Cisco Module 5

8/2/2019 Cisco Module 5

1/51


2/51

Development of Ethernet

Grew out of Alohanet developed at

University of Hawaii in 70s. Multiple access to shared radio frequency

Formed basis for Ethernet MAC method

CSMA/CD (Carrier Sense Multiple Access /Collision Detect

Original Ethernet was first LAN

First Standard published as consortium ofDigital Equipment Company, Intel, Xerox (DIX)in 1980


3/51

IEEE Standards

IEEE standardized original Ethernet

standards

IEEE official Ethernet standards are802.3

802.3 standard has been supplementedas improvements in physical media haveoccurred. Still remains 802.3

Reason Ethernet is scalable


4/51

IEEE Ethernet Naming Rules

Ethernet refers to family: Ethernet, Fast

Ethernet, Gigabit Ethernet (Gig-E), and10-Gb Ethernet

As Ethernet is expanded, IEEE issues new

supplement. New supplements are given 1 or 2 letter

designation and an abbreviateddescription (Identifier) 802.3u = Fast Ethernet

10BASE2 (IEEE 802.3a)

1000BASE-TX (IEEE 802.3X)


5/51

IEEE Ethernet Naming Rules (Contd)

Description consists of three parts

Number indicating bps Tx Word BASE indicating Baseband TX Numbers referring to Coax Cable segment

length

One or more letters indicating type of mediumused F = Fiber optic cable T = Copper unshielded twisted pair

Baseband signaling whole bandwidth usedfor signal. Used in Ethernet Broadband signaling Analog carrier signal is

modulated by data signal


6/51

IEEE

Standards making organization

Goals

Supply information to build devices to

comply with Ethernet standard Not stifle innovation

Equipment Mfgs not required to

meet standards


7/51

802.3/Ethernet and OSI Model

Standards specify

Physical media and connectors Communication via Data Link Layer

Encapsulation of protocol-specific traffic

Data link layer broken into 2-sublayers Media Access Control (MAC) [802.3]

How to Tx frames on physical medium,physical addressing, line discipline, network

topology Logical Link Control (LLC) [802.2]

Logically ID different protocols forencapsulation.


8/51

Layer 2 comparisons

Layer 1 Limitation Layer 2 Solution

Cannot communicate with theupper-level layers

Communicates with upper-level layers via the LLC sublayer

Can only describe stream ofbits

Uses framing to organize orgroup bits. (makes bits havemeaning

Cannot identify computers Identifies computer usingMAC Addressing

Cannot decide whichcomputer will Tx data from

group of computers trying toTX at same time

Uses MAC sublayer toaccomplish decisions


9/51

LLC and MAC

MAC is concerned with physical

media

LLC is independent of physical

media Allows multiple Layer 3 protocols (IP,

IPX, AppleTalk) to be supported along

with multiple frame types


10/51

MAC Addressing

All computers on a network must be

able to be uniquely identified inorder for frame delivery

Media Access Control (MAC) addressis located on NIC Hardware address

NIC address

Layer 2 address

Ethernet address


11/51

MAC addressing (Contd)

MAC address is 48 bits

Expressed at 12 hexadecimal digits

First 6 digits are assigned to Mfg by IEEE

Organizat ionally Unique I dent ifier (OUI )

Last 6 digits are assigned by vendor

OUI Vendor Assigned

00 60 2F 3A 07 BC Cisco Particular Device


12/51

MAC addressing (Contd)

Data link layer adds H e a d e r and

T r a i l e r to upper level data Header and Trailer contain control

information for corresponding Layer 2at destination

Upper-layer data is encapsulatedbetween Layer 2 Header and Trailer


13/51

Broadcast LANs

Ethernet and 802.3 are B r o a d c a s t

Networks All stations see all frames

Each station checks every frame to

determine if frame is destined for thatstation

Part of data contained in frame isdestination MAC address On finding matching address, destination

passes data in frame to upper-levelprotocols


14/51

Framing

Framing allows essential

information to encoded into the bitstream

Which computer is talking to whichcomputer

Where data starts, stops, what protocol

to use


15/51

Generic Frames

Different types of frames described

by different standards

Generic Frame Fields

Frame Start Address

Length / Type / Control

Data

Frame Check Sequence (FCS)


