Chapter 7

Type/Length

This 2 byte field identifies the upper layer protocol encapsulated in the Ethernet frame. common values are in hexadecimal. -Referred to as EtherType, Type, or Length.

Destination MAC address Field

-6 byte field of the identified recipient to the frame. contains the MAC address and can be unicast, multicast and broadcast address.

Characteristics of a MAC address

-Must be globally unique.

Frame Forwarding Method for Cisco Switches

-Two different methods are used and are used based on specific situations.

Unicast MAC Address

-the unique address used when a frame is sent from a single transmitting device to a single destination device. In other words the other machines that are not the intended location will not see the frame. This is done because a destination IP is sent in the frame along with the MAC address. The router will send the information directly to the intended machine. Address Resolution Protocol (ARP) determines the address for IPv4 while Neighbor Discovery (ND) does this for IPv6.

Which function or operation is performed by the LLC sublayer

It communicates with upper protocol layers.

MAC Address and Hexadecimal

MAC Addresses are displayed in Hexadecimal. -Represented by 0x(e.g., 0x73) to distinguish between decimal and hexadecimal values in documentation. -Hexadecimal may also be represented by a subscript 16, or the hex number followed by an H(e.g., 73H)

Frame Processing

MAC addresses are sometimes referred to as burnt in address (BIA) because the address is hard coded into read-only memory (ROM) on the NIC. Software exists to change the MAC address of the machine. This is useful when attempting t gain access to a network that filters based on the BIA.

Preamble and Start Frame

-First 8 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.

Ethernet Encapsulation

-One of two LAN technologies the other being Wireless LAN or WLAN. -Ethernet operates in the dat link layer and the physical layer. -part of the family of networking technologies defined in IEEE 802.2 and 802.3 standards. Supports speeds from 10 Mbps- 100,000 ( 100 Gbps)

Ethernet Frame Order

1.Preamble and SFD, 2.Destination MAC address, 3.Source MAC address, 4.Type/Length, 5.Data, FCS

Ethernet MAC address

48-bit binary value expressed as 12 hexadecimal digits also said to be 6 bytes in length. 8bits = 1 byte. -MAC addresses are unique to the Ethernet device or Ethernet interface. -All vendors that sell Ethernet devices must register with the IEEE to obtain a unique 6 hexadecimal code called the organizationally unique identifier (OUI). -First 6 hexadecimal digits (3 bytes) must be the OUI identifier and the last 6 hexadecimal digits (3 bytes) are the unique value assigned by the vendor.

Source MAC address Field

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

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

An address that reaches a specific group of hosts.

Data Link Layer

-802.2 IEEE Ethernet - Logical Link Layer LLC and Media Access control MAC

Frame check Sequence Field

- 4 bytes of data used for frame errors. 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.

Physcial

-802.3 IEEE Ethernet

Switch Fundamentals

-A layer 2 Ethernet switch uses layer 2 MAC addresses to make forwarding decisions. -The switch is unaware of the protocol portion of the frame, it simply sends or discards the frame solely on the Layer 2 Ethernet MAC addresses. -An Ethernet switch examines the MAC address table to make a forwarding decision for each frame.

Filtering Frames

-A switch examines the and populates the MAC address table by examining the source MAC address of every frame. When the Switch also has the destination MAC address listed on the MAC address table, the switch can forward the data on a specific port directly to the intended host MAC address rather than to multiple other Host in an effort to find the correct host.

which two functions or operations are preformed by the MAC sublayer?

-Adds a header and trailer to form an OSI layer 2 PDU - It is responsible for the Media Access Control.

Multicast MAC Address

-All devices are sent the frame, but only some of the devices are the intended destination for the frame. -Destination MAC address 01-00-5E for IPv4 packets and 33-33 when data is for IPv6 packets. -Flooded from all external ports unless the switch is configured for multicast snooping. - Not forwarded by a router, unless the router is configured to route multicast packets. -devices that belong to a multicast group (host group) are assigned a multicast group ip address.

Data Link Sublayers

-LLC Sublayer communicates between the networking software at the upper layers and the device hardware at the lower layers. -Places information in the frame that identifies which network layer protocol is being used for the frame. -The information allows multiple Layer 3 protocols such as IPv4 and IPv6 to use the same network interface and media. -MAC sublayer- This sublayer (IEEE 802.3, 802.11 or 802.15) is implemented in hardware and is responsible for data encapsulation and media access control. It provides data link layer addressing and is integrated with various physical layer technologies.

Two Characteristics of Ethernet technology

-Supported by IEEE 802.3 -Uses unique MAC addresses to ensure that data is sent to the appropriate destination.

