resumen ccna cap 2

7/30/2019 Resumen ccna cap 2

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several competing LAN standards existed, including Token Ring, Fiber Distributed Data

Interface (FDDI), and Asynchronous Transfer Mode (ATM)

The termEthernetrefers to a family of standards that together define the physical and data

link layers

the most commonly used Ethernet standards allow the use of inexpensive unshieldedtwisted-pair (UTP) cabling

for the data link layer, the IEEE separates the functions into two sublayers: The 802.3 Media Access Control (MAC) sublayer The 802.2 Logical Link Control (LLC) sublayer

Because the network uses a single bus, if two or more electrical signals were sent at thesame time, they would overlap and collide, making both signals unintelligible. So,

unsurprisingly, Ethernet also defined a specification for how to ensure that only one device

sends traffic on the Ethernet at one time. Otherwise, the Ethernet would have been

unusable. The carrier sense multiple access with collision detection (CSMA/CD) algorithm,defines how the bus is accessed.

CSMA/CD algorithm can be summarized as follows:

A device that wants to send a frame waits until the LAN is silentin other words, noframes are currently being sentbefore attempting to send an electrical signal.

If a collision still occurs, the devices that caused the collision wait a random amount oftime and then try again.

10BASE-T called for the use of Ethernet hubs

Hubs are essentially repeaters with multiple physical ports.

The original Ethernet LANs created an electrical bus to which all devices connected.10BASE2 and 10BASE5 repeaters extended the length of LANs by cleaning up theelectrical signal and repeating ita Layer 1 functionbut without interpreting the

meaning of the electrical signal.

Hubs are repeaters that provide a centralized connection point for UTP cablingbutthey still create a single electrical bus, shared by the various devices, just like 10BASE5and 10BASE2.

Because collisions could occur in any of these cases, Ethernet defines the CSMA/CDalgorithm, which tells devices how to both avoid collisions and take action when collisionsdo occur.Siempre Par 1 = Blue, Par 2 = Orange, Par 3 = Green y Par 4 = Brown

La diferencia de T568A y T568B es la ubicacion de los pares 2 y 3 intercambiados.


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Gigabit Ethernet does have a concept of straight-through and crossover cables, with a

minor difference in the crossover cables. The pinouts for a straight-through cable are thesamepin 1 to pin 1, pin 2 to pin 2, and so on.

The crossover cable crosses the same two-wire pair as the crossover cable for the other

types of Ethernetthe pair at pins 1,2 and3,6as well as crossing the two other pairs (the pair at pins 4,5 with the pair at pins 7,8).

Cisco switches have a feature called auto-mdix that notices when the wrong cablingpinouts are used. This feature readjusts the switchs logic and makes the cable work.

Logica del hub

Step 1 The network interface card (NIC) sends a frame.

Step 2 The NIC loops the sent frame onto its receive pair internally on the card.

Step 3 The hub receives the electrical signal, interpreting the signal as bits so that it can

clean up and repeat the signal.

Step 4 The hubs internal wiring repeats the signal out all other ports, but not back to the

port from which the signal was received.

Step 5 The hub repeats the signal to each receive pair on all other devices.


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The CSMA/CD algorithm works like this:

Step 1 A device with a frame to send listens until the Ethernet is not busy.

Step 2 When the Ethernet is not busy, the sender(s) begin(s) sending the frame.

Step 3 The sender(s) listen(s) to make sure that no collision occurred.

Step 4 If a collision occurs, the devices that had been sending a frame each send a jammingsignal to ensure that all stations recognize the collision.

Step 5 After the jamming is complete, each sender randomizes a timer and waits that long

before trying to resend the collided frame.

Step 6 When each random timer expires, the process starts over with Step 1.

CSMA/CD does not prevent collisions, but it does ensure that the Ethernet works well even

though collisions may and do occur. However, the CSMA/CD algorithm does create someperformance issues.

First, CSMA/CD causes devices to wait until the Ethernet is silent before sending data.

This process helps avoid collisions, but it also means that only one device can send at any

one instant in time. As a result, all the devices connected to the same hub share thebandwidth available through the hub.

The logic of waiting to send until the LAN is silent is calledhalf duplex. This refers to thefact that a device either sends or receives at any point in time, but never both at the sametime.