16/51

Frame Fields Frame Start

Tells NIC where frame starts

Address Source and Destination MAC Addresses

Length and Type If required describes length of the frame and

protocol type

Data Field Contains data being Tx

Frame Check Sequence Field Number based on data in frame. Sending computer

calculates the value and places it in FCS. Rxcomputer calculates number and checks it againstFCS value


17/51

Ethernet Frame Structure

Ethernet II Frame

Structure

Oct et s in Each Fr am e Field Fr am e Field

8 Preamble

6 Destinatin MAC Address

6 Source MAC Address

2 Type Field

46 to 1500 Data and Pad

4 Frame Check Sequence (CRC

Checksum)


18/51

Ethernet Frame Fields Preamble

Alternating 1s and 0s used in timing for 10 Mbps and slower

Start Frame Delimiter (SFD) Marks end of Start Frame

Destination Address Destination MAC Address

Source Address Source MAC Address

Length/Type If value 1536 indicates Type

Decoded per protocol indicated Data and Pad

Any length > 64 octets and < 1518 octets that does not exceedMaximum Transmission Unit (MTU)

Frame Check Sequence


19/51

Ethernet Operation

Carrier Sense Multiple Access /

Collision Domain (CSMA/CD) Provides media access control

strategies

Media Access Control protocolsthat determine which computer on a

shared domain (collision domain) isallowed to Tx.


20/51

Media Access Control

MAC and LLC comprise Layer 2

2 categories of MAC

Deterministic

Token Ring, FDDI Nondeterministic (probalistic)

Ethernet / 802.3


21/51

Deterministic MAC Protocols

Token Ring

Hosts in ring

Token circulates around ring

Grabs Token

Tx data for limited time

Releases Token


22/51

Nondeterministic MAC Protocols

First come / First served (FCFS)

CSMA/CD

Listen for quiet

Begin Tx

More than 1 Tx at same time collision

Frame is lost

All other hosts hear collision

Wait random time (backoff) Retransmit


23/51

Topologies

Ethernet

Logical bus, physical star, extendedstar

Token Ring Logical ring, physical star

FDDI

Logical ring, physical dual-ring


24/51

Ethernet Broadcast Broadcast every host on net receives

every packet Only device that has matching MAC address

will pay attention to packet.

Remaining devices drop the packet

Ethernet not concerned with Layer 3 Checks packet for errors

Detects error packet is dropped

Destination does notnotify Source of droppedpacket

Connectionless architecture

Best-effort delivery system


25/51

Simplex, Half-, Full-Duplex

Simplex Unidirectional: data only

travels in one direction Half-duplex Data travels in both

directions but only one direction at

a time. Full-duplex Data travels both

directions at the same time With switching, no collisions occur

Can achieve full bandwidth


26/51

Ethernet Timing


27/51

Types of collisions

Single collisions

A collision detected while trying totransmit a frame

Multiple collisions

Same frame collided repeatedly beforebeing successfully Tx

Deferred Tx No collision frame was delayed in

being Tx because of busy medium


28/51

Result of Collisions

As a result of collisions, corrupted

or partial frames can occur


29/51

Local, Remote, Late collisions

Local collision

On coax, signals colliding in eachdirection overlap and cancel/emphasizewaveform causing overvoltage

(collision) On UTP, detects signal on RX at same

time sending on Tx (Half-duplex only)

Excessive crosstalk and be perceived ascollision


30/51

Remote collision

Typically a frame that is a runt and

FCS is invalid Results from collisions on far side of a

repeated connection


31/51

Late Collision Occurs after the first 64 octets of Tx have

been sent. Theoretical limits of network propagation have

been exceeded by that point

802.3 allows automatic retransmission of latecollided frames, but not required

Explicitly denies automatic retransmission inGigabit Ethernet

Occurs after slot t imehas elapsed and on farside of a repeater


32/51

Ethernet Errors Late collisions are considered to be errors

Frequency of errors will indicate urgency of problem

Simultaneous Tx before slot time Collision or runt

Simultaneous Tx after slot time Late collision

Excessively or Illegally Tx Jabber, long frame, range errors

Illegally short Tx Short Frame, Collision fragment, runt

Corrupted Tx FCS error

Insufficient or Excessive Number of Bits Tx Alignment error

Mismatch of actual or reported number of Octets in frame Range error

Unusually long preamble or jam event Ghost or jabber


33/51

Jabber

Tx of at least 20,000 to 50,000 bit-

times in duration More properly called Long frame

May or may not have valid FCS


34/51

Long Frame

Greater than maximum legal size

Takes into account if frame is tagged Does not count as an error if frame is

802.1Q tagged


35/51

Short Frame

< minimum 64 octets and good FCS

Sometimes called runts


36/51

FCS Errors

FCS Error occurs when Checksum

values differ by 1 or more bits fromwhat was Tx.

