CNT 140 Chapter 7 Ethernet Switching
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
Ethernet Addressing
The Ethernet frame includes a frame check sequence (FCS) trailer used for error detection
Ethernet Error detection
Ethernet communications standard: IEE 802.3u
Fast Ethernet
Which IP addresses are represented using the decimal base ten number system and the binary base 2 number system ?
IPv4
If you see the hexadecimal value of 0x86DD in the type field in a Ethernet frame, what upper layer protocol is encapsulated in the frame ?
IPv6
Which addresses are represented using hexadecimal base sixteen number system ?
IPv6 and Ethernet / MAC addresses
If the source MAC address of a frame does not exit in a switch MAC address table/ CAM table , what does it do with that frames information ?
If the source MAC address does not exist, the source MAC address will be add to the MAC/CAM table along with the incoming port
When a devices receives a frame and that frames destination MAC address does not match the devices MAC address that is in RAM, what happens to the frame ?
If there is no match, the device discards the frame.
True or False ? Most CISCO switches and Ethernet NICs default to autonegotiation for speed and duplex ?
True
What is the interface configuration command used to enable auto-MDIX on a switch ?
mdix auto
8 bits equals how many bytes ?
1 byte
It the destination unicast MAC address is not in the table , and the switch has to forward it all ports , what kind of unicast MAC address is this known as ?
An unknown unicast MAC address
What feature automatically negotiates the best speed and duplex setting between interconnected devices ? Auto-MDIX autobots autonegotiation autotune
Autonegotiation
What is autonegotiation ?
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. Full-duplex is chosen it both devices have the capability along with their highest common bandwidth.
What are some ways to avoid having a duplex mismatch ?
Both sides of a link should have autonegotiation on, or both sides should have it off. Best practice is to configure both Ethernet switch ports as full-duplex
What type of addresses in Ethernet LANs can only be used as the destination of a packet and never a source address ?
Broadcast and Multicast address
What does BIA stand for ?
Burned in address or MAC address
Sometimes the MAC address is referred to as what ?
Burned-in address (BIA)
The MAC address table is sometimes referred to as what ?
Content Addressable Memory (CAM)
what is a function of the MAC sublayer ( choose three ) ? Controls access to the media Checks for errors in received bits Uses CSMA/CD or CSMA/CA to support Ethernet Technology communicates between software at the upper layers and the device hardware at the lower layer allows multiple Layer 3 protocols to use the same network interface and media
Controls access to the media checks for errors in received bits Uses CSMA/CD or CSMA/CA to support Ethernet Technology
A direct connection between a router and a host requires what type of cable ?
Cross-over cable
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.
Cut-through switching
Which switching method can be implemented using fast-forward switching or fragment-free switching ?
Cut-through switching
What protocol for IPv4 is an example of a protocol that uses Ethernet and IPv4 broadcast address ?
DHCP
This files (46-1500 bytes ) contains the encapsulated data from a higher layer , which is a generic layer 3 PDU, or more commonly, an IPv4 packets. All frames must be at least 64 bytes long. If a small packet is encapsulated, additional bits called pas are used to increase the size of the frame to this minimum size
Data field
Which part of an Ethernet Frame uses a pad to increase field to at least 64 bytes ? Ethertype Preamble Start of Frame Delimiter Data Field
Data field
Why is it a good thing that a switch discards a frame the fails the error check, this is known as a frame fragment ?
Discarding frames with errors reduces the amount of bandwidth consumed by corrupt data .
What is one of the most common causes of performance issues on 10/100 Mbps Ethernet links ?
Duplex mismatch
What are some reasons for why a duplex mismatch might occur ?
Duplex mismatch occurs when one or both ports on a link are reset, and the auonegotiation process does not result in both link partners having the same configuration. It also occurs when users reconfigure one side of a link and forget to reconfigure the other.
IEEE 802.3 or Ethernet data encapsulation includes the following:
Ethernet Frame Ethernet Addressing Ethernet Error Detection
Any device that is the source of destination of an Ethernet frame, will have an _______ and therefore, a _____ ?
Ethernet NIC and MAC address
What are the two LAN technologies used today ?
Ethernet and Wireless LANS
The range of IPv6 multicast addresses begins with what ?
FF00::/8
This switching method offers the lowest level of latency. It immediately forwards a packet after reading the destination address. Because this switching method starts forwarding before the entire packet has been received, there may be times when the 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. This switching method is the typical cut-through method of switching .
Fast-Forward switching
What are the two variants of cut-though switching ?
Fast-forward switching Fragment-free switching
In this switching method, the switch stored the first 64 bytes of the frame before forwarding . This switching can be viewed as a compromise between store-and-forward switching and fast-forward switching. The reason this switching method stores only the first 64 bytes of the frame is that most network errors and collisions occur during the first 64 bytes. The switching method 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. This switching method 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.