The term collision domain defines the set of devices whose frames could collide

switches do the following:

Switches interpret the bits in the received frame so that they can typically send the frameout the one required port, rather than all other ports

If a switch needs to forward multiple frames out the same port, the switch buffers theframes in memory, sending one at a time, thereby avoiding collisions

The switchs logic requires that the switch look at the Ethernet header, which is considered

a Layer 2 feature. As a result, switches are considered to operate as a Layer 2 device,whereas hubs are Layer 1 devices.

Buffering also helps prevent collisions. Imagine that PC1 and PC3 both send a frame toPC4 at the same time. The switch, knowing that forwarding both frames to PC4 at the same

time would cause a collision, buffers one frame (in other words, temporarily holds it in

memory) until the first frame has been completely sent to PC4.

switch features provide significant performance improvements as compared with usinghubs. In particular:

If only one device is cabled to each port of a switch, no collisions can occur.Devices connected to one switch port do not share their bandwidth with devicesconnected to another switch port. Each has its own separate bandwidth, meaning that a

switch with 100-Mbps ports has 100 Mbps of bandwidthper port.Shared Ethernet means that the LAN bandwidth is shared among the devices on the LAN

because they must take turns using the LAN because of the CSMA/CD algorithm


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The term switched Ethernet refers to the fact that with switches, bandwidth does not have

to be shared, allowing for far greater performance

LAN switches with only one device cabled to each port of the switch allow the use offull-duplex operation. Full duplex means that an Ethernet card can send and receive

concurrently

Ethernet LAN addressing identifies either individual devices or groups of devices on aLAN. Each address is 6 bytes long, is usually written in hexadecimal, and, in Ciscodevices, typically is written with periods separating each set of four hex digits. For

example, 0000.0C12.3456 is a valid Ethernet address.

UnicastEthernet addresses identify a single LAN card

To ensure a unique MAC address, the Ethernet card manufacturers encode the MACaddress onto the card, usually in a ROM chip.

The first half of the address identifies the manufacturer of the card. This code, which isassigned to each manufacturer by the IEEE, is called the organizationally unique identifier(OUI).

Each manufacturer assigns a MAC address with its own OUI as the first half o f the address,

with the second half of the address being assigned a number that this manufacturer hasnever used on another card.

The IEEE defines two general categories of group addresses for Ethernet:

Broadcast addresses: The most often used of the IEEE group MAC addresses, thebroadcast address, has a value of FFFF.FFFF.FFFF (hexadecimal notation). The broadcastaddress implies that all devices on the LAN should process the frame.

Multicast addresses: Multicast addresses are used to allow a subset of devices on aLAN to communicate. When IP multicasts over an Ethernet, the multicast MAC addresses

used by IP follow this format: 0100.5exx.xxxx, where any value can be used in the last halfof the address.

The termframing refers to the definition of the fields assumed to be in the data that is

received. In other words, framing defines the meaning of the bits transmitted and receivedover a network.


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when a device receives such an Ethernet frame, that receiving device needs to know what

type of L3 PDU is in the Ethernet frame. Is it an IP packet? an OSI packet? SNA? and so

on.

most data-link protocol headers, including Ethernet, have a field with a code that definesthe type of protocol header that follows. These fields in data -link headers are calledType

fields. For example, to imply that an IP packet is inside an Ethernet frame, the Type field

would have a value of hexadecimal 0800 (decimal 2048).

If the 802.3 Type/Length field (in Figure 3-14) has a value less than hex 0600 (decimal1536), the Type/Length field is used as a Length field for that frame, identifying the length

of the entire Ethernet frame

when sending IP packets, the Ethernet frame has two additional headers:

An IEEE 802.2 Logical Link Control (LLC) headerAn IEEE Subnetwork Access Protocol (SNAP) header

Note that the SNAP header Type field has the same purpose, with the same reservedvalues, as the Ethernet Type/Length field.

Error detection

The Ethernet Frame Check Sequence (FCS) field in the Ethernet trailerthe only field in

the Ethernet trailerallows a device receiving an Ethernet frame to detect whether the bits

have changed during transmission.

To detect an error, the sending device calculates a complex mathematical function, with theframe contents as input, putting the result into the frames 4-byte FCS field.

The receiving device does the same math on the frame; if its calculation matches the FCS

field in the frame, no errors occurred. If the result doesnt match the FCS field, an erroroccurred, and the frame is discarded.

resumen ccna cap 2

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