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Ethernet

34 important questions on Ethernet

What does the Ethernet MAC sublayer do?

MAC constitutes the lower sublayer of the data link layer. MAC is implemented by hardware, typically in the computer NIC. The specifics are listed in the IEEE 802.3 standards.

What are the two primary responsibilities of the Ethernet MAC sublayer?

Data encapsulation

Media access control

Explain the data encapsulation process of the Ethernet MAC sublayer.

The data encapsulation process includes frame assembly before transmission, and frame disassembly upon reception of a frame. In forming the frame, the MAC layer adds a header and trailer to the network layer PDU.

What are the three primary functions of the data encapsulation process of the Ethernet MAC sublayer?

Data encapsulation provides three primary functions:

Frame delimiting - The framing process provides important delimiters that are used to identify a group of bits that make up a frame. These delimiting bits provide synchronization between the transmitting and receiving nodes.

Addressing - The encapsulation process contains the Layer 3 PDU and also provides for data link layer addressing.

Error detection - Each frame contains a trailer used to detect any errors in transmissions.

Explain media access control (responsibility of the MAC sublayer)

The second responsibility of the MAC sublayer is media access control. Media access control is responsible for the placement of frames on the media and the removal of frames from the media. As its name implies, it controls access to the media. This sublayer communicates directly with the physical layer.

What is a contention-based method of networking?

The underlying logical topology of Ethernet is a multi-access bus; therefore, all nodes (devices) on a single network segment share the medium. Ethernet is a contention-based method of networking. A contention-based method means that any device can try to transmit data across the shared medium whenever it has data to send.

What is the minimum and maximum Ethernet frame size?

The minimum Ethernet frame size is 64 bytes and the maximum is 1518 bytes. This includes all bytes from the Destination MAC Address field through the Frame Check Sequence (FCS) field. The Preamble field is not included when describing the size of a frame.

When is a frame sonsidered a "collision fragment" or "runt frame", and what happens to it?

Any frame less than 64 bytes in length is considered a “collision fragment” or “runt frame” and is automatically discarded by receiving stations.

When is a frame considered a "jumbo" or "baby giant frame"

Frames with more than 1500 bytes of data are considered “jumbo” or “baby giant frames”.

When is a frame considered invalid?

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 and are therefore considered invalid.

What is an Ethernet MAC address?

An Ethernet MAC address is a 48-bit binary value expressed as 12 hexadecimal digits (4 bits per hexadecimal digit).

How is Hexadecimal represented in text?

Hexadecimal is usually represented in text by the value preceded by 0x (for example 0x73) or a subscript 16. Less commonly, it may be followed by an H(for example 73H). However, because subscript text is not recognized in command line or programming environments, the technical representation of hexadecimal is preceded with "0x" (zero X). Therefore, the examples above would be shown as 0x0A and 0x73 respectively.

Name two rules the IEEE requires a vendor to follow hen selling ethernet devices.

All MAC addresses assigned to a NIC or other Ethernet device must use that vendor's assigned OUI as the first 3 bytes.

All MAC addresses with the same OUI must be assigned a unique value in the last 3 bytes.

What is a burned-in address?

The MAC address is often referred to as a burned-in address (BIA) because, historically, this address is burned into ROM (Read-Only Memory) on the NIC. This means that the address is encoded into the ROM chip permanently.

What command can be used to identify the MAC address of an Ethernet adapter on a Windows host?

On a Windows host, the ipconfig /all command can be used to identify the MAC address of an Ethernet adapter. In Figure 1, notice the display indicates the Physical Address (MAC) of the computer to be 00-18-DE-DD-A7-B2. If you have access, you may wish to try this on your own computer. On a MAC or Linux host, the ifconfig command is used.

What does a broadcast packet contain?

A broadcast packet contains a destination IPv4 address that has all ones (1s) in the host portion. This numbering in the address means that all hosts on that local network (broadcast domain) will receive and process the packet. Many network protocols, such as DHCP and ARP, use broadcasts.

What does a multicast MAC Address do?

Multicast addresses allow a source device to send a packet to a group of devices. Devices that belong to a multicast group are assigned a multicast group IP address. The range of IPv4 multicast addresses is 224.0.0.0 to 239.255.255.255. The range of IPv6 multicast addresses begin with FF00::/8. Because multicast addresses represent a group of addresses (sometimes called a host group), they can only be used as the destination of a packet. The source will always be a unicast address.

