CCENT Exam Notes

8/4/2019 CCENT Exam Notes

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CCENT (Cisco Certified Entry Network

Technician) Certification Exam Notes

A. OSI Model

1. The 7 layers of OSI model are:

1. The Application Layer: Application layer is responsible for identifying and establishing the

availability of intended communication partner and verifying sufficient resources exist for

communication. Some of the important application layer protocols are: WWW, SMTP, FTP

etc.

2. The Presentation Layer: This layer is responsible for presenting the data in standard

formats. This layer is responsible for data compression, decompression, encryption, and

decryption. Some Presentation Layer standards are: JPEG, MPEG, MIDI, PICT, Quick Time,TIFF.

3. The Session Layer: Session Layer is responsible for co-ordinating communication between

systems/nodes. The following are some of the session layer protocols and interfaces: a)

Network File System (NFS), SQL, RPC (Remote Procedure Call), X-Windows, ASP, DNA

SCP.

4. The Transport Layer: The Transport Layer is responsible for multiplexing upper-layer

applications, session establishment, and tearing-down of virtual circuits. This layer does

"flow control" to maintain data integrity. Flow Control prevents the problem of a sending

host on one side of the connection overflowing the buffers in the receiving host.

5. The Network Layer: There can be several paths to send a packet from a given source to a

destination. The primary responsibility of Network layer is to send packets from the source

network to the destination network using a pre-determined methods. Routers work at

Network layer.

6. The Data Link Layer:

Data Link Layer is layer 2 of OSI reference model. This layer is divided into two sub-layers:

A. Logical Link Control (LLC) sub-layer.

B. Media Access Control (MAC) sub-layer.The LLC sub-layer handles error control, flow control, framing, and MAC sub-layer

addressing.

The MAC sub-layer is the lower of the two sub-layers of the Data Link layer. MAC sub-layer

handles access to shared media, such a Token passing or Ethernet.

7. Physical Layer: The actual flow of bits take place through Physical layer. At Physical

layer, the interface between the DTE and DCE is determined. The following are some of the

standard interfaces are defined at Physical layer: A> EIA/TIA-232, EIA/TIA-

449,V.24,V.35,X.21,G.703,HSSI (High Speed Serial Interface).

2. Few Important things to note about OSI layer functions:


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1. Application layer is responsible for ensuring the availability of intended communication

partner. This layer is also responsible for synchronizing sending and receiving applications.

2. Presentation Layer is responsible for negotiating data transfer syntax.

3. Session layer is responsible for co-coordinating communication between systems

4. Transport layer is responsible for multiplexing upper-layer applications. It is also

responsible for establishment and tear down of virtual circuits.

3. Data Link Layer is layer 2 of OSI reference model. This layer is divided into two sub-

layers:

1. Logical Link Control (LLC) sub-layer.

2. Media Access Control (MAC) sub-layer.

The LLC sub-layer handles error control, flow control, framing, and MAC sub-layer

addressing.

The MAC sub-layer is the lower of the two sub-layers of the Data Link layer. MAC sub-layer

handles access to shared media, such a Token passing or Ethernet.

4. DOD Model maps to OSI model as below:Process/ Application maps to OSI's Application, Presentation, Session layers (layers 7,6,5).

Host-to-Host maps to ISO's Transport layer (layer 4).

Internet maps to ISO's Network layer (layer 3).

Network Access maps to ISO's Data Link and Physical Layers (layers 2, 1).

5. Switches work at layer 2 of ISO model, which is Data Link Layer. A switch looks at the

destination MAC address before forwarding the frame.

6. TCP and UDP work at transport layer of OSI model or the Host-to-Host layer of DOD

Model. SNMP uses UDP over IP.

7. The following are frequently used OSI layer 2 (data link) protocols:

a. PPP

b. SLIP

c. Frame Relay

d. HDLC

e. Ethernet

f. Token Ring

8. The following are some Presentation Layer standards:

Graphic and Visual Image: PICT, TIFF, JPEG

Movies and Sound: MIDI, MPEG, Quick Time

The following are Session layer standards:

NFS, SQL,RPC,X-Windows.

9. Physical layer is responsible for bit synchronization. Repeaters work at Physical layer (Layer 1),

Bridges and simple switches work at Data Link Layer (Layer 2),

Routers work at Network Layer (Layer 3) of ISO Reference Model.

10. Segments is associated with Transport layer, Packets is associated with Network Layer

and

Frames is associated with Data Link Layer

B. TCP/UDP concepts

1. FTP (File Transfer Protocol) is used for transferring files from and to a client computer to a

host. FTP is a connection oriented protocol. It uses TCP/IP for file transfer. Compare thiswith UDP protocol used by TFTP.


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2. Telnet:

Telnet command works at Layer 7 of your TCP/IP stack. If you are able to Telnet to your

remote router, you can be sure that the TCP/IP stack is properly installed.

Other commands, such as Ping, trace, and ipconfig work at lower layers of the TCP/IP stack.

