CNA 101 - Module 7 - Ethernet Switching

Ethernet frame Ethernet Addressing Ethernet Error detection

IEEE 802.3 data encapsulation includes the following:

1. Preamble and SFD (Start Frame Delimiter) 2. Destination MAC Address 3. Source MAC Address 4. Type/Length 5. Data 6. FCS (Frame Check Sum)

Identify the fields

Ethernet Error detection

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

Ethernet Addressing

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.

Frame Check Sequence Field

The Frame Check Sequence (FCS) field (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.

Preamble and Start Frame Delimiter Fields

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.

Type / Length

This 2-byte field identifies the upper layer protocol encapsulated in the Ethernet frame. Common values are, in hexadecimal, 0x800 for IPv4, 0x86DD for IPv6 and 0x806 for ARP.

Source MAC Address Field

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

Destination MAC Address Field

This 6-byte field is the identifier for the intended recipient. As you will recall, 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.

Data Field

This field (46 - 1500 bytes) contains the encapsulated data from a higher layer, which is a generic Layer 3 PDU, or more commonly, 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.

Ethernet frame

This is the internal structure of the Ethernet frame.

The source Layer 2 address of incoming frames.

What addressing information is recorded by a switch to build its MAC address table?

64-1518

What are the two sizes (minimum and maximum) of an Ethernet frame?

1. It is responsible for Media Access Control. 2. It adds a header and trailer to form an OSI Layer 2 PDU.

What are two functions or operations are performed by the MAC sublayer?

The frame is dropped.

What happens to runt frames received by a Cisco Ethernet switch?

1. Controls access to the media. 2. checks for errors in received bits. 3. Uses CSMA/CD or CSMA/CA to support Ethernet technology.

What is a function of the MAC sublayer?

A feature that detects Ethernet cable type.

What is auto-MDIX?

01-00-5E-xx-xx-xx

What is the special value assigned to the first 24 bits of a multicast MAC address?

They must be globally unique.

What statement describes a characteristic of MAC addresses?

An address that reaches a specific group of hosts.

What type of address is 01-00-5E-0A-00-02?

It will discard the frame.

What will a host on an Ethernet network do if it receives a frame with a destination MAC address that does not match its own MAC address?

LLC

Which data link sublayer controls the network interface through software drivers?

LLC

Which data link sublayer works with the upper layers to add application information for delivery of data to higher level protocols?

It communicates with upper protocol layers.

Which function or operation is performed by the LLC sublayer?

Switch

Which network device has the primary function to send data to a specific destination based on the information found in the MAC address table?

Switch

Which network device makes forwarding decisions based on the destination MAC address that is contained in the frame?

EtherType Field

Which part of an Ethernet Frame describes the higher-layer protocol that is encapsulated?

Preamble

Which part of an Ethernet Frame notifies the receiver to get ready for a new frame?

Data Field

Which part of an Ethernet Frame uses a pad to increase the frame field to at least 64 bytes?

FCS (Frame Check Sequence)

Which part of an Ethernet frame detects errors in the frame?

The first three bytes are used by the vendor assigned OUI.

Which statement is true about MAC addresses?

1. It is supported by IEEE 802.3 standards. 2. It uses the CSMA/CD access control method.

Which two characteristics describe Ethernet technology?