Chapter 7: Ethernet Switching
Hexadecimal numbering system uses numbers _-_ and letters _-_?
0-9 and A-F
What is the hexadecimal range?
00 to FF
8 bits equal how many bytes?
1 byte
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 100 Mbps 1000 Mbps (1Gbps) 10000 Mbps (10Gbps) 40000 Mbps (40Gbps) 100000 Mpbs (100 Gbps)
How many hexadecimal values make up a 48 bit Ethernet MAC address?
12 hexadecimal digits
What is the Ethernet maximum size a frame can be from the destination MAC address field through the frame check sequence (FCS) field?
1518 bytes
How many hexadecimal digits are in one byte?
2 hexadecimal digits
The range of IPv4 multicast address is what?
224.0.0.0 to 239.255.255.255
A single hexadecimal digit represents how many binary bits?
4 bits
An Ethernet MAC address consists of how many bits?
48 bits
By default, how long does Ethernet switch keep an source MAC address entry for?
5 minutes
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 (FCS) field?
64 bytes
If the encapsulated data is an IP multicast packet, the devices that belong to a multicast group are assigned what type of IP address?
A multicast group IP address
This unique address that is used when a frame is sent form a single transmitting device to a single destination device.
A unicast MAC address
If you see the hexadecimal value of 0x806 in the Type/Length field of an Ethernet Frame, what upper layer protocol is being 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 (ARP)
How do vendors ensure that every Ethernet device or Ethernet interface is unique?
All vendors must obtain an organizationally unique identifier (OUI) from the IEEE
What is a collision fragment or a runt frame?
Any frame that is less than 64 bytes in length
What does auto-MDIX stand for?
Automatic medium-dependent interface crossover
What feature automatically negotiates the best speed and duplex setting between interconnecting devices?
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 speeds and duplex capabilities. Full-duplex is chosen if both devices have the capability along with their highest common bandwidth.
What are some ways to prevent duplex mismatch?
Both sides of the 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 be used for source?
Broadcast Multicast
What does bia stand for?
Burned in Address or MAC address
Sometimes the MAC address is referred to as what?
Burned-in address (BIA)
Dropped frames are likely to be the result of what two events?
Collisions Other unwanted signals Note: They are considered invalid
The MAC address table is sometimes referred to as what?
Content Addressable Memory (CAM)
What is the 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 layers 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?
Crossover 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
Ethernet is defined by which two layers of the OSI model?
Data link Layer Physical Layer
What fields are included when describing the size of the Ethernet frame?
Destination MAC Source MAC Type/Length Data FCS
When a NIC receives an Ethernet frame, it examines what to see if it matches the physical MAC address that is stored in RAM?
Destination MAC address
When a device is forwarding a message to an Ethernet network, the Ethernet header includes these:
Destination MAC address Source MAC address
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 determine if a frame is address 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.
Destination MAC address Field
Why is it good that a switch discards frames with errors?
Discarding frames with errors reduces the amount of bandwidth consumed by corrupted 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 a duplex mismatch?
Duplex mismatch occurs when one or both ports on a links are reset, and the autonegotiation process does not result in both link partners having the same configuration. It also can occur when users reconfigure one side of the link and forget to reconfigure the other.
The Ethernet frame includes both a destination and source MAC address to deliver the Ethernet frame
Ethernet Addressing
What are two LAN technologies used today?
Ethernet LANs Wireless LANs
IEEE 802.3 data encapsulation includes the following:
Ethernet frame Ethernet Addressing Ethernet Error Detection
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 (48 ones in binary)?
FF-FF-FF-FF-FF-FF
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.
Fast-forward switching
What are the two variants of cut-through switching?
Fast-forward switching 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 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 endure 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.
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 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.
Frame Check Sequence Field (FRC)
Both ends of the connection can send and receive simultaneously.
Full-duplex
Gigabit Ethernet ports only operate in what duplex mode?
Full-duplex
There are two types of duplex setting used for communications on an Ethernet netwrok:
Full-duplex Half-duplex
Ethernet communications standard: IEEE 802.3z
Gigabit Ethernet over Fiber
Ethernet communications standard: IEEE 802.3ab
Gigabit Ethernet over copper
Only one end of the connection can send at a time.
Half-duplex
Hexadecimal numbers are often represented by what two values?