Telnet is used for terminal emulation that runs programs remotely.

FTP is used to transfer files. TCP/IP is the protocol used when you are Telnetting to a remote host. HTTP is used for

accessing the World Wide Web services.

3. The default port numbers used for some important services are:

FTP:21

Telnet:23

SMTP:25

WWW:80

SSL:443 (Secure socket layer)

When you are accessing a Web page, the browser sends the request over the default port

number (For WWW, the default port number is 80). For any reason, If you change the defaultport number that a Web server listens to, users need to specify the port number in the URL.

4. The distinguishing characteristics of a TCP frame are Sequence Number, Acknowledgment

Number, and Window.

5. TCP and UDP Protocol Characteristics:

TCP is a full-duplex, connection-oriented protocol. It incorporates error checking as well.

UDP (User Datagram Protocol): UDP is a thin protocol. UDP is a connectionless protocol.

It doesn't contact the destination before sending the packet and doesn't care whether the

packet is reached at the destination. UDP uses port number 6.

6. The port numbers used by different programs are as below:

FTP : Port #21

Telnet: Port #23

SMTP: Port #25

SNMP: Port #161

It is also worthwhile to know that FTP, Telnet, SMTP use TCP; whereas TFTP, SNMP use

UDP.

7. UDP is known as unreliable protocol because there is no way for the source to know if any

packets are lost during transit to the destination. Important fields that are available with TCPheader, but not available with UDP header include sequence number, acknowledgement

number, and window size.

C. IP Addressing and Subnetting

1. IP addresses are written using decimal numbers separated by decimal points. This is called

dotted decimal notation of expressing IP addresses.

The different classes of IP addresses is as below:

Class Format Leading Bit Network address Maximum Maximum hosts pattern Range

networks / nodes

A N.H.H.H 0 0 - 126 127 16,777,214B N.N.H.H 10 128 - 191 16,384 65,534


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C N.N.N.H 110 192 - 223 2,097,152 254

Network address of all zeros means "This network or segment".

Network address of all 1s means "all networks", same as hexadecimal of all Fs.

Network number 127 is reserved for loopback tests.

Host (Node) address of all zeros mean "This Host (Node)".

Host (Node) address of all 1s mean "all Hosts (Nodes)" on the specified network.

2. Class B network has the form N.N.H.H, the default subnet mask is 16 bits long. There is

additional subnet mask of 7 bits long.

7 bits of subnet mask corresponds to (2^7)=128 subnets.

9 bits (16-7) of host addresses corresponds to (2^9-2)=512-2 = 510 hosts.

Some times, the subnet mask is specified with the bits available in the default subnet mask. In

this case the bits available in default subnet mask is 16. Therefore, total number of bits

available in the subnet mask are 16+7=23. If you are given a subnet mask of 23 bits long for

a class B address, it is understood that it contains the bits from the default subnet mask as

well.

Class C IP address has the form N.N.N.H. You have 8 bits available for host portion, ofwhich 4 bits are used for subnetting. This means, you have 2^4 = 16 subnets. Similarly, you

have 4 bit available for host Ids. This equals 2^4 = 16 host Ids. Out of this, host Ids of all 1s'

and all 0s' are reserved. This leaves us with 14 host Ids per subnet.

3. The address range and the corresponding binary values for the first octet of IP classes are

as given below:

Class A: 0-127 binary: 00000000-01111111

Class B: 128-191 binary: 1000000010111111

Class C: 192-223 binary: 1100000011011111

Class D: 224-239 binary: 1110000011101111

Note that the IP address 172.54.17.6 belongs to class B, and hence the applicable binary

address range is 1000000010111111.

4. The following are true about the IP address 132.145.12.5/25

a. Subnet mask: 255.255.255.128

b. Subnet address: 132.145.12.0

c. Broadcast address: 132.145.12.127

d. Lowest valid host address in the given subnet: 132.145.12.1 (A host address of all zeros

cannot be used)

e. Highest valid host address in the given subnet: 132.145.12.126 (A host address of all ones

cannot be used)f. Valid host address range: 192.145.12.1 - 192.145.12.126

5. The number of distinct IPs required in an IP network are:

1. One each per client computer

2. One each per server computer

3. One each per router interface.

6. The range of numbers from 224.0.0.0 to 239.255.255.255 are used for multicast packets.

This is known as Class D address range.

7. Subnetting:Subnetting is nothing but creating networks within a network. Subnetting allows an


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organization with a single IP address (Class A /Class B /Class C) to have multiple

subnetworks, thus allowing several physical networks with in the organization.

The subnet mask is computed as below:

1. Find the Class of the IP address, in this case it is a class B network. Class B network has

the form N.N.H.H. Therefore, we have a total of 16 bits (two octets) for assigning to internal

networks and hosts. The minimum number of host addresses required is 500 (see thequestion). The last octet corresponds to 2^8 = 256 hosts which is still less than 500 Hosts..

