network fundamental ch 7
What is the special value assigned to the first 24 bits of a multicast MAC address transporting an IPv4 packet?
01-00-E
Ethernet supports data bandwidths of the following:
10 Mbps 100 Mbps 1000 Mbps (1 Gbps) 10,000 Mbps (10 Gbps) 40,000 Mbps (40 Gbps) 100,000 Mbps (100 Gbps)
What are the two sizes (minimum and expected maximum) of an Ethernet frame? (Choose two.)
64 bytes 1518 bytes
Full-duplex
Both ends of the connection can send and receive simultaneously.
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.
IEEE 802.3 data encapsulation includes the following:
Ethernet frame Ethernet Addressing Ethernet Error detection
Examine the Source MAC Address
Every frame that enters a switch is checked for new information to learn. It does this by examining the source MAC address of the frame and the port number where 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 that entry. By default, most Ethernet switches keep an entry in the table for 5 minutes.
two variants of cut-through switching:
Fast-forward switching - Fragment-free switching
Fast-forward switching
Fast-forward switching offers the lowest level of latency. Fast-forward switching immediately forwards a packet after reading the destination address. Because fast-forward switching starts forwarding before the entire packet has been received, there may be times when packets are relayed with errors. This occurs infrequently, and the destination NIC discards the faulty packet upon receipt. In fast-forward mode, latency is measured from the first bit received to the first bit transmitted. Fast-forward switching is the typical cut-through method of switching.
Port-based memory
Frames are stored in queues that are linked to specific incoming and outgoing ports. A frame is transmitted to the outgoing port only when all the frames ahead in the queue have been successfully transmitted. It is possible for a single frame to delay the transmission of all the frames in memory because of a busy destination port. This delay occurs even if the other frames could be transmitted to open destination ports.
two types of duplex settings used for communications on an Ethernet network:
Full-duplex Half-duplex
Find the Destination MAC Address
If the destination MAC address is a unicast address, the switch will look for a match between the destination MAC address of the frame and an entry in its MAC address table. If the destination MAC address is in the table, it will forward the frame out the specified port. If the destination MAC address is not in the table, the switch will forward the frame out all ports except the incoming port. This is called an unknown unicast.
Fragment-free switching
In fragment-free switching, the switch stores the first 64 bytes of the frame before forwarding. Fragment-free switching can be viewed as a compromise between store-and-forward switching and fast-forward switching. The reason fragment-free switching stores only the first 64 bytes of the frame is that most network errors and collisions occur during the first 64 bytes. Fragment-free switching tries to enhance fast-forward switching by performing a small error check on the first 64 bytes of the frame to ensure that a collision has not occurred before forwarding the frame. Fragment-free switching is a compromise between the high latency and high integrity of store-and-forward switching, and the low latency and reduced integrity of fast-forward switching.
features of an Ethernet broadcast are as follows:
It has a destination MAC address of FF-FF-FF-FF-FF-FF in hexadecimal (48 ones in binary). It is flooded out all Ethernet switch ports except the incoming port. It is not forwarded by a router.
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?
LLC
Recall that LLC and MAC have the following roles in the data link layer:
LLC Sublayer MAC Sublayer
Layer 2 Ethernet switch uses
Layer 2 MAC addresses to make forwarding decisions.
Half-duplex
Only one end of the connection can send at a time.
Ethernet header includes these
Source MAC address Destination MAC address
Switches use one of the following forwarding methods for switching data between network ports:
Store-and-forward switching Cut-through switching
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.
features of an Ethernet multicast are as follows
There is a destination MAC address of 01-00-5E when the encapsulated data is an IPv4 multicast packet and a destination MAC address of 33-33 when the encapsulated data is an IPv6 multicast packet. There are other reserved multicast destination MAC addresses for when the encapsulated data is not IP, such as Spanning Tree Protocol (STP) and Link Layer Discovery Protocol (LLDP). It is flooded out all Ethernet switch ports except the incoming port, unless the switch is configured for multicast snooping. It is not forwarded by a router, unless the router is configured to route multicast packets.
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. Note: You may also see this field referred to as EtherType, Type, or Length.
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.
LLC Sublayer
This IEEE 802.2 sublayer communicates between the networking software at the upper layers and the device hardware at the lower layers. It places information in the frame that identifies which network layer protocol is being used for the frame. This information allows multiple Layer 3 protocols, such as IPv4 and IPv6, to use the same network interface and media.
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
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.0-
Store-and-forward switching
This frame forwarding method receives the entire frame and computes the CRC. CRC uses a mathematical formula, based on the number of bits (1s) in the frame, to determine whether the received frame has an error. If the CRC is valid, the switch looks up the destination address, which determines the outgoing interface. Then the frame is forwarded out of the correct port.
Ethernet header Destination MAC address
This is the MAC address of the destination device NIC.
Ethernet frame
This is the internal structure of the Ethernet frame.
MAC Sublayer
This sublayer (IEEE 802.3, 802.11, or 802.15 for example) 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.
organizationally unique identifier (OUI).
To ensure this, all vendors that sell Ethernet devices must register with the IEEE to obtain a unique 6 hexadecimal (i.e., 24-bit or 3-byte) code called the
When a vendor assigns a MAC address to a device or Ethernet interface, the vendor must do as follows:
Use its assigned OUI as the first 6 hexadecimal digits. Assign a unique value in the last 6 hexadecimal digits.
direct connection between a router and a host requires
a cross-over connection.
