature of a mesh topology?

Systems are directly connected to every other system. Signup and view all the answers

In which access control method does each node have its designated time on the medium?

Controlled access Signup and view all the answers

What is a primary benefit of star and extended star topologies?

They are easy to troubleshoot and scalable. Signup and view all the answers

Which technology is primarily associated with half-duplex communication?

Wireless LANs Signup and view all the answers

What type of address is always used as the source in multicast communication?

Unicast address Signup and view all the answers

How does a Layer 2 Ethernet switch make forwarding decisions?

Based on Layer 2 MAC addresses Signup and view all the answers

What happens when a switch receives a frame with a source MAC address that already exists in its table but on a different port?

The existing entry is updated with the new port number Signup and view all the answers

What happens to the MAC address table when the switch is powered on?

It is empty until frames are received Signup and view all the answers

What is the function of the refresh timer in a MAC address table?

To automatically delete inactive MAC addresses Signup and view all the answers

What characterizes a multicast IP address?

It represents a group of addresses Signup and view all the answers

In what memory format is the MAC address table referred to?

Content Addressable Memory (CAM) Signup and view all the answers

What occurs when a switch checks a frame's source MAC address upon receipt?

It adds or updates the source MAC in its table Signup and view all the answers

What is a major advantage of discarding frames with errors in networking?

Reduces the used bandwidth Signup and view all the answers

What is a disadvantage of the cut-through method of switching?

No error checking before forwarding packets Signup and view all the answers

How does port-based memory buffering in switches operate?

Stores frames in linked queues specific to incoming and outgoing ports Signup and view all the answers

What unique advantage does shared memory buffering provide?

Enhances transmission of larger frames with fewer drops Signup and view all the answers

What is the primary function of the data link layer?

To control how data is placed and received on the media Signup and view all the answers

Which type of switching method offers the lowest level of latency?

Cut-through switching Signup and view all the answers

What consequence can result from a busy destination port in port-based buffering?

Delayed transmission of all other frames in memory Signup and view all the answers

Which of the following describes the LLC sublayer?

Facilitates communication between networking layers and hardware Signup and view all the answers

What is one characteristic of asymmetric switching as mentioned in the content?

Allows different data rates on different ports Signup and view all the answers

What information does an Ethernet frame contain?

Source and destination MAC addresses Signup and view all the answers

Which networking technique reduces the amount of bandwidth used but may increase latency?

Store-and-forward switching Signup and view all the answers

How does a switch build its MAC address table?

By analyzing incoming frames to learn the source MAC addresses Signup and view all the answers

What is the role of the MAC sublayer in the data link layer?

To facilitate media access control and data encapsulation Signup and view all the answers

What characteristic distinguishes WAN from LAN regarding media access control methods?

LANs typically have lower latency compared to WANs Signup and view all the answers

Which statement best describes the function of error detection in the data link layer?

It checks for corrupted frames and rejects them before passing to upper layers Signup and view all the answers

What is the significance of switch speeds and forwarding methods in a Layer 2 switch?

They influence how quickly data frames can be processed and forwarded Signup and view all the answers

What is the purpose of a unicast MAC address in Ethernet communication?

To send frames from a single transmitting device to a single destination device Signup and view all the answers

What MAC address is used in an Ethernet broadcast frame?

FF-FF-FF-FF-FF-FF Signup and view all the answers

Which protocol is used by a source host to determine the destination MAC address for an IPv4 address?

Address Resolution Protocol (ARP) Signup and view all the answers

How does an Ethernet switch handle broadcast frames?

It floods the frame out all ports except the incoming port Signup and view all the answers

What destination MAC address is used for IPv4 multicast packets?

01-00-5E Signup and view all the answers

What is true about the source MAC address when sending an Ethernet frame?

It must always be a unicast address Signup and view all the answers

What happens when a multicast frame is received by a device not part of the multicast group?

It ignores the packet Signup and view all the answers

Which of the following statements accurately describes an Ethernet multicast frame?

It contains a MAC address of 33-33 for IPv6 multicast packets Signup and view all the answers

What is the primary role of autonegotiation in Ethernet switches?

To automatically negotiate the best speed and duplex capabilities Signup and view all the answers

What issue can arise from a duplex mismatch on Ethernet links?

Performance issues on the link Signup and view all the answers

Which statement about duplex settings is accurate?

It is best practice to configure both ports as full-duplex Signup and view all the answers

How does auto-MDIX benefit connection setups between devices?

It automatically detects and configures cable types for connections Signup and view all the answers

In what scenario is a crossover cable required?

For direct connections between a router and a host Signup and view all the answers

Why is it important for duplex and bandwidth settings to match on a connection?

To prevent speed degradation on the link Signup and view all the answers

What is a key feature of Gigabit Ethernet ports regarding duplex operation?

They only operate in full-duplex mode Signup and view all the answers

What could be a consequence of improperly configuring only one side of a link?

Performance issues due to duplex mismatch Signup and view all the answers

Study Notes

Data Link Layer

The data link layer prepares communication for transmission on specific media.
It compares media access control methods on WAN and LAN topologies.

