CH5 continued – Flashcards
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An Ethernet MAC address is a____-bit binary value expressed as ____ hexadecimal digits
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48 12 (4 bits per hexadecimal digit).
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Just as decimal is a base ten number system, hexadecimal is a base ______ system.
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sixteen The base sixteen number system uses the numbers 0 to 9 and the letters A to F.
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It is important to distinguish hexadecimal values from decimal values regarding the characters 0 to 9, how do you do this?
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Hexadecimal is usually represented in text by the value preceded by 0x (for example 0x73) or a subscript 16. Less commonly, it may be followed by an H(for example 73H).
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****************************** Hexadecimal is used to represent what?
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Ethernet MAC addresses and IP Version 6 addresses.
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Why were MAC addresses created?
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because Ethernet was once a half-duplex topology operating on multi-access bus or hubs and this meant that all nodes would receive every frame transmitted. MAC addressing prevent excessive overhead by identifying the actual source and destination.
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The MAC address value is a direct result of _____-enforced rules for vendors to ensure globally unique addresses for each Ethernet device. What do these rules require vendors to do?
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IEEE Requires that all MAC addresses assigned to a NIC or Ethernet device use the first 3 bytes of the OUI (Organizationally Unique Identifier) that was provided to them by the IEEE. This OUI must be unique or reassigned. *It is possible for duplicate MAC addresses to exist due to mistakes during manufacturing or in some virtual machine implementation methods.
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What is historically referred to as a "burned-in address"?
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MAC address because historically, ON OLDER PC's, this address was burned into ROM (Read-Only Memory) on the NIC. This meant that the address is encoded into the ROM chip permanently. **On modern PC operating systems and NICs, it is possible to change the MAC address in software.
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When the computer starts up, the first thing the NIC does is?
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copy the MAC address from ROM into RAM. When a device is forwarding a message to an Ethernet network, it attaches header information to the packet. The header information contains the source and destination MAC address.
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When a NIC receives an Ethernet frame, it examines the destination MAC address to see if it matches the device's physical MAC address stored in the receivers _____.
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RAM *If there is no match, the device discards the frame. If there is a match, it passes the frame up the OSI layers, where the de-encapsulation process takes place.
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******************** Ethernet NICs will also accept frames if ?
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the destination MAC address is a broadcast or a multicast group of which the host is a member.
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On a Windows host, the _______command can be used to identify the MAC address of an Ethernet adapter
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ipconfig /all
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The process that a source host uses to determine the destination MAC address is known as?
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Address Resolution Protocol (ARP).
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the destination MAC address can be a unicast, broadcast, or multicast address, the source MAC address must always be a?
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Unicast
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A broadcast packet contains a destination IPv4 address that has all _____ in the host portion.
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ones (1s) (binary 1's (1111) = hexadecimal F) This numbering in the address means that all hosts on that local network (broadcast domain) will receive and process the packet.
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Many network protocols, such as _____ & ____, use broadcasts.
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DHCP and ARP
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When the IPv4 broadcast packet is encapsulated in the Ethernet frame, the destination MAC address is the broadcast MAC address of ______________ in hexadecimal (48 ones in binary).
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FF-FF-FF-FF-FF-FF (f= binary for 1111)
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_______________ addresses allow a source device to send a packet to a group of devices.
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Multicast
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Devices that belong to a multicast group are assigned a multicast group IP address. The range of IPv4 multicast addresses is _______________ to ____________
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224.0.0.0 to 239.255.255.255.
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Another word for Host group?
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Multicast
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The multicast MAC address is a special value that begins with ___-__-__ in hexadecimal. The remaining portion of the multicast MAC address is created by converting the lower 23 bits of the IP multicast group address into 6 hexadecimal characters.
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01-00-5E
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the multicast hexadecimal address 01-00-5E-00-00-C8. The last byte, or eight bits, of the IP address 224.0.0.200, is the decimal value 200. The easiest way to see the hexadecimal equivalent is to first convert it to ?
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binary with a space between each four bits, 200 (decimal) = 1100 1000 (binary). Using the binary to hexadecimal conversion chart shown earlier, 1100 1000 (binary) = 0xC8.