Fragment-Free switching
Which part of an Ethernet frame detects errors in the frame ? Preamble Start of Frame Delimiter Frame check Sequence
Frame Check Sequence
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 dat 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.
Frame Check Sequence Field
Both ends of the connection can send and receive simultaneously .
Full-duplex
Gigabit Ethernet port operate only in what duplex mode ?
Full-duplex
What are the two types of duplex setting used for communications on an Ethernet network ?
Full-duplex Half-duplex
Only one end of the connection can send at a time.
Half-duplex
What is a major advantage of using store-and -forward switching ?
It determines if a frame has errors before propagating the frame . When an error is detected in a frame , the switch discards the frame.
When does a duplex mismatch occur ?
It occurs when one port on the link operates at half-duplex while the other port operates at full-duplex, as shown in the figure.
When a devices a frame and that frames destination MAC address matches the devices MAC address that is in RAM, what happens to the frame ?
It passes the frame up the OSI layers, where the de-encapsulation process takes place.
What does MAC addressing provide at the data link layer of the OSI model ?
It provides a method for device identification.
How does a Ethernet switch handle multiple incoming frames ?
It uses memory buffering to store frames before forwarding them.
If the size of a transmitted frame is less than the minimum, or greater than the maximum, the receiving device what with the frame ?
It will drop the frame
It the destination unicast MAC address is in the table , what does the Switch do with the frame ?
It will forward the frame out the specified port associated with the destination unicast MAC address of the frame.
This sublayer (IEEE 802.., 802.11, 802.15 for example ) is implemented in hardware and is responsible for data encapsulation and media access control. It provides data link layers addressing and is intergrade with various physical layer technologies.
MAC Sublayer
Which two Ethernet standards support Jumbo frames ?
Most Fast Ethernet and Gigabit Ethernet
Frames are stored in queues that are linked to specific incoming and outgoing ports. A frame is transited 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.
Port-based memory
What are the two memory buffering techniques used by an Ethernet switch ?
Port-based memory Shared memory
Which two types of memory buffering techniques are used by switches ? Long term memory buffering port-based memory buffering shared memory buffering short-term memory buffering
Port-based memory buffering shared memory buffering
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.
Shared memory
In asymmetric switching , which memory buffer method is used ?
Shared memory buffering also results in the ability to store larger frames with potentially fewer dropped frames. This is important with asymmetric switching which allows for different data rates on different ports such as when connecting a server to a 10 Gbps switch port and PCs to 1 Gbps ports.
How do you show the MAC address table on a switch ?
Show mac address-table
What are the two frame forwarding methods used by Cisco switches ?
Store-and-Forward Cut-through switching
Which frame forwarding method is required for QoS analysis on converged networks ?
Store-and-forward switching is required for quality of service analysis on converged networks where frame classification for traffic prioritization is necessary .
This frame forwarding method receives the entire frame and computes the CRC. Cyclical redundancy Check 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 the correct port.
Store-and-forward swithching
In cut-through switching how much of the frame does it buffer before it forwards the frame to an out going port ?
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.
How does a switch filter frames ?
The switch uses its populated MAC/CAM address table to filter frames by identifying what destination MAC addresses are associated with witch ports and sends the frame out that single port.
It the destination unicast MAC address is not in the table , what does the Switch do with the frame ?
The switch will forward the frame out all ports except the incoming port. This is called an unknown unicast
How does a switch look for a match in it MAC/CAM address table it the destination MAC address is an 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 .
Give two reasons why an Ethernet switch would use a buffering technique ?
To store frames before forwarding them and it may be used when the destination port is busy because of congestion.
Ture or False ? The auto-MDIX feature is enable by default on switches running Cisco IOS release 12.2.(18) SE or later .
True
True or False ? In cut-through switching, the switch acts upon the data as soon as it is received, even if the transmission is not complete ?
True, 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.
True or False ? Some switches are configured to perform cut-through switching on a per-port basis until a user-defined error threshold is reached, and then they automatically change to store-and-forward.
True. When the error rate falls below the threshold, the port automatically changes back to cut-through switching
What are two of the most basic setting that need to be set up for the communication between a switch and an end device ?
Two of the most basic settings on a switch are the bandwidth (sometimes referred to as "speed") and duplex settings for each individual switch port. It is critical that the duplex and bandwidth settings match between the switch port and the connected devices, such as a computer or another switch.
What does auto-MDIX do ?
When enabled, the switch automatically detects the type of cable attached to the port and configured the interfaces accordingly. Therefore, you can use either a crossover or a straight-through cable for connections to a copper 10/100/1000 port on the switch, regardless of the type of device on the other end of the connection.
