Cisco Three Layer Model

Ø Access Layer

Ø Distribution Layer

Ø Core Layer

Access Layer

ü Also called desktop layer

ü End user are connected

ü User and resources are locally available

ü Entry point to the network

ü Bottom most layer at the Cisco hierarchical model

ü Devices available are: switches, hubs, etc.

Distribution Layer

ü Primary functions are: -routing, filtering, and WAN access.

ü Other functions are: -

o Aggregation of access layer devices;

o Routing traffic to provide departmental or workgroup access;

o Segmenting the network in to multiple broadcast/multicast domains;

o Translating between different media types, such as Token Ring and Ethernet;

o Providing security and filtering services;

o After choosing the path, forwards the request to the Core layer.

Core Layer

ü Also called backbone layer

ü Switches traffic to the appropriate service (e.g. e-mail, internet access, videoconfrencing)

ü Provides quick transport to the desired enterprise service.

The Three-Layer Hierarchical Network Model

IC/32

Cisco Switch Product Line Cisco Router Product Line

IC/35 IC/36\

Layer Name	Function	Example
1.Application (Layer 7) (Associated word: Shared)	Provides communication services to various applications e.g. word processor Deals with user interface Provides n/w services to the user e.g. email, file & print services, terminal emulation, login validation, file format translation.	File Transfer-TFTP, FTP, NFS. E-mail- SMTP Remote Login-Telnet, r log in. Network Management-SNMP.
2.Presentation (Layer 6) (Associated word: Format)	Translation of data Define data format, presentation Define encryption e.g. FTP enables us to choose binary or ASCII transfer	JPEG, ASCII*, EBCDIC, TIFF, GIFF, PICT, MPEG, MIDI
3.Session (Layer 5) (Associated word: Dialog)	Keeping different application data separate. Define-how to start, how to control and how to end conversation (session) b/w presentation layer entities. Includes-control and management of multiple bi-directional messages. Coordinates-communication b/w systems and serves to organize their communication by offering three different modes: -simplex, half-duplex, and full duplex. Allows-presentation layer to have seamless view of incoming stream of data. Basic Function: - ü Connection Establishment ü Connection Maintenance ü Connection teardown	RPC, SQL, NFS, Net BIOS names, SCP e.g.ATM machines do not teardown sessions unless all the activities are performed.
4. Transport (Layer4) (Associated word: Reliability)	Segmentation of data stream Provide-end to end data transport/delivery services Establish-logical connection b/w sending host and receiving host. Provide –choice of protocol that do or do not provide error recovery, multiplexing of data, session establishment, teardown of VCs Reordering- of incoming data stream, when out of packets are arrived. Basic Functions: - ü Flow control ü Connection oriented communication ü Windowing ü Acknowledgement ü Sequencing	TCP, UDP, SPX
5.Network (Layer 3) (Associated word: Datagram or Segment)	Segmentation of contention networks Define: -end to end delivery of packets, logical addressing, how routing works, how to fragment a packet in to smaller packets to accommodate media with smaller MTU.	IP, IPX, Appletalk
6.Datalink (Layer 2) (Associated word: Frame)	Define – delivery across a one particular link Organize the streams of bits sent by physical layer into logical groups called frames. Adds up control information (SA, DA, frame length, information about upper layer protocol etc.) at the upper end called frame header. Gives following details to the traffic: -where to go, what to do on reaching destination Basic Functions: ü Combines bits in to bytes and bytes into frame. ü Access to the media using MAC address. ü Error detection and error recovery	IEEE-802.2, 802.3,802.5, HDLC, FR, PPP, FDDI

7.Physical

(Layer 1)

(Associated word:

Bits)

Defines –physical characteristic (media type, connector type, signaling type) of the transmission media.
Specifies- electrical, mechanical, procedural and functional requirements for activating, maintaining and deactivating the physical link between end systems.
Other physical characteristics: voltage level, data rates, MTU and physical connectors.

EIA/TIA-232, V.35, EIA/TIA-449, V.24, RJ45, IEEE-802.3, 802.5,

FDDI

*ASCII-American Standard Code for Information Interchange

Router Boot Sequence

Step1: Power on Self Test (POST)

It is an h/w test to verify that all components are operational and present.
POST is stored and run from ROM.

Step 2: Load and run bootstrap code

Bootstrap code is used to execute subsequent programs.
Finds the IOS s/w, loads and run it.
Not used until next time router is reloaded.

Step2.1: Find the IOS software

Bootstrap code finds the IOS image /software from the flash memory by default.
Configuration register and configuration file in the NVRAM help determine IOS image.

Step.2.2: Load the IOS software

Bootstrap code loads the IOS image into RAM and starts IOS running.

Step 3: Find the configuration

By default IOS looks in NVRAM for a valid configuration file.
File is called start-up configuration.
If no configuration exists or is ignored, router enters in to setup mode.

Router Components

Internal configuration components

RAM
NVRAM
Flash
ROM
Configuration Register

External configuration components

Interfaces- makes physical connections to the outside world.

Ethernet and fast Ethernet
Asynchronous & synchronous serial
Token Ring
FDDI
ATM
Console and Auxiliary port

1.RAM (Random Access Memory)

It is read and write memory.
It contains software and data structure.
It is principal software running the Cisco IOS image and Running-Configuration.
Some Routers (e.g.2500 series) run IOS from Flash and not RAM.

2.NVRAM (Nonvolatile RAM)

Stores initial or startup configuration file.
It is not erased when the router/switch is reloaded.
Uses a battery to maintain the data when power is removed from the router.

3.Flash Memory

Stores fully functional IOS image.
Not erased when the router is reloaded.
It is an EEPROM created by Intel.
Can be used to store configuration files on Cisco 7500 series platforms.
Some routers (e.g.2500 series) run the IOS image directly from flash and not from RAM.

4.ROM (Read Only Memory)

Contains micro codes to start and maintain the router
Stores bootable IOS image
Contains the code that is used to boot the router until the router knows where to get the full IOS image.

5.Configuration Register

It’s a 16-bit software register, written into NVRAM.
Used to control how the router boots up.
Value can be seen with the show version command and typically is 0X2102,which tells the router to load the IOS from flash memory.

6.Interfaces

Relax Infotech

CHAPTER 1 ROUTER & LAYER