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An Ethernet switch is a Layer 2 device, explain.
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It uses MAC addresses to make forwarding decisions therefor it is a layer 2 device. It is completely unaware of the protocol being carried in the data portion of the frame, such as an IPv4 packet.
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an Ethernet _____ consults a MAC address table to make a forwarding decision for each frame.
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switch
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an Ethernet _____ repeats bits out all ports except the incoming port
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hub
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This is sometimes referred to as a content addressable memory (CAM) table.
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MAC address table
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How does a switch MAC address table work?
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It makes a table by examining the source MAC addresses of what is plugged in and the frames received on ports. It adds addresses and ports to a list. Default these tables last for 5 minutes It then looks for a match in its table between the destination and source. If the destination MAC address is not in the table, it will forward the frame out all ports except the incoming port as an "unknown unicast". When the switch's MAC address is able to filter the frame it will send it out just a single port.
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A switch can have multiple MAC addresses associated with a single port. This is common when?
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the switch is connected to another switch.
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When a device has an IP address that is on a remote network, the Ethernet frame cannot be sent directly to the destination device. What happens to the frame?
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the Ethernet frame is sent to the MAC address of the default gateway, the router.
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When a router receives a packet from a remote network, what will the frames destination MAC address be of?
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The router that receives the packet .
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Switches use one of the following forwarding methods for switching data between network ports? Also, What is the predominant switching method used on Cisco Switches?
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Store-and-forward switching: receives entire frame, stores it as buffers, and as it is storing it computes CRC before forwarding out port. The switch will discard packet if faulty Cut-through switching: Predominant switching method on Cisco switches. Forwards before frame is entirely received and just looks at destination to save time. The switch buffers just enough of the frame to read the destination MAC address so that it can determine to which port to forward the data. The destination network adapter discards the faulty packet upon receipt.
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How does CRC (cyclic redundancy check) work?
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CRC uses a mathematical formula, based on the number of bits (1s) in the frame, to determine whether the received frame has an error.
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Why would you want to discard frames with errors?
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to reduce the amount of bandwidth
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_________________ switching is required for Quality of Service (QoS) analysis on converged networks where frame classification for traffic prioritization is necessary. For example, voice over IP data streams need to have priority over web-browsing traffic.
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Store-and-forward switching
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The destination MAC address is located in the?
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first 6 bytes of the frame following the preamble.
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most network errors and collisions occur during the first ____ bytes.
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64
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When might an Ethernet switch use memory buffering?
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when the destination port is busy due to congestion and cannot yet be transmitted
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there are two methods of memory buffering:
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port-based: Frames stored in ques and linked to ports shared memory: deposits all frames into a common memory buffer that all the ports on the switch share. The amount of buffer memory required by a port is dynamically allocated.
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The_____ keeps a map of frame to port links showing where a packet needs to be transmitted.
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switch **The map link is cleared after the frame has been successfully transmitted.
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what permits larger frames to be transmitted with fewer dropped frames.
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The number of frames stored in the buffer is restricted by the size of the entire memory buffer and not limited to a single port buffer.
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what is Asymmetric switching?
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allows for different data rates on different ports. This allows more bandwidth to be dedicated to certain ports, such as a port connected to a server. **memory buffering is very important here
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Two of the most basic settings on a switch are?
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bandwidth and duplex setting: Must match between devices **for each individual switch port**
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Autonegotiation is an optional function found on most Ethernet switches and NICs. What does it do?
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enables two devices to automatically exchange information about speed and duplex capabilities. The switch and the connected device will choose the highest performance mode. *They will go as fast as they both can match*
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One of the most common causes of performance issues on 10/100 Mb/s Ethernet links occurs when?
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one port on the link operates at half-duplex while the other port operates at full-duplex
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When the auto-MDIX (medium-dependent interface crossover) feature is enabled on switches, what happens?
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the switch detects the type of cable attached to the port, and configures 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. *IF THE SWITCH FEATURES auto-MDIX* The auto-MDIX feature is enabled by default on switches running Cisco IOS Release 12.2(18)SE or later.
