Chapter 5 Cisco Netacad
What is the correct order a switch takes to built its MAC address table and correctly forward frames?
1) A switch receives a broadcast frame from host 1 with Source and Destination. 2) The switch enters the LEARNED source MAC address and the switch port that receives the frame into the MAC table. (Show Mac address-table) command on a switch reveals MAC addresses learned. 3) The switch floods the broadcast frame out all ports except on the port from it was receives from because its looking for Destination Mac-Address and is announcing "Where are you?" 4) The destination device replies to the broadcast with a unicast frame addressed to host 1. 5) The switch enters the Replies source MA address of the device replying to host 1 and switch port it is connected to into the MAC table. 6) The switch can now forward messages between the two devices without flooding the network because it has recorded the MAC addresses and associated switch ports in its MAC address table.
How would you configure Auto-MDIX on an interface?
1) Configure terminal 2)Interface interface -id EX: int Fa0/1 3) speed auto (needs to be Auto for MDIX to work) 4) duplex auto (needs to be Auto for MDIX to work) 5) mdix auto (On by default but if Speed or Duplex are not Auto, then MDIX is off)
What is the IPv4 address range for multicast addresses?
224.0.0.0 to 239.255.255.255
What ethernet frame field name uses Pad to increase this frame field to at least 64 bytes?
802.2 Header and Data
Explain how a layer 2 switch operates.
A layer 2 LAN switch performs switching and filtering based only on the OSI data link layer (layer 2) MAC address and depends upon routers to pass data between independent IP subnetworks
What are the two primary function of Media Access Control? (Not sublayer)
A) Control of frame placement on and off the media B) Media recovery (ex. Collision Management)
List and explain the two variants of cut-through switching
A) Fast-forward switching (fastest but with most errors - Good in stable environments). 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. B) 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 the 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.
Explain the difference between fixed configuration switches and modular switches.
A) Fixed configuration switches- features and options are limited to those that originally come with the switch. B) Modular switches- The chassis accepts various different line cards that contain carious port types
Data encapsulation provides three primary functions. These are:
A) Frame delimiting (structuring frames) B) Addressing (put address info inside Frame) C) Error Detection (Does FCS match my computation)
There are two ways that a device can gather MAC addresses. Explain both.
A) Monitor incoming traffic. As a device receives frames from the media, it can record the Source IP and MAC address as a mapping in the ARP table. B) An ARP request is a layer 2 broadcast to all devices on the local network. The ARP request contains the IP address of the destination host and the broadcast MAC address, FFFF.FFFF.FFFF.
List and explain the two methods of memory buffering
A) Port-based Memory Buffering (used by cheaper switches). In port-based memory buffering, frames are stored in queues that are linked to specific incoming and outgoing ports. In port-based memory buffering frames are stored in queues that are linked to specific incoming ports. A frame is transmitted to the outgoing port only when all the frames ahead of it 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. B) Shared Memory Buffering. Shared memory buffering deposits all frames into a common memory buffer that all the ports on the switch share. Shared memory buffering deposits all frames into a common memory buffer which all the ports on the switch share. the amount of buffer memory required by a port is dynamically allocated. The frames in the buffer are linked dynamically to the destination port. this allows the packet to be received on one port and than transmitted on another port, without moving it to a different queue. Shared memory is more sophisticated and more $$$.
List and explain two types of switch packet forwarding methods
A) Store-and-forward switching (Slowest but with least errors). The switch receives the frame, it stores the data in buffers until the complete frame has been received. During the storage process, the switch analyzes the frame for information about its destination. In this process, the switch also performs an error check using the Cyclic Redundancy Check (CRC) trailer portion of the ethernet frame. B) Cut-through. A cut-through switch 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.
List and describe the major types of Layer 3 interfaces
A) Switching Virtual Interface (SVI)- logical interface on a switch associated with a virtual local area network (VLAN) B) Routed Port- Physical port on a layer 3 switch configured to act as a router port C) Layer 3 EtherChannel- logical interface on a Cisco device associated with a bundle of routed ports
A switch port can be configured to be a layer 3 routed port and behave like a regular router interface. What is needed to accomplish this?
A) The no switchport command B) assign an IP address to the port
What are the two basic functions of the ARP protocol?
A) resolving IPv4 addresses to MAC addresses B) Maintaining a table of mappings
What are the two main components of CEF operation?
CEF is mainly used to increase packet switching speed reduced by the overhead and delayed introduced by other routing techniques. CEF consists of two key components: The Forwarding Information Base (FIB) and adjacencies The FIB is similar to the routing table generated by multiple routing protocols, maintaining only the next-hop address for a particular IP-route. The adjency table maintains layer 2 or switching information linked to a particular FIB entry, avoiding the need for an ARP request for each table lookup
Cisco devices which support Layer 3 switching utilize what Cisco forwarding method?
Cisco Express Forwarding (CEF)
The MAC address table is sometimes referred to as a what?