Therefore, you have to borrow one more bit from the third octet to make it 256*2 = 512

Hosts. This leaves 7 bits in the third octet for assigning subnet addresses. This is equal to

2^7=128 subnets.

2. Write the 7 bits available for subnetting in third octet in the form 11111110 (last bit being

the Host bit). The decimal equivalent of the first seven bits is

2^7+2^6+2^5+2^4+2^3+2^2+2^1

= 128 + 64 +32 + 16 + 8 + 4 + 2 = 254.

3. Now the subnet mask required is 255.255.254.0.

8. Consider a host IP 156.16.3.47: The available hosts with a 11 bit subnet mask, may becomputed as below:

11 bit subnet mask is equivalent to 255.255.255.224; It will accommodate 2,046 subnets,

each with 30 hosts. The host address of 156.16.3.47 is in the 32 subnet range. The valid hosts

for subnet 32 are 33-62. The broadcast address is 63.

9. To find the subnet that a host 56.16.3.52 with 12-bit subnet mask belongs to:

A 12-bit subnet mask gives us 255.255.255.240; 4094 subnets, each with 14 hosts. (Host

addresses of all zeros and all 1s can't be assigned). The 156.16.3.52 is in the 48 subnet range.

The valid range is through 49 - 62. 63 is a broadcast address.

Here is how you get the subnet range:

1. Find the subnet mask. In this case, default subnet mask for Class B address is 255.255.0.0.

There are additional; 12 bits in the subnet mask. Now the subnet mask looks like:

11111111.11111111.11111111.11110000. This is equal to 255.255.255.240.0.

2. Now, deduct the lowest value octet (Which is non zero), from 256. Here, (256-240)=16.

This is the value that the subnets are incremented. Therefore, you will have hosts with values

from:.

156.16.3.1 to 156.16.3.14 (All 0s and all 1s host addresses can not be used)

156.16.3.17 to 156.16.3.30

156.16.3.33 to 156.16.3.46

156.16.3.49 to 156.16.3.62 and so on.

It is important to know that subnets are incrementing by a factor of 16.

10. The subnet Id of a host with IP address 165.212.18.5/20 is 165.212.16.0

The available host address range is 165.212.16.1 - 165.212.31.254

Broadcast address: 165.212.31.255

11. Variable-Length Subnet Masking:

Variable-length subnet masking (VLSM) means implementing more than one mask on the

same major class of a network. It allows for a more efficient use of IP address space both in

terms of hosts and subnets, and can be essential on a network that does not have an

abundance of IP address space.

The following routing protocols support VLSM:1. RIP v2


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2. EIGRP

3. IS-IS

4. OSPF

5. BGP

D. Router commands

1. Key word: COPY This command copies configuration information to specified location. The following are

some frequently used COPY commands:

COPY RUNNING-CONFIGURATION STARTUP-CONFIGURATION (alternatively, you

can use an older version of the command, WRITE MEMORY): This command saves the

current configuration to NVRAM.

Alternatively, we can issue the command using short form:

COPY RUNNING STARTUP - Copies configuration from RAM to NVRAM

COPY STARTUP RUNNING - This command merges configuration from NVRAM to

RAM. COPY FLASH TFTP - Copies current IOS from router flash memory to TFTP server.

COPY TFTP FLASH - Copies image file from TFTP server to flash. This is used to upgrade

the IOS image file to a newer version, or if your IOS image becomes corrupt.

2. "sh int " is a very useful command. It displays the following information:

1. Hardware address

2. Status of interface and the line protocol - carrier detect brings up the serial port(which

means that physical layer connections are working) and keep alive bring up the line protocol

(which means that Data link layer protocol is working)..

3. MTU, BW, DLY, rely, and load metrics.

4. Encapsulation type (layer 2, Data link layer) - HDLC is the default.

3. SHOW command is extensively used for seeing the status and configuration information of

the router. Some of the frequently used commands are:

1. SHOW RUNNING-CONFIGURATION -This command displays the router's active

configuration file, passwords, system name, and interface settings, interfaces IP addresses etc.

2. SHOW INTERFACE - Shows status and configuration information of the local interfaces.The first line says something like TokenRing1 is up, line protocol is up. The first part

TokenRing1 is up describes the physical layer components such as electrical cabling and

signaling are OK. The second part line protocol is up means that the router is detectingkeep-alive messages. The router may be put into administratively down status, at which point

the line would read, TokenRing1 is administratively down, line protocol is down.

3. SHOW INTERFACE SERIAL 0 - Shows the serial 0 configuration.

4. SHOW INTERFACES - Displays statistics for all interfaces configured on the switch.

5. SHOW PROCESS - Displays a routers CPU utilization.

6. SHOW CONFIG - Displays information on the startup configuration.

7. SHOW VERSION - Displays information about the system hardware (RAM/ROM),

software version, names of configuration files, and boot-images. This command will also

show the current configuration register value.

8. Show IP protocol: This command will show information on RIP timers including routing

update timer (30sec default), hold-down timer (default 180sec). It also displays the number ofseconds due for next update (this is fraction of update timer). This command also gives the


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network number for which IP RIP is enabled, Gateway, and the default metric.