Broadcast MAC Address

-The Frame is sent to all devices on the LAN, they all receive and process the frames. -Destination Address MAC is always FF-FF-FF-FF-FF-FF -Flooded out all Ethernet switch ports expect incoming ports. -Not forwarded by a router. The encapsulated data is an IPv4 broadcast packet, meaning the packet contains a destination IPv4 address that has all ones in the host portion.

Auto-MDIX (Automatic Medium-dependent interface crossover)

-The switch can automatically detect the type of cable attached to the port and configures the interfaces accordingly when enabled, regardless of the type of device on the other end of the connection. -enabled by default and can be disabled and re-enabled by using the mdix auto interface configuration command.

Switch Learning and Forwarding

-Using a MAC address table the switch examines the frame on the received port and forwards the frame to the match destination MAC address in the frame with the entry on it's MAC address table. -The switch examines the source MAC address of the frame and the port number when the frame entered the switch. If the source MAC address does not exist, it is added to the table along with the incoming port number. If the source MAC address does exist the switch updates the refresh timer for the entry in the table. Most switches keep the entry in the table for 5 minutes.

Functions of the MAC sublayer

-controls access o the media - checks for errors in received bits -uses CSMA/CD or CSMA/CA to support Ethernet technology.

Memory buffering on Switches

-storing frames before forwarding them. Port-based memory: -Frames are stored in queues that are linked to specific incoming and outgoing ports. -Frames are forwarded only have the proceeding frame has been successfully transmitted. Shared Memory: -Deposits all frames into a common memory buffer shared by all switch ports and the amount of buffer memory required by a port is dynamically allocated. -The frames in the buffer are dynamically linked to the destination port enabling a packet to be received on one port and then transmitted on another port, without moving it to a different queue.

Data Field

Contains the encapsulated data from a higher layer which is a generic layer 3 PDU or more commonly, and IPv4 packet. -Frames must be 64 bytes long, so padding can occur when dealing with a small packet. This padding will extend the size to the minimum 64 bytes.

Duplex and Speed Settings

Full-duplex: both ends of the connection can send and receive simultaneously. half-duplex: only one side of the connection will send data while the other side must wait to receive the data before sending it's own data. Auto-negotiation: an optional function found on most Ethernet switches and NICs, allowing two devices to automatically negotiate the best speed and duplex capabilities. Full-duplex is chosen if both devices have the capability along with their highest common bandwidth. -Duplex mismatch is one of the most common causes of performance issues on 10/100Mbps Ethernet links, it occurs when one switch is able to send full-duplex but the other switch is unable to do so and the full-duplex switch continues to send messages whenever it wants without waiting for the half-duplex switch to respond.

Mac Sublayer

Responsible for data encapsulation and accessing the media. -Ethernet frame is the internal structure of the Ethernet frame. -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 Local area network.

Ethernet frame fields

The Minimum Ethernet frame size is 64 bytes and expected maximum is 1518 bytes. -It includes all bytes from the destination MAC address field through the frame check sequence field. -Frames can be larger if containing VLAN tagging. -Frames less than 64 bytes in length are considered a collision fragment or runt frame and automatically discarded by the receiving station. -Frames larger or smaller are dropped by receiving stations. -Jumbo frames are supported by fast Ethernet and Gigabit Ethernet switches and NICs.

What statement is true about a MAC address

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

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

The source layer 2 address of incoming frames

Cut- through switching

This frame forwarding method forwards the frame before it is entirely received. At a minimum, the destination address of the frame must be read before the frame can be forwarded. -Variations include Fast-Forward switching: offers the lowest level of latency. Packet is immediately forwarded after reading the destination address. When errors are sent via this method, the NIC is responsible for discarding packets with errors. Fragment-free switching: The switch stores the first 64 bytes of the frame before forwarding and checks it for errors. Since most network errors occur in the first 64 bytes of a frame, this method is splitting the difference between latency and store-and-forward switching.

Store-and-forward switching

This frame forwarding method receives the entire frame and computes the CRC using a mathematical formula, based on the number of bits in the frame, to determine whether the received frame has an error. -If CRC is valid the switch looks up the destination address, which determines the outgoing interface. Then the frame is forwarded out the correct port. -Provides for quality of service (QofS) analysis on converged networks where frame classification for traffic prioritization is necessary. -A big advantage is that if a frame has errors the switch will discard the frame, reducing the amount of bandwidth consumed by corrupt data.

What is Auto-MDIX

a feature that detects the Ethernet cable type