Ethernet Frames

Ethernet sublayers are related to frame fields.
Ethernet MAC addresses are described.
Switches build MAC address tables and forward frames.
Switch forwarding methods and port settings on Layer 2 switch ports are noted.

Purpose of the Data Link Layer

Enables upper layers to access the media.
Accepts Layer 3 packets and controls how data is placed and received on the media.
Exchanges frames between endpoints over the network media.
Receives encapsulated Layer 3 packets and sends them to proper upper-layer protocols.
Performs error detection and rejects corrupt frames.

Data Link Sublayers

LLC Sublayer: Communicates between networking software and device hardware. Places information in the frame that identifies which network layer protocol is being used.
MAC Sublayer: Responsible for data encapsulation and media access control. Its internal structure is the Ethernet frame. Ethernet frames have a source and destination MAC address, and a frame check sequence to ensure accuracy. MAC sublayer also handles media access control (allowing devices to communicate over a shared medium).

Providing Access to Media

Packets exchanged between nodes may experience numerous data link layer and media transitions.
At each hop, a router accepts a frame, de-encapsulates it to a packet, re-encapsulates it into a new frame, and forwards it.

WAN Topologies

Three common physical WAN topologies include:
- Point-to-point: simplest, permanent link between two endpoints.
- Hub and spoke: star topology where a central site links branch sites via point-to-point links.
- Mesh: every end system interconnected to every other for high availability, requiring many connections.
Hybrid: a variation or combination of any topologies.

LAN Topologies

End devices are typically interconnected using a star or extended star topology.
Early Ethernet and Legacy Token Ring provide bus and ring topologies.

Half and Full Duplex Communication

Half-duplex: One device sends or receives at a time on a shared medium. Used on WLANs and legacy bus topologies with Ethernet hubs.
Full-duplex: Both devices transmit and receive simultaneously on a shared medium. Ethernet switches operate in full-duplex mode.

Access Control Methods

Contention-based: All nodes in half-duplex compete for medium access. Examples include legacy bus-topology Ethernet and wireless LANs (CSMA/CD and CSMA/CA.)
Controlled access: Deterministic access; each node has its predetermined time on the medium, used in legacy networks like Token Ring and ARCNET.

CSMA/CD and CSMA/CA

CSMA/CD: Devices detect collisions, wait randomly, and retransmit data. Used by legacy Ethernet.
CSMA/CA: Devices include time duration and use collision avoidance to control when sending data. Used by IEEE 802.11 WLANs.

Ethernet Frame Fields

Preamble & SFD: Notification to the receiving node.
Destination MAC Address: Receiving node’s physical address.
Source MAC Address: Sending node’s physical address.
Type/Length: Information about upper layer protocols, data about its purpose.
Data: Encapsulated data from upper layers.
FCS: Error detection using cyclic redundancy check.

Ethernet MAC Addresses

Unicast: Unique address used for single-to-single communication.
Broadcast: Destination MAC address of FF-FF-FF-FF-FF-FF, processed by every device on the LAN.
Multicast: Destination MAC address of 01-00-5E and 33-33, processed by a group of devices in the same multicast group.
MAC address table identifies connected devices for switch to route data from a device’s MAC address.

The MAC Address Table

Properties: Table is empty at start-up, referred to as content addressable memory (CAM).
Switches use MAC addresses to make forwarding decisions.
Learning: Switches examines source MAC addresses to determine incoming port, and updates the table based on those connections.

Switch Learning and Forwarding

Learning: Switch checks frames and adds the source MAC address to the table.
- If source MAC is not found, adds to the table
- If exists, updates the entry and refresh timer.
Forwarding: If the destination MAC is in the table, the switch forwards to the correct port.
If the destination MAC is not in the table, forwards to all ports except the incoming port

Port-based memory and shared memory

Port-based memory: Stores frames in queues linked to specific incoming and outgoing ports, transmitting each frame in the queue in order.
Shared memory: Stores frames into a common buffer, all ports share for quick and efficient packet transfer, dynamically allocated, allowing transmission with minimal delay.

Switch Speeds and Forwarding Methods

Store-and-forward switching: Waits for complete frame before checks for errors. Higher latency.
Cut-through switching: Forwards quickly, as soon as destination address is found; Lower latency, but no error checking.
Fast forwarding: Lowest latency cut-through method. Only checks if destination is available on the MAC address table.
Fragment-free: Stores and forwards when the frame is completely checked for errors.

Duplex and Speed Settings

Duplex: describes whether communications can occur in both directions simultaneously (full-duplex) or whether only one device can send at any given time (half-duplex).
Speed: refers to the data transfer rate.
Auto-negotiation: automatically configures the best speed and duplex capabilities for both computers, essential for optimal performance. Note: Gigabit Ethernet ports only operate in full-duplex mode.

Auto-MDIX

Automatic medium-dependent interface crossover that automatically detects cable type and configures interfaces, eliminating the need for specific cables, like crossover or straight-through.

End

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Description

This quiz explores the Data Link Layer, focusing on its role in preparing communication for transmission over different media. It covers Ethernet frames, MAC addresses, and the functionalities of the LLC and MAC sublayers. Understand how data is managed at Layer 2 and the principles behind error detection and frame exchanging.