Show IP route: This command will display the IP routing table entries. In addition, it displays

the Gateway of last resort (if one is assigned). It also displays the codes used for various

types of routes. Some of the important codes are:

C: directly connected;

S: Statically connectedI : IGRP

R : RIP

show IP interface: This command shows you interface-wise information such as IP address

assigned to each interface, whether the interface is up, MTU etc.

Debug IP RIP: Debug IP RIP will turn the RIP debugging ON. This will display a continuous

list of routing updates as they are sent and received. This leads to lot of overhead, which is

the reason that you use "undebug ip rip" to turn-off debugging as soon as you finish with

debugging.

9. show version: This command displays the current version of the Cisco IOS. In addition,

this command displays the following important information:

- How long the router has been up (length of time since boot-up).- How the system was started (power on etc.)

- From where the system was loaded from (booted via flash , or tftp etc.)

- The contents of configuration register.

10. sh hosts ---> displays the host names and related IP addresses.

11. sh int s0 ---> Among other things, you can see the encapsulation type (layer 2) used.

4. The auxiliary password is used to set the password for the auxiliary port. Assuming that

you are at # prompt, the sequence of commands are:

1. RouterA#config t

2. RouterA(config)#line aux 0

3. RouterA(config-line)#login

4. RouterA(config-line)#password

Now you are set with a password . Type "Z " to take you to the # prompt or

"exit" to go back to global configuration "RouterA(config)#" prompt.

Similar procedure is applicable for setting vty and console passwords as well.

5. A banner is displayed whenever anyone logs in to your Cisco router. The syntax for

configuring the banner is

"banner motd # " . MOTD stands for "Message Of The Day".

# symbol signifies the start of the banner message to the router. You will be prompted for themessage to be displayed. You need to enter "#" symbol at the end of the message, signifying

that the msg has ended.

There are five different types of passwords:

1. ENABLE PASSWORD - A global command that restricts access to privileged exec mode.

This is a non-encrypted password.

2. ENABLE SECRET - Assigns a one-way encryptographic secret password, available in

versions 10.3 and up. This secret password is used instead of the enable password when it

exists.

3. Virtual Terminal Password (vty password): The virtual terminal password is used for

Telnet sessions into the router. The password can be changed at any time. It can be set upwhen you configure the router from the console. There can be five distinct passwords


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corresponding to each vty (vty0 to vty4) or there can be a single password for all vtys.

4. Auxiliary Password: Auxiliary password is used to set password to the auxiliary port. This

port is used to access a router through a modem.

5. Console Password: Console password is used to set the console port password.

E. Cisco IOS

1. Cisco router loads the IOS image based on the boot field and the boot system commands inthe configuration. Various boot system commands are:

1. boot system flash ;IOS image is loaded from flash memory

2. boot system tftp ;Boots the system image from the

TFTP server.

3. boot system rom ; IOS is loaded from rom

4. The command to save the running configuration to NVRAM is:

copy running-config startup-config (write memory)

2. At times it may be necessary to upgrade the IOS software. You cannot load the router from

Flash, and upgrade the flash at the same time. You need to load from other sources such as

tftp server or ROM. To load the image from ROM, there are two ways:

BY using config-register 0X2101 command. This will enable the router to load the IOS

from the ROM and Flash image is not used. This way we can load the upgrade to flash, and

set back the configuration register to load from flash as usual after the upgrade is completed.

Another way is by adding the command BOOT SYSTEM ROM to the startup configuration

file.

3. Boot system - This is a global command that allows you to specify the source of the IOS

software image to load. If you configure more than one source, attempts are made to load the

IOS from the first command in the configuration to the last successively. If the first fails, the

second boot command is used.

Boot system rom - Loads IOS from ROM.

Boot system flash - Loads the first file from flash memory.

Boot system tftp < tftp_address > - Loads IOS with a filename

from a TFTP server.

Both "copy star run" and "reload" will load Cisco router configuration into RAM.

4. Router Memory Components:

ROM (Read Only Memory); Memory containing micro-code for basic functions to start and

maintain the router. ROM is not typically used after the IOS is loaded. RXBOOT is located

here.

RAM/DRAM: stores the running configuration, routing tables, and packet buffers. Some

routers, such as the 2500 series, run IOS from Flash, not RAM.

NVRAM (Non-Volatile Ram): Memory that does not lose information when power is lost.

Stores the systems configuration file and the configuration register. NVRAM uses a battery

to maintain the data when power is turned off.

Flash Memory: Stores the compressed IOS (IOS stands for Cisco Internetwork Operating

System) image. Flash memory is either EEPROM or PCMCIA card. Flash memory enables

you to copy multiple versions of IOS software. This allows you to load a new lever of the


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operating system in every router in your network and then, to upgrade the whole network to

that version at a convenient time.