High number from single stationindicates faulty NIC, bad drivers, badcabling at station

Errors associated with many hostsusually indicates: bad cabling, baddrivers, faulty hub port, induced noise


37/51

Alignment Error

Data does not end at octet

boundary has extra bits left over Extra bits truncated and if FCS fails

then error reported

Usually caused by:

Bad drivers or collision

Read/Write error in software

Can overload router CPU leading to crash


38/51

Range Error

Legal value in field length but

doesnt match actual number ofoctets in data field


39/51

Ghost

Noise on the cable that appears to

be a frame, but isnt >= 72 octets long

Ghosting network is slow for no apparent

reason


40/51

Autonegotiation Allows interfaces to match Tx and Rx

speeds with other interfaces 10BASE-T required a link pulse every 16 ms.

Turned into Normal Link Pulse (NLP)

Series of NLP Tx called Fast Link Pulse (FLP)

FLP consists of 33 NLP bursts = 16 bit datacode

Pages can be added representing additionalmore sophisticated negotiation and link

parameters After decoding an FLP an acknowledgement is

sent. Communicating partners can keep movingup pages to establish best link parameters


41/51

Full, Half-duplex link establishment

Autonegotiation optional for most

implementations Required for Gigabit networks

Two ways of establishing Full duplex Completed autonegotiation cycle

Administratively set

If setting one, m u s t set the other Gigabit does not support half-duplex


42/51

Collision and Broadcast Domains

3 types of media environments

Shared-media Multiple hosts have access to same media

Extended shared-media

Networking devices have extendednetworking to longer cable lengths ormore hosts

Point-to-Point One device only connected to one device


43/51

Indirectly connected networks Circuit-switched

Point-to-Point communication on an electricalcircuit that exists for duration of theconnection.

Not shared environment no collisions Telephone system

Packet-switched Packets sent with sufficient address info to

reach destination. Packets may travel throughdifferent circuits

Logical point-to-point connection Not shared environment no collisions

Cell phones and Internet


44/51

Collisions and Collision Domains

Collision occurs when 2 hosts Tx at

same time on shared medium Collision domain connected

physical segments where collisionscan occur

Layer 1 devices do not break up

collision domains Layer 2 & 3 break up collision domains

Segmentation


45/51

Layer 1 Devices and collision domains

Layer 1 devices simply extend

Ethernet cable segments More potential users

All traffic is passed on through layer 1

devices Greater number of potential collisions

Four repeater rule in place so timing

conditions assure all hosts hear collisions Repeater, NIC, Propagation delay increase

latency


46/51

Segmentation

Layer 2 and 3 devices segment

large collision domains into more,smaller collision domains

Decisions based on MAC addresses

Signals only propagated to correctsegment

Less traffic on segment

Increased bandwidth for hosts on segment


47/51

Layer 2 Broadcasts When node needs to communicate with

all hosts on network, sends broadcastwith MAC destination address0xFFFFFFFFFFFF. Every NIC must

recognize this address. Layer 2 devices must forward all

broadcast traffic to all segments

Accumulation of broadcast and multicasttraffic called broadcast radiation.


48/51

Layer 2 Broadcasts 3 sources of broadcast traffic

Workstations broadcast ARP requests in orderto locate MAC address

Routers broadcast routing protocol updatesperiodically to keep routing tables corrects

RIP broadcasts updates every 30 seconds

IP multicasts send streaming multimedia data.

Unless Layer 3 is used, is sent to all segments


49/51

Broadcast domains Grouping of collision domains

connected by Layer 2 devices. Must be controlled at Layer 3

Layer 3 devices do not forward broadcasts

to other segments Layer 3 forwarding based on IP address,

not MAC address


50/51

Data Flow Layer 1 devices always forward a

frame Layer 2 devices forward frame

unless prevented

Layer 3 devices block frames unlesstold to forward


51/51

Network Segment Segment can have different meanings

when referring to networking Layer 4 context means breaking up data

stream from Layer 5

Network context means a section of a networkbounded by bridges, switches, and routers

LAN topology means continuous circuit oftenconnected to other segments by repeaters

Cisco Module 5

Documents

Transcript of Cisco Module 5