What does auto-MDIX stand for ?
automatic medium-dependent interface crossover feature .
What are two methods for switching data between ports on a switch ? (Choose two )
cut-through switching Store-and-forwarding switching
What command is used to show ARP information on a switch ?
show ARP or show ip arp
What command is used to show mad address information for VLAN 1 ?
show interfaces vlan 1
The hexadecimal numbering system uses the number what to what and letters what to what ?
0 to 9 and letters A to F
What is the hexadecimal range ?
00 to ff is the hexadecimal range
Hexadecimal numbers are oftnen represented by what to values ?
0x before a number and H after a number
Ethernet communications standard: IEEE 802.3ae
10 Gigabit Ethernet over Fiber
Ethernet 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 supports data bandwidths of the following:
10 Mbps 100Mbps 1000Mbps (1Gbps) 10000 Mbps (10 Gbps) 40000 Mbps (40 Gbps) 100,000 Mbps (100 Gbps)
How many hexadecimal values make up a 48-bit Ethernet MAC address ?
12 hexadecimal values
What is the maximum size a Ethernet frame can be from the destination MAC address field through the frame check sequence ?
1518 bytes
How many hexadecimal digits make 1 byte ?
2 hexadecimal digits
The range of IPV4 multicast addresses is what ?
224.0.0.0 to 239.255.255.255
A single hexadecimal digit represents how many binary bits ?
4 binary bits
An Ethernet MAC address consists of how many bits ?
48 bit binary value
By default, how long does a Ethernet switch keep an Source MAC address entry for ?
5 mins
How many bytes make up a MAC address ?
6 bytes
What is the minimum size a Ethernet frame can be from the destination MAC address field through the frame check sequence ?
64 bytes
Ethernet NICs will also accept frames if the destination MAC address is sent by what two groups ?
A broadcast or a multicast group of which the host is a member
How does a layer 2 Ethernet switch make forwarding decisions ?
A layer 2 Ethernet switch uses layer 2 MAC address to make forwarding decisions.
As with the unicast and broadcast address, the multicast IP address requires what type of corresponding MAC address to deliver frames on a local network?
A multicast MAC address
The first 3 bytes or 6 hexadecimal numbers of a MAC address is know as what ?
A unique code called the Organizationally unique identifier (OUI)
If you see the hexadecimal value of 0x806 in the type field in a Ethernet frame, what upper layer protocol is encapsulated in the frame ?
ARP or address resolution protocol
The process that a source host uses to determine the destination MAC address associated with an IPv4 address is known as what ?
Address Resolution Protocol
If the encapsulated data in an IP multicast packet, the deices that belong to a multicast group are assigned what type of IP address ?
An multicast group ip address
What happens to a frame that is 64 bytes in length ?
Any frame less than 64 bytes is automatically discarded by receiving stations.
What is the MAC sublayer responsible for ?
Data encapsulation and accessing the media
Ethernet is defined by which two layer in the OSI model ?
Data link layer and the physical layer.
When a device is forwarding a message to an Ethernet network, the Ethernet header includes these:
Destination MAC address Source MAC address
Which fields are included when describing the size of the Ethernet frame ?
Destination MAC address Source MAC address Type/Length data FCS
This 6-bytes field is the ID for the intended recipient. As you will recall, this address is used by layer 2 to assist devices in determining if a frame is addresses 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 unicast, multicast or broadcast address
Destination MAC address Field
Which part of an Ethernet Frame describes the higher layer protocol that is encapsulated ? Ethertype Preamble Start of Frame Delimiter Frame Check Sequence
EtherType
This is the internal structure of the Ethernet Frame
Ethernet Frame
Not all Ethernet broadcasts carry an IPv4 broadcast packet , give an example of this ?
For example, ARP Requests do not use IPv4, but the ARP message is sent as an Ethernet broadcast.
Ethernet communications standard : IEEE 802.3ab
Gigabit Ethernet over Copper
Ethernet communications standard: IEEE 802.3z
Gigabit Ethernet over Fiber
If you see the hexadecimal value of 0x800 in the type field in a Ethernet frame, what upper layer protocol is encapsulated in the frame ?
IPv4
What is a collision fragment or runt frame ?
It is any frame the is less than 64 bytes in length.
Frames with more than 1500 bytes of data are considered what ?
Jumbo or baby giant frames
Which data link sublayer controls the network interface through software drivers ? MAC LLC
LLC
Which data link sublayer works with the upper layer to add application information for delivery of data to higher level protocols ? MAC LLC
LLC
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 ids which network layer protocol is being used for the frame. This info allows multiple Layer 3 protocols, such as IPv4 and IPv6, to use the same network interface and media
LLC Sublayer
Which two separate sublayers of the data link layer are used by IEEE 802 LAN/MAN protocols, including Ethernet, use the following two separate sublayers of the data link layer to operate ?