5. When you first switch on a router, you enter Setup mode. Setup mode is different from

configuration mode in that setup mode appears when there is no configuration file present.

Upon entering setup mode, you can supply some basic configuration parameters to Ciscorouter.

6. Router modes of operation:

1. User EXEC mode:- This is the LOWEST level of access. This allows examination of

router status, see routing tables, and do some diagnostics. However, you cannot change the

router configuration, view the configuration files, or control the router in any way. The

prompt in this mode is "Router>".

2. Privileged (enable) EXEC mode:- This mode allows you to have all the privileges of

EXEC (user) mode plus commands that enable you to view configuration files, change the

router configuration, perform troubleshooting that could potentially disrupt traffic. Thedefault prompt for this mode is "Router#".

When you are working in the privileged mode (at # prompt), you can get back to user mode

by typing "disable" at the "#" prompt.

7. The Cisco router can be configured from many locations.

1. Console port: During the initial installation, you configure the router from a console

terminal connected to the "Console port" of the router.

2. Virtual Terminals (vty): A virtual terminal (vty) is typically accessed through Telnet. A

router can be accessed through vty after it the initial installation in the network. There are

five virtual terminals, namely, vty0, vty1, vty2, vty3, vty4.

3. Auxiliary Port: you can configure a router through auxiliary port. Typically, a modem is

used to configure the modem through aux port.

4. TFTP Server: Configuration information can be downloaded from a TFTP server over the

network.

5. NMS (Network Management Station): You can also manage router configuration through

NMS such as CiscoWorks or HP OpenView.

8. There may be times that you have lost the passwords for accessing the router and need to

recover from the lost passwords. ROM Monitor (rommon), stored ROM is used for password

recovery. By hitting special Break key within the first 60 seconds of the router power-on, it ispossible to recover from lost passwords. Broadly, the sequence of operations is as below (For

a 2501 router):

Typical sequence of steps involved in password recovery (on 2501 router) is:

1. Power cycle the router by turning it OFF and ON.

2. Press Break key (key configuration depends on the router type) within 60 seconds of power

on.

3. Use the rommon command o/r 0X2142

4. Use rommon command INITIALIZE to load IOS

5. Skip the setup mode (by choosing NO to configuration commands).

6. Enter privileged mode


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7. After entering the privileged mode, you can view the passwords by issuing appropriate

config commands.

9. During password recovery, the config register and NVRAM are modified. ROM hold the

bootstrap code to start up the router and Flash contain the IO image.

Each exclamation mark (!) represents that one UDP segment has been successfullytransferred. While erasing, you get 'e' displayed on screen.

10. To change the enable password, you use "enable password password".

To change the enable secret, you use "enable secret password".

11. The following are some important commands that can be used to edit and review

command history buffer. It will be useful to practice these commands.

A : Move to the beginning of the command line

E : Move to the end of the command line

F : Move forward one character, same as using "Right Arrow".

B : Move backward one character, same as using "Left Arrow". P : Repeat Previous command, same as using "Up Arrow".

N : Repeat Next (more recent) command, same as using "Down Arrow".

B : Moves to beginning of previous word.

F : Moves to beginning of next word.

R : Creates new command prompt, followed by all the characters typed at the last one.

12. To configure your router to do look-up for domain name server, the correct syntax is:

ip name-server

Here the ipaddress_of_server is the ip address of the DNS server.

13. To enable the Cisco IOS to forward packets destined for obscure subnets of directly

connected networks onto the best route, you use "ip classless" command.

14. By default, Cisco routers support 5 simultaneous telnet sessions. This number can be

configured using ios commands.

i. CDP

1. CDP, short for Cisco Discovery Protocol runs over Layer 2 (the data link layer) on all

Cisco routers, bridges, access servers, and switches. CDP allows network management

applications to discover Cisco devices that are neighbors of already known devices. CDP runson all LAN and WAN media that support SubNetwork Access Protocol (SNAP).

2. The no cdp enable instruction has to be given at the interface configuration mode, where

CDP needs to be disabled. (in this case S0/0).

3. The command "show CDP neighbors" displays all the neighboring devices connected and

their capability. Several capability codes are:

R - Router H - Host

T - Trans Bridge I - IGMP

B - Source Route Bridge r - repeater

S - Switch


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This command displays the following:

1. Neighbor Device ID : The name of the neighbor device;

2. Local Interface : The interface to which this neighbor is heard

3. Capability: Capability of this neighboring device - R for router, S for switch, H for Host

etc.

4. Platform: Which type of device the neighbor is. (2500 router or anything else)5. Port ID: The interface of the remote neighbor you receive CDP information

6. Holdtime: Decremental hold time in seconds

4. The command "terminal no editing" stops all advance editing features.

5. Important CDP Commands:

The command cdp run in global configuration mode enables CDP globally on the router.

The command cdp enable is used to enable CDP on a particular interface/port.

The command used to disable CDP is no cdp enable.

The command ip default-gateway 192.168.1.1 in global configuration mode can be used

to sets the default gateway on a Cisco switch/router.