Content addressable memory (CAM) table. On SW# sho mac-address-table
Cisco 3 layer Hierarchical Model
Core layer: this layer is considered the backbone of the network and includes the high-end switches and high-speed cables such as fiber cables. This layer of the network does not route traffic at the LAN. In addition, no packet manipulation is done by devices in this layer. Rather, this layer is concerned with speed and ensures reliable delivery of packets. Distribution layer: this layer includes LAN-based routers and layer 3 switches. This layer ensures that packets are properly routed between subnets and VLANs in your enterprise. this layer is also called the Workgroup layer. Access layer: this layer includes hubs and switches. This layer is also called the desktop layer because it focuses on connecting client nodes, such as workstations to the network. This layer ensures that packets are delivered to end user computers.
What ethernet frame field assists a host in determining if the frame received is addressed to it?
Destination address field
On ethernet networks, how is the broadcast MAC address displayed in hexadecimal?
FF-FF-FF-FF-FF-FF
What ethernet frame field detects errors in an ethernet frame?
Frame Check Sequence
What duplex setting is required when using 1000Mb/s speeds?
Full duplex
What is half duplex communication?
Half-duplex communication relies on unidirectional data flow where sending and receiving data are not performed at the same time. (one pipe)
explain how CSMA/CD operates
In CSMA/Collision Detection, the device monitors the media for the presence of a data signal. If a data signal is absent, the device transmits the data. If signals are then detected that show another device was transmitting at the same time, all devices stop sending and try again later at a random time interval.
What is full duplex communication?
In full duplex communication, data flow is bidirectional, so data can be sent and received at the same time thus you get 200mbs on a 100mbs link (in theory) (two pipes)
Explain how a layer 3 switch differs from a layer 2 switch
Instead of using only the layer 2 MAC address information for forwarding decisions, a layer 3 switch can also use IP address information.
What is a unicast MAC address?
It is the unique address used when a frame is sent from a single transmitting device to a single destination device
What is the purpose and responsibility of the MAC sublayer?
MAC is the lower sublayer of the Data Link Layer and responsible for data encapsulation and Media Access Control.
List and explain two potential issues with ARP
Overhead on the Media) As a broadcast frame, an ARP request is received and processed by every device on the local network. If a large number of devices were to be powered up and all start accessing network services at the same time, there could be some reduction in performance for a short period of time. Security) ARP spoofing, or ARP poisoning is a technique used by an attacker to inject the wrong MAC address association into a network by issuing fake ARP requests.
What ethernet frame field notifies destinations to get ready for a new frame?
Preamble
What command is used on a Cisco router to display the ARP table?
Show ip arp
What ethernet frame field contains the originating NIC or interface MAC address?
Source Address field
what ethernet frame field synchronizes sending and receiving devices for frame delivery?
Start of Frame Delimiter
What happens if no device responds to the ARP request?
The ARP request is dropped because a frame cannot be created
What is the purpose of the LLC sublayer?
The Ethernet LLC sublayer handles the communication between the upper layers and the lower layers. So Layer 2 needs to talk to layer 3 via LLC Sublayer
If the destination IPv4 hosts is on the local network, the frame will use the MAC address of this device as the destination MAC address. If the destination IPv4 host is not on the local network, what happens?
The source node will use the MAC address of the gateway as the destination address for frames containing an IPv4 packet addressed to hosts on other networks.
Why were MAC addresses created?
To prevent the excessive overhead involved in the processing of every frame, a unique identifier called a MAC address was created to identify the actual source and destination nodes within an ethernet network.
What ethernet frame field describes which higher-level protocol has been used?
Type
Explain in detail what the Auto_MDIX feature does.
When the auto-MDIX feature is enabled, the switch detects the required cable type for copper ethernet connections and configured the interfaces electronically to support needed connection accordingly. Therefore, you can use either a crossover or 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 since auto-MDIX is Auto-Sensing.
Explain what a "collision fragment" or "runt frame" is and what happens to it on the network.
any frame less than 64 bytes in length is automatically discarded by receiving stations. It is left at 64 bytes because a collision occurred that collapsed generating a full frame.
What command is used in Windows 7 to display the ARP table?
arp -a
Stackable Configuration Switches
can be interconnected using a special cable that provides high-bandwidth throughput between the switches. Cisco StackWise technology allows the interconnection of up to nine switches. Switches can be stacked one on top of the other with cables connecting the switches in a daisy chain fashion. The stacked switched effectively operate as a single larger switch. Stackable switches are desirable where fault tolerance and bandwidth availability are critical and a modular switch is too costly to implement.
Entries in the ARP table are time stamped. What does this mean?
if a device does not receive a frame from a particular device by the time the time stamp expires, the entry for this device is removed from the ARP table.
explain how CSMA/CA operates
in CSMA/Collision Avoidance, the device examines the media for the presence of a data signal. If the media is free, the device sends a notification across the media of its intent to use it. The device then sends the data. This method is used by 802.11 wireless networking technologies.
on a Windows host, what command can be used to identify the MAC address of an ethernet adapter?
ipconfig /all or getmac