Logical Link Control (LLC) and Media Access Control (MAC)
The process that a source host uses to determine the destination MAC address associated with an IPv6 address is known as what ?
Neighbor Discovery (ND)
Which part of an Ethernet Frame notifies the receiver to get ready for a new frame ? Start of Frame Delimiter Frame Check Sequence Preamble Data Field
Preamble
Which Ethernet frame field is not included when describing the size of the frame ?
Preamble and Start Frame Delimiter
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.
Preamble and Start Frame Delimiters Fields
When the computer boots up, the NIC copies its MAC address from ROM to where ?
RAM
This 6 -bytes field ids the originating NIC or interface of the frame
Source MAC address Field
What is used to identify the physical source an destination devices (NICs)on the local network segment ?
The MAC address
When a NIC receives an Ethernet frame, it examines what to see it it matches the physical MAC address that is stored in RAM ?
The destination MAC address
How does the frame know if it needs to be a multicast frame ?
The multicast MAC address is associated with, and uses addressing information from, the IPv4 or IPv6 multicast address.
An Ethernet multicast frame is received and processed by what group of devices on the Ethernet LAN ?
The multicast group
If the encapsulated data is an IPv4 broadcast packet, this means the destination IPv4 address will contain what in the host portion of the address ?
The packet contains a destination IPv4 address that has all ones in the host portion. EX ( 192.168.1.255) / 24
How does the switch use the MAC address table or Content addressable memory table ?
The switch forwards frames by searching for a match between the destination MAC address in the frame and an entry in the CAM or MAC address table.
If the source MAC address does exist in the MAC/Content Addressable Memory table but is now on a different port, what does the switch do with this information ?
The switch treats this as a new entry. The entry is replace using the same MAC address but with the more current port number.
How does the switch build it MAC address table ?
The switch uses the source MAC address of the frames received on a port to build the table. It does this dynamically.
The 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 address for when the encapsulated data in not IP, such as Panning 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 in not forwarded by a router, unless the router is configured to route multicast packets.
If a packet with a destination IPv4 address of 192.168.1.255 / 24 is sent out, in the local network or broadcast domain, all hosts will do what with the packet ?
They will receive and process the packet
True or False ? A Layer 2 Ethernet switch is completely unaware of the data (protocol) being carried in the data portion of the frame, such as an IPv4 packet, an ARP message, or an IPv6 ND packet.
True, The switch makes its forwarding decisions based solely on layer 2 Ethernet MAC address or Media Access Control address.
How do vendors ensure that every Ethernet device or Ethernet interface is unique ?
Vendors must obtain organizationally unique identifiers from the IEEE (OUI)
Is it possible for you to change you MAC address ?
Yes, On modern PC operating systems and NICs, it is possible to change the MAC address in software. This is useful when attempting to gain access to a network that filters based on BIA. Consequently, filtering or controlling traffic based on the MAC address is no longer as secure.
Dropped frames are likely to be the result of what two events ?
collisions or other unwanted signals. note : they are considered invalid
The IPv4 destination address is a broadcast address, 192.168.1.255. When the IPv4 broadcast packet is encapsulated in the Ethernet frame, the destination MAC address is the broadcast MAC address of what in hexadecimal ?
FF-FF-FF-FF-FF-FF or 48 ones in binary
If the source MAC address of a frame exist in the MAC/CAM address table , what does the switch do with that frames information ?
If the source MAC address does exist, the switch updates the refresh times for that entry in the table.
The 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 port except for the incoming port It is not forwarded by the router
Where is the MAC address stored ?
It is hard coded into read-only memory (ROM) on the NIC.\\ Note: this means that the address is encode into the RO chip permanently
An Ethernet broadcast frame is received and process by who on the Ethernet LAN ?
It is process and received by every devices on the Ethernet LAN
Every time a frame enters a switch , the switch will check the frame for what information ?
It will examine the source MAC address of the frame and the port number where the frame entered the switch.
Unlike legacy Ethernet hubs that repeat bits out all ports except the incoming port. An Ethernet Switch examines what to make a forwarding decision for each frame ?
It will look at its MAC address table to make a forwarding decision for each frame.
If a frame has VLAN tagging what does it do to the frame ?
It will make the frame larger due to the additional information that was add.
This 2-bytes field ids 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 Ether Type, Type, or Length
Type/Legnth
List the three types of different layer 2 MAC address used for commutation in Ethernet ?
Unicast Broadcast\ Multicast
This unique address is used when a frame is sent from a single transmitting device to a single destination device ?
Unicast MAC address
The source MAC address must always be what type of mac address ?
Unicast address
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
The last 6 hexadecimal digits of an MAC address as assigned by who ?
Vendor assigned or vendor code