F. Routing Concepts

1. Routing protocols job is to maintain routing tables and route packets appropriately.

Examples of routing protocols are RIP, IGRP, EIGRP, OSPF. Routers can support multiple

independent routing protocols and can update and maintain routing tables for each protocol

independently.

Routed protocols are used to transport user traffic from source node to destination node.

Examples of routed protocols are IP, IPX, AppleTalk.

2. There are broadly three types of routing protocols:

1. Distance Vector (Number of hops) - Distance vector routing determines the direction

(vector) and distance to any link in the internetwork. Typically, the smaller the metric, the

better the path. EX: Examples of distance vector protocols are RIP and IGRP. Distance vector

routing is useful for smaller networks. The limitation is that any route which is greater than

15 hops is considered unreachable. One important thing that differentiates distance vector

with Link state is that distance vector listens to second hand information to learn routing

tables whereas, Link state builds its routing tables from first hand information. Distance

vector algorithms call for each router to send its entire routing table to each of its adjacent

neighbors.2. Link State Routing: Link State algorithms are also known as Shortest Path First (SPF)

algorithms. SPF recreates the exact topology of the entire network for route computation by

listening at the first hand information. Link State takes bandwidth into account using a cost

metric. Link State protocols only send updates when a change occurs, which makes them

more attractive for larger networks. Bandwidth and delay are the most heavily weighed parts

of the metric when using Link-State protocols. EX: OSPF and NLSP.

Benefits of Link State protocols:

Allows for a larger scalable network

Reduces convergence time

Allows supernetting

3. Balanced Hybrid - Balanced Hybrid combines some aspects of Link State and Distance


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Vector routing protocols. Balanced Hybrid uses distance vectors with more accurate metrics

to determine the best paths to destination networks. EX: EIGRP.

3. Distance vector protocol depends only on Hop count to determine the nearest next hop for

forwarding a packet. One obvious disadvantage is that, if you have a destination connected

through two hops via T1 lines, and if the same destination is also connected through a singlehop through a 64KBPS line, RIP assumes that the link through 64KBPS is the best path!

4. All RIP, RIP2, and IGRP use distance vector algorithms.

RIP2 transmits the subnet mask with each route. This feature allows VLSM (Variable

Length Subnet Masks) by passing the mask along with each route so that the subnet is exactly

defined.

5. In RIP, after route convergence, all routes are distributed to the neighbors by a router.

6. Maximum hop count supported by RIP is 15. A hop count of 16 or greater is considered

unreachable.

7. The command syntax for configuring RIP version1 on a router is:

Router rip

Network

Note that the network numbers are specified as either class A, or Class B or Class C; and

contain no subnetting information.

Example:

Router rip

Network 156.14.0.0

Network 196.12.12.0

G. NAT

1. Given below are the four important forms of NAT (Network Address Translation):

1. Static NAT: It is a one-to-one mapping between an unregistered IP address and a registered

IP address.

2. Dynamic NAT: Usually, Dynamic NAT is implemented, where a pool of public IP

addresses is shared by an entire private IP subnet. When a private host initiates a connection,

a public IP address is selected. The mapping of the computers non -routable IP address

matched to the selected IP address is stored in the NAT Table. As long as the outgoingconnection is maintained, the private host can be reached by incoming packets sent to the

specified public address. When the binding expires, the address is returned to the pool for

reuse.

3. Overloading: A variation of Dynamic NAT, also known as Network Address Port

Translation (NAPT) maps multiple unregistered IP addresses to a single registered IP address

by multiplexing streams differentiated by the TCP/UDP port number.

4. Overlapping: When Overlapping is employed, the IP addresses used on the internal

network are registered IP addresses utilized on another network. To avoid conflict, a NAT

Table is built to translate these redundant internal addresses to a unique IP address. Vice

versa, when sending packets into the private network, the registered addresses must be

translated to an address unique in the network.


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2. Important Network Address Translation (NAT) terms of inside local, inside global, outside

local, and outside global are explained below:

a. Inside local: A private IP address assigned to a host on the inside network.

b. Inside global: A public IP address that represents one or more inside local IP addresses to

the outside world.c. Outside local: The IP address of an outside host as it appears to the inside network. Not

necessarily a legitimate address, it is allocated from an address space routable on the inside.

d. Outside global: The IP address assigned to a host on the outside network. The address is

allocated from a globally routable address or network space.

H. Switching

1. Switches are data link layer devices that enable multiple physical LAN segments to be

interconnected into a single larger network. Two widely used switching methods are store-

and-forward switching and cut-through switching.

2. Switches forward packets based on the physical address (such as MAC address) whereas,

routers forward packets based on logical address (such as IP address). A frames MAC

address doesnt change when being forwarded through a switch.

3. Important types of switching used in Cisco devices:

Store-and-Forward switching: Here the LAN switch copies the entire frame into its buffers

and computes the CRC. The frame is discarded if there are any CRC errors. Giant (more than

1518 bytes0 and Runt (less than 64 bytes) frames are also dropped, if found.