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*********************************There are two primary addresses assigned to a device on an Ethernet LAN:
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Physical address (the MAC address) (Layer 2) - Used for Ethernet NIC to Ethernet NIC communications to deliver the data link frame with the encapsulated IP packet from one NIC to another NIC on the same network. Logical address (the IP address)(Layer 3) - Used to send the packet from the original source to the final destination.
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____ addresses are used to identify the address of the original source and the final destination.
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IP
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Most applications use _______ to determine the IP address when given a domain name such as www.cisco.com.
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DNS (Domain Name System)
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****************************** The Layer 2 Ethernet frame contains:
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Destination MAC address - This is the MAC address of the file server's Ethernet NIC. Source MAC address - This is the MAC address of PC-A's Ethernet NIC.
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************************** The Layer 3 IP packet contains:
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Source IP address - This is the IP address of the original source, PC-A. •Destination IP address - This is the IP address of the final destination, the file server.
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When the destination IP address is on a remote network, the destination MAC address will be the address of the host's _________ __________.
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default gateway (router)
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How are the IP addresses of the IP packets in a data flow associated with the MAC addresses on each link along the path to the destination?
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This is done through a process called Address Resolution Protocol (ARP).
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every device with an IP address on an Ethernet network also has an ______ ______ address.
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Ethernet MAC
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A frame contains what two addresses?
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Destination MAC and source MAC. *IP is encapsulated so it is in the packet, not the frame*
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To determine the destination MAC address, the device uses?
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ARP
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ARP provides two basic functions:
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Resolving IPv4 addresses to MAC addresses Maintaining a table of mappings
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When a packet is sent to the data link layer to be encapsulated into an Ethernet frame, how does the device determine MAC addressing?
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the device refers to a table in its memory to find the MAC address that is mapped to the IPv4 address. *This table is called the ARP table or the ARP cache. Stores in RAM of device
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After a packet is framed for delivery, what is different between same network and different network?
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If the packets destination is on another network.. the ARP needs to address destination IPv4 to the default gateway
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If there is no entry is found in the ARP table, what happens?
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then the device sends an ARP request to learn the MAC address of destination. Destination responds and sends reply. Others ignore. ARP table is logged with new address
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ARP messages have a type field of ?
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0x806: This informs the receiving NIC that the data portion of the frame needs to be passed to the ARP process.
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What type of casts are ARP requests?
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broadcasts They are messages that are encapsulated directly in the frame that includes the target IPv4 address and target MAC address
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ARP REQUEST: When the source and destination IP addresses are on the same network, and the MAC address is unknown, the destination MAC address in the header will be that of?
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the destination IP address
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Will routers forward ARP requests out of their ports?
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No
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How do ARP replies work?
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The ARP request is a Broadcast so all devices on the LAN get it, however only one will reply because it will have the destination IPv4 address. It will generate an ARP reply encapsulated in a frame in UNICAST form. The source device will add information into its ARP cache.
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IPv6 uses a similar process to ARP for IPv4, known as?
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ICMPv6 neighbor discovery. IPv6 uses neighbor solicitation and neighbor advertisement messages, similar to IPv4 ARP requests and ARP replies.
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ARP REQUEST: when the destination IPv4 address is on a remote network, and the MAC address is unknown, the destination MAC address in the header will be that of?
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ITs default gateway
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For each device, an ____ _____ ________ removes ARP entries that have not been used for a specified period of time.
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ARP cache timer or commands show ip arp in router or arp -a in command promt
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What can flood local media?
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ARP broadcasts
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Ethernet standards define both the Layer __ protocols and the Layer _ technologies.
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2 1
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There are two styles of Ethernet framing, What are they and what are the differences?
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IEEE 802.3 Ethernet standard and the Ethernet II. The most significant difference between the two standards is the addition of a Start Frame Delimiter (SFD) and the change of the Type field to a Length field
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The multicast MAC address is a special value that begins with __-__-__ in hexadecimal. It allows a source device to send a packet to a group of devices.
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01-00-5E
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On Ethernet networks, the hexadecimal address FF-FF-FF-FF-FF-FF represents the ?
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Broadcast MAC address