When would Multicast addresses be used?

Multicast addresses would be used in remote gaming, where many players are connected remotely but playing the same game. Another use of multicast addresses is in distance learning through video conferencing, where many students are connected to the same class.

What is the MAC address table used for?

Unlike legacy Ethernet hubs that repeat bits out all ports except the incoming port, an Ethernet switch consults a MAC address table to make a forwarding decision for each frame. In the figure, the four-port switch was just powered on. It has not yet learned the MAC addresses for the four attached PCs.

Switches use one of the following forwarding methods for switching data between network ports:

Store-and-forward switching

Cut-through switching

Explain Memory Buffering on Switches

An Ethernet switch may use a buffering technique to store frames before forwarding them. Buffering may also be used when the destination port is busy due to congestion and the switch stores the frame until it can be transmitted.

There are two types of duplex settings used for communications on an Ethernet network:

Full-duplex – Both ends of the connection can send and receive simultaneously.

Half-duplex – Only one end of the connection can send at a time.

There are two primary addresses assigned to a device on an Ethernet LAN:

Physical address (the MAC address) – Used for Ethernet NIC to Ethernet NIC communications on the same network.

Logical address (the IP address) – Used to send the packet from the original source to the final destination.

What is the purpose of Layer 2 addresses?

Layer 2 or physical addresses, like Ethernet MAC addresses, have a different purpose. These addresses are used to deliver the data link frame with the encapsulated IP packet from one NIC to another NIC on the same network. If the destination IP address is on the same network, the destination MAC address will be that of the destination device.

What does a layer 2 ethernet frame contain?

Destination MAC address – This is the MAC address of the file server’s Ethernet NIC.

Source MAC address – This is the MAC address of PC-A’s Ethernet NIC.

What does a layer 3 IP packet contain?

Source IP address – This is the IP address of the original source, PC-A.

Destination IP address – This is the IP address of the final destination, the file server.

Recall that every device with an IP address on an Ethernet network also has an Ethernet MAC address. When a device sends an Ethernet frame, it contains these two addresses:

Destination MAC address - The MAC address of the Ethernet NIC, which will be either the MAC address of the final destination device or the router.

Source MAC address - The MAC address of the sender’s Ethernet NIC.

To determine the destination MAC address, the device uses ARP. ARP provides two basic functions:

Resolving IPv4 addresses to MAC addresses

Maintaining a table of mappings

What is the ARP table?

Resolving IPv4 Addresses to MAC Addresses
When a packet is sent to the data link layer to be encapsulated into an Ethernet frame, the device refers to a table in its memory to find the MAC address that is mapped to the IPv4 address. This table is called the ARP table or the ARP cache. The ARP table is stored in the RAM of the device.

ARP messages are encapsulated directly within an Ethernet frame. There is no IPv4 header. The ARP request message includes:

Target IPv4 address – This is the IPv4 address that requires a corresponding MAC address.

Target MAC address - This is the unknown MAC address and will be empty in the ARP request message.

Only the device with an IPv4 address associated with the target IPv4 address in the ARP request will respond with an ARP reply. The ARP reply message includes:

Sender’s IPv4 address – This is the IPv4 address of the sender, the device whose MAC address was requested.

Sender’s MAC address – This is the MAC address of the sender, the MAC address needed by the sender of the ARP request.

The ARP reply is encapsulated in an Ethernet frame using the following header information:

Destination MAC address – This is the MAC address of the sender of the ARP request.

Source MAC address – This is the sender of the ARP reply’s MAC address.

Type - ARP messages have a type field of 0x806. This informs the receiving NIC that the data portion of the frame needs to be passed to the ARP process.

What happens when an ARP reply is received?

Only the device that originally sent the ARP request will receive the unicast ARP reply. Once the ARP reply is received, the device will add the IPv4 address and the corresponding MAC address to its ARP table. Packets destined for that IPv4 address can now be encapsulated in frames using its corresponding MAC address.

What is ARP spoofing?

In some cases, the use of ARP can lead to a potential security risk known as ARP spoofing or ARP poisoning. This is a technique used by an attacker to reply to an ARP request for an IPv4 address belonging to another device, such as the default gateway, as shown in the figure. The attacker sends an ARP reply with its own MAC address. The receiver of the ARP reply will add the wrong MAC address to its ARP table and send these packets to the attacker.

The question on the page originate from the summary of the following study material:

CCNA 1

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