Cut-Through (Real-Time) switching: Here, the LAN switch copies only the destination

address into its buffers. It immediately looks up the switching table and starts forwarding the

frame. The latency is very less because, the frame is forwarded as soon as the destination

address is resolved.

Fragment-Free switching: Here, the switch waits for the collision window before

forwarding the entire frame. The collision window is 64 bytes long.

4. The Catalyst 1900 and 2820 series switches support three types of switching methods:

FastForward (Cut-through): In this type of switching, the packet is forwarded as soon as the

destination address is read. This has least latency.

FragmentFree (Modified cut-through): This type of switching is useful when your network

is experiencing large number of collisions. FragmentFree switching has a latency in between

FastForward and the Store-and Forward. Store-and-Forward: This method stores the entire frame and checks for errors before

forwarding it on to another port. Store-and-forward has the highest latency compared with

both FastForward and FragmentFree.

The default switching method used by Catalyst 1900 series switches is FastForward.

5. In store-and-forward switching, an entire frame must be received before it is forwarded.

This means that the latency through the switch is relative to the frame sizethe larger the

frame size, the longer the delay through the switch. Cut-through switching allows the switch

to begin forwarding the frame when enough of the frame is received to make a forwarding

decision. This reduces the latency through the switch. Store-and-forward switching gives the

switch the opportunity to evaluate the frame for errors before forwarding it. This capability tonot forward frames containing errors is one of the advantages of switches over hubs. Cut-


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through switching does not offer this advantage, so the switch might forward frames

containing errors.

6. The store and forward switch works by first receives the whole packet before

retransmission. Cut-through switch mode retransmits a frame as soon as it reads the

destination address. Fragment-free switch mode reads the first 64 bytes beforeretransmission. Therefore, the latency of cut-through and fragment-free switch modes is

constant, whereas that of store-and-forward varies depending on the length of the packet.

7. Cisco 2950 Switch System LED:

The system LED shows whether the system is receiving power and functioning properly.

LED Off: System is not powered up.

LED color is Green: System is operating normally.

LED color is Amber: System is receiving power but is not functioning properly. The amber

color on a Cisco2950 series switch indicates that the system is malfunctioning.

8. The purpose of port security is to prevent access to the LAN from un-authorized hosts.

9. If no matching MAC entry is found in the MAC table of a switch, the frame will be

broadcast to all other ports except the port through which it had arrived.

10. The MAC address table of a switch would be empty to begin with. However, it builds the

MAC table learning from the frames that arrive at its ports by adding the MAC address and

the corresponding port that it had arrived to the MAC table.

11. Spanning Tree Protocol (STP) 802.1d is used to prevent routing loops. In Cisco Catalyst

5000 series switches, use BDPUs (Bridge Protocol Data Units) to determine the spanning tree

topology. STP uses a Tree Algorithm (STA) to prevent loops, resulting in a stable network

topology.

I. Wireless concepts

1. Wired Equivalent Privacy (WEP) is a protocol used to secure IEEE 802.11 wirelessnetworks. It is part of the IEEE 802.11 wireless networking standard. Wireless networks

broadcast messages using radio, and therefore more susceptible to eavesdropping than wired

networks. WEP was intended to provide confidentiality comparable to that of a traditional

wired network.

2. Wi-Fi Protected Access (WPA and WPA2) is a class of systems to secure wireless (Wi-Fi)computer networks. WPA is forward compatible with the IEEE 802.11i standard, and was

intended as an intermediate measure to take the place of WEP while 802.11i was prepared.

802.11i includes dynamic key exchange, stronger encryption, and user authentication. It is

not backward compatible with WPA. The 802.11i standard is widely known as WPA2.

3. WPA stands for Wi-Fi Protected Access. The key features of the protocol are given below:

1. It supports both static and dynamic key distribution

2. It provides Device Authentication, as well as User Authentication.

3. It uses TKIP (Temporal Key Integrity Protocol) encryption for dynamic key exchange.

Note that WPA2 uses AES encryption where as WPA uses TKIP. AES encryption is a


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stronger encryption protocol.

4. WPA is forward compatible with WPA2.

J. Security

1. The following are the important characteristics of SDM:

1. SDM doesnt use Telnet/SSH for communicating with the router. Actually, a web server

will be running on the router, and the client software will be running on the host computer.

2. SDM uses web interface on a PC, and the user needs to connect to the router over an IP

network and not through Console.

3. The configuration will be written to the routers running configuration file only after the

Finish button is pressed on the SDM wizard. Note that the configuration is not written to the

start-up configuration.

4. SDM configuration wizard allows DHCP client services to be configured, with an option to

add PAT services or not.

2. The Internet architecture provides an unregulated network path to attack innocent hosts.

Denial-of-service (DoS) attacks exploit this to target mission-critical services. DoS attacks,

are explicit attempts to block legitimate users system access by reducing system availability.