Hexadecimal numbers are often represented by the value preceded by and H after a number
If you see the hexadecimal value of 0x800 in the Type/Length field of an Ethernet Frame, what upper layer protocol is being encapsulated in the frame?
IPv4
Which IP addresses are represented the decimal base ten number system and the binary baser 2 number system?
IPv4
If the source MAC address of a frame exits in the MAC address/CAM table, what does the switch do with that frame's information?
If the source MAC address does exist, the switch updates the refresh timer of that entry in the table.
If the MAC address of a frame does not exist in a switches MAC address table/CAM table what does it do with that frame's information?
If the source MAC address does not exist, the source MAC address will be added to the table along with the incoming port number.
When a devices receives a frame and that frame's destination MAC does match the devices MAC address that is in RAM what happens to the frame?
If there is a match, it passes the frame up the OSI layers, where the de-encapsulation process takes place.
Unlike legacy Ethernet hubs that repeat bits out all ports except the incoming port, An Ethernet does what?
It examines its MAC address table to make a forwarding decision for each frame.
The features of an Ethernet broadcast are as follows:
It has a destination MAC address of FF-FF-FF-FF-FF-FF It is flooded out all Ethernet switch ports except the incoming port It is not forwarded by a router
What happens to a frame that is less than 64 bytes in length?
It is automatically discarded by receiving stations.
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.
How does a layer 2 Ethernet switch make forwarding decisions?
It uses Layer 2 MAC addresses to make forwarding decisions
If 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.
Frames with more than 1500 bytes of data are considered what?
Jumbo or baby giant frames
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.
LLC Sublayer or Logical Link
Which data link sublayer controls the network interface through software drivers? MAC LLC
LLC or Logical Link Control
This sublayer (IEEE 802.3, 802.11, 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.
MAC Sublayer
Any device that is the source or destination of an Ethernet frame, will have an Ethernet NIC and therefore, a what?
MAC address
What is used to identify the physical source and destination devices (NICs) on the local network segment?
MAC address
When the computer boots up, the NIC copies what from ROM into RAM?
MAC address
How does a Ethernet switch handle multiple incoming frames?
Memory buffering
An Ethernet multicast frame is received an processed by a what group of devices on the Ethernet LAN?
Multicast Group
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?
Multicast MAC address
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)
The first 3 bytes or 6 hexadecimal (24 bits) numbers of a MAC address is known as what?
Organizationally Unique Identifier (OUI)
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
Frames are stored in ques 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 ot open destination ports
Port-based memory
What are the two memory buffering methods used by a Ethernet switch?
Port-based memory Shared memory
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
What Ethernet Frame field is not included when describing the size of the frame?
Preamble and SFD (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 receiver a new frame.
Preamble and Start Frame Delimiter Fields
An Ethernet broadcast frame is received and processed by what?
Processed by every device on the Ethernet LAN
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.
This 6 byte field identifies the originating NIC or interface of the frame.
Source MAC address Feild
What are the two frame forwarding methods used by Cisco Switches?
Store and forwad switching Cut through switching
What is one big advantage of store-and-forward switching vs cut-through switching?
Store and forward switching determines if a frame has errors before propagating the frame. When an error is detected in a frame, the switch discards the frame.
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.
Store-and -forward switching
Which frame forwarding method is required for quality of service (QoS) analysis on converged networks where frame classification for traffic prioritization is necessary.
Store-and-forward switching
What are two methods for switching data between ports on a switch?
Store-and-forward switching Cut-through switching
Where is the MAC address stored?
The address is hard coded into read-only memory (ROM) on the NIC Note: This means that the address in encoded into the ROM chip permanently
When a devices receives a frame and that frame's destination MAC does not match the devices MAC address that is in RAM what happens to the frame?
The device discards the frame
If a frame has VLAN tagging, what does it do to the frame size?
The frame size will become larger
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 Example: 192.168.1.255 /24
If the size of a transmitted frame is less than the minimum, or greater than the maximum, the receiving device does what with the frame?
The receiving device drops the frame.
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 to the data.
How does the switch build its MAC address table?
The switch dynamically builds the MAC address table by examining the source MAC address of the frames received on a port.
How does the switch use the MAC address table?
The switch forwards frames by searching for a match between the destination MAC address in the frame and an entry in the MAC address table.
If the source MAC address does exist in the MAC address/Content Addressable Memory table but on a different port, what does the switch do with this information?