What is auto-MDIX?
a feature that detects Ethernet cable type
what type of address is 01-00-5E-0A-00-02?
an address that reaches a specific group of hosts
IPv6 addresses and Ethernet addresses
are represented using the hexadecimal base sixteen number system.
What feature automatically negotiates the best speed and duplex setting between interconnecting devices?
autonegotiation
What is a function of the MAC sublayer? (Choose three.)
control access to the media checks for errors in received bits uses CSMA/CD or CSMA/CA to support ethernet technology
Which switching method can be implemented using fast-forward switching or fragment-free switching?
cut through switching
What are two methods for switching data between ports on a switch? (Choose two.)
cut through switching store and forward switching
Which part of an Ethernet Frame uses a pad to increase the frame field to at least 64 bytes?
data field
Which part of an Ethernet Frame describes the higher-layer protocol that is encapsulated?
ether type
MAC sublayer is responsible
for data encapsulation and accessing the media.
Which part of an Ethernet frame detects errors in the frame?
frame check sequence
Ethernet header Source MAC address
his is the MAC address of the source device NIC.
Ethernet NICs will also accept frames
if the destination MAC address is a broadcast or a multicast group of which the host is a member.
range of IPv4 multicast addresses
is 224.0.0.0 to 239.255.255.255
encapsulated data
is an IP multicast packet, the devices that belong to a multicast group are assigned a multicast group IP address
Autonegotiation
is an optional function found on most Ethernet switches and NICs. It enables two devices to automatically negotiate the best speed and duplex capabilities.
multicast MAC address
is associated with, and uses addressing information from, the IPv4 or IPv6 multicast address
auto-MDIX feature
is enabled by default on switches running Cisco IOS Release 12.2(18)SE or later. However, the feature could be disabled. For this reason, you should always use the correct cable type and not rely on the auto-MDIX feature
Ethernet
is one of two LAN technologies used today, with the other being wireless LANs (WLANs). Ethernet uses wired communications, including twisted pair, fiber-optic links, and coaxial cables.operates in the data link layer and the physical layer. It is a family of networking technologies defined in the IEEE 802.2 and 802.3 standards
Ethernet multicast frame
is received and processed by a group of devices on the Ethernet LAN that belong to the same multicast group.
MAC address table
is sometimes referred to as a content addressable memory (CAM) table. While the term CAM table is fairly common, for the purposes of this course, we will refer to it as a MAC address table.
Hexadecimal
is used to identify an Ethernet address because a single hexadecimal digit represents four binary bits. Therefore, a 48-bit Ethernet MAC address can be expressed using only 12 hexadecimal values.
crossover cable
is used when connecting like devices, and a straight-through cable is used for connecting unlike devices.
Which function or operation is performed by the LLC sublayer?
it communicates with upper protocol layers
Which two functions or operations are performed by the MAC sublayer? (Choose two.)
it is responsible for media access control it adds a header and trailer to form an OSI layer 2 PDU
Which two characteristics describe Ethernet technology? (Choose two.)
it is supported by IEEE 802.3 standards it uses unique MAC addresses to ensure that data is sent to the appropriate destination
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
frame size
may be larger if additional requirements are included, such as VLAN tagging. VLAN tagging is beyond the scope of this course.
Ethernet switch
may use a buffering technique to store frames before forwarding them. Buffering may also be used when the destination port is busy because of congestion. The switch stores the frame until it can be transmitted.
Ethernet frame size is
minimum 64 bytes and the expected 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 the frame
Ethernet MAC address consists
of a 48-bit binary value
Duplex mismatch is one
of the most common causes of performance issues on 10/100 Mbps Ethernet links. It occurs when one port on the link operates at half-duplex while the other port operates at full-duplex
Gigabit Ethernet ports only
operate in full-duplex
Which two types of memory buffering techniques are used by switches? (Choose two.)
port based memory buffering shared memory buffering
Which part of an Ethernet Frame notifies the receiver to get ready for a new frame
preamble
Ethernet broadcast frame is
received and processed by every device on the Ethernet LAN.
Ethernet MAC address
s a 48-bit address expressed using 12 hexadecimal digits,Because a byte equals 8 bits, we can also say that a MAC address is 6 bytes in length. address is used to identify the physical source and destination devices (NICs) on the local network segment. MAC addressing provides a method for device identification at the data link layer of the OSI model.
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?
switch
Which statement is true about MAC addresses?
the first three bytes are used by the vendor assigned OUI
what happens to runt frames received by a Cisco Ethernet switch?
the frame is dropped
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
the switch acts upon the data as soon as it is received, even if the transmission is not complete. The switch buffers just enough of the frame to read the destination MAC address so that it can determine to which port it should forward out the data.switch looks up the destination MAC address in its switching table, determines the outgoing interface port, and forwards the frame onto its destination through the designated switch port. The switch does not perform any error checking on the frame.
unicast MAC address is
the unique address that is used when a frame is sent from a single transmitting device to a single destination device.
What statement describes a characteristic of MAC addresses?
they must be globally unique
Sometimes the MAC address is referred
to as a burned-in address (BIA) because the address is hard coded into read-only memory (ROM) on the NIC. This means that the address is encoded into the ROM chip permanently.
IEEE 802 LAN/MAN protocols, including Ethernet, use the following
two separate sublayers of the data link layer to operate. They are the Logical Link Control (LLC) and the Media Access Control (MAC)
Auto-MDIX can be re-enabled
using the mdix auto interface configuration command.
range of IPv6 multicast addresses begins
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.