The switch treats this as a new entry. The entry is replaced using the same MAC address but with the more current port number.
How does a switch filter frame?
The switch uses its populated MAC/CAM address table to filter frames by identifying what destination MAC addresses are associated with which ports and send the frame out that single port.
Every frame that enters a switch, the switch will check the frame for what information?
The switch will examine the source MAC address of the frame and the port number of where the frame entered the switch.
If 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 its MAC/CAM address table 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.
The last 6 hexadecimal digits of a MAC address are assigned by who?
They are vendor assigned or vendor code
If a packet with a destination IPv4 address of 192.168.1.255 /24 is sent out in the local network (broadcast domain) all host will do what with the packet?
They will all receive and process the packet
Give two examples of why a Ethernet switch may use a buffering technique?
To store frames before forwarding them and it may also be used when the destination port is busy because of congestion. The switch stores the frame until it can be transmitted.
True of False: The auto-MDIX feature is enabled by default on switches running Cisco IOS Release 12.2(18)SE or later.
True
True or False. Most Cisco switches and Ethernet NICs default to autonegotiation for speed and duplex.
True
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. When the error rate falls below the threshold, the port automatically changes back to cut-through switching.
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 to the data.t lookin at the rest of the frame or even waiting for the rest of the frame.
True or False. A layer 2 Ethernet switch is completely unaware of the data (protocols) 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 the Layer 2 MAC addresses
What are two of the most basic settings that need to be set for 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.
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
Type/Length
The source MAC address must always be what?
Unicast
In Ethernet, different MAC addresses are used for Layer 2 communications they are :
Unicast Broadcast Multicast
If the destination Unicast MAC address is in the table, it will forward the frame out the specified port. If the destination Unicast MAC address is not in the table, the switch will forward the frame out all ports except the incoming port. What kind of unicast MAC address is this known as?
Unknown unicast
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
How does the frame know if it needs to be multicast frame?
Uses addressing information from, the IPv4 or IPv6 multicast address.
What does auto-MDIX do?
When enabled, the switch automatically detects the type of cable attached to the port and configures the interfaces accordingly. Therefore, you can use either 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.
Is it possible for you change your 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.
The range of IPv6 multicast addresses begins with what?
ff00::/8
What is the interface configuration command used to enable auto-MDIX on a switch?
mdix auto
What command is used to show ARP information on a switch?
show arp or show ip arp
What command is used to show MAC address information for vlan 1?
show interfaces vlan 1
What command shows the mac address table on a switch?
show mac address-table
What protocol for IPv4 uses Ethernet and IPv4 broadcast address?
DHCP
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 frame 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.
Data Field
Which part of an Ethernet Frame uses a pad to increase the frame to at least 64 bytes? EtherType Preamble Start of Frame Delimiter Data Field
Data Field
What is the MAC sublayer responsible for?
Data encapsulation Accessing the Media
Which part of an Ethernet Frame descries the higher-layer protocol that is encapsulated? EtherType Preamble Start of Frame Delimiter Frame Check Sequence
EtherType
The Ethernet frame includes a frame check sequence (FCS) trailer used for error detection
Ethernet Error Detection
This is the internal structure of the Ethernet frame
Ethernet Frame
Ethernet communications standard: IEEE 802.3u
Fast Ethernet
Which two Ethernet Standards support jumbo frame?
Fast Ethernet Gigabit Ethernet
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
If you see the hexadecimal value of 0x86DD in the Type/Length field of an Ethernet Frame, what upper layer protocol is being encapsulated in the frame?
IPv6
Which addresses are represented using hexadecimal base sixteen number system?
IPv6 and MAC address
Ethernet NICs will also accept frames if the destination MAC address is a sent to what two groups?
If the destination MAC address is a broadcast or multicast group of which the host is a member.
Which data link sublayer works with the upper layers to add application information for delivery of data to higher level protocols? MAC LLC
LLC or Logical Link Control
IEE 802 LAN/MAN protocols, including Ethernet, use which two separate sublayers of the data link layer to operate?
LLC sublayer or Logical Link Control MAC sublayer or Media Access Control
What does MAC addressing provide at the data link of the OSI model?
Provides a method for device identification
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 encapsulated data is an IPv6 multicast packet There are other reserved multicast destination MAC address for when the encapsulated data is not IP , such a 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.