Any physical or host-based intrusions are generally addressed through hardened security

policies and authentication mechanisms. Although software patching defends against some

attacks, it fails to safeguard against DoS flooding attacks, which exploit the unregulated

forwarding of Internet packets.

K.Miscellaneous

i. Cables

1. There are different types of cables:

1. Straight through cable: This is also known as patch cable. A straight through cable is used

to connect a computer to a hub or a switch.

2. Crossover cable: A crossover cable is used to connect one computer to the other computer

B without using a switch or hub.

3. Rollover cable: A rollover (also known as console cable) is used to connect to routers for

configuration purpose.

Also, you cant connect the Ethernet port on PC to another Ethernet port on a router using astraight through cable. You need crossover cable for this purpose. Also, the PC need to have

a network card, and appropriate networking protocol stack properly installed and configured.

2. For connecting a switch to another switch, a crossover cable is required. For connecting a

host to a switch, a straight-through cable is used. For connecting a Cisco Router/Cisco Switch

console port, a rollover cable is required.

3. 100BaseT (Fast Ethernet) uses IEEE803.2u standard which incorporates CSMA/CD

protocol.

4. 10Base2, and 10Base5 use coaxial cable for transmission of digital signals. Therefore, itcant support simultaneous transmission and reception. On the other hand, 10BaseT and


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100BaseT use RJ-45 connectors and Cat 3/4/5 cables. These cables have multiple pairs of

wires, and hence support full duplex transmission (Note that the other parts of the network

should also support full duplex, like the network card).

5. In normal course, a DTE device is expected to communicated with a DCE device. In such

event, the connections between the DCE and DTE devices is straight. However, if thecommunication need to happen between two DTEs or two DCEs, the signals need to be

rolled. This is achieved by rolling the pins in the cable or in the DB-25 adapter. Typically, a

DTE to DTE communication can be achieved by:

1. By connecting DTE to a rolled RJ-45 cable to a straight DB-25 adapter to DTE, OR

2. By connecting a straight RJ-45 cable to a rolled DB-25 adapter to DTE.

Similarly, a DTE to DCE communication can be achieved by:

1. Connecting a DTE to a straight RJ-45 cable to a straight DB-25 adapter to DCE, OR

2. Connecting a rolled RJ-45 cable to a rolled DB-25 adapter to DCE.

Note that rolling the signals twice is same as using straight through cables or adapters without

any rolling at all.

6. The maximum allowed cable lengths are as given below:

1. 10Base5: 500 meters

2. 10Base2: 185 meters

3. 10BaseT: 100 meters



7. The cable determines the DTE/DCE combination. The router with DCE cable needs to be

configured with clock rate for proper functioning of the WAN link.

ii. ARP and RARP

1. ARP (Address Resolution Protocol): IP works at network layer. IP address is a logical

address. If a packet is to be delivered to a destination machine, its physical address (MAC

address) needs to be known. ARP is a protocol, which enables a machine to obtain its MAC

address from a known IP address.

2. RARP (Reverse Address Resolution Protocol): RARP is used to obtain IP address from a

known MAC address.

iii. Static and Default Routing

1. There are three ways a router learns how to forward a packet:

1. Static Routes - Configured by the administrator manually. The administrator must also

update the table manually every time a change to the network takes place. Static routes are

commonly used when routing from a network to a stub (a network with a single route)

network.

The command is

ip route network mask address/interface [distance]

ex: ip route 165.44.34.0 255.255.255.0 165.44.56.5Here, 165.44.34.0 is the destination network or subnet


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255.255.255.0 is the subnet mask

165.44.56.5 is the default gateway.

2. Default Routes - The default route (gateway of last resort) is used when a route is not

known or is infeasible. The command is

ip route 0.0.0.0 0.0.0.0 165.44.56.5The default gateway is set to 165.44.56.5

3. Dynamic Routes - As soon as dynamic routing is enabled, the routing tables are

automatically updated. Dynamic routing uses broadcasts and multicasts to communicate with

other routers. Each route entry includes a subnet number, the interface out to that subnet, and

the IP address of the next router that should receive the packet. The commands to enable rip

are:

router rip

network .

2. ISP router is normally configured with static route.

3. A static route is one that is hard coded into the routing tables. Here, the destination

network/host, and the next hop information are entered by hand. A static route on a Cisco

router is most commonly defined by using the command:

Router(config)# ip route A.B.C.D (destination network/host) A.B.C.D (subnet mask)

A.B.C.D (next hop)

You can also use the port identifier such as e0, s1 etc. to define the next hop address.

Optionally, the "distance metric" can be added at the end of the command to change the

default weight.

4. The command to configure a static route to network 192.204.1.64/28 from assuming that

the default gateway is 192.204.1.2 is:

ip route 192.204.1.64 255.255.255.240 192.204.1.2

Syntax: ip route [administrative_distance]

5. Default route is used when there is no other known route to a given IP packets destination.

Default route is also known as gateway of last resort.

6. The correct syntax for setting default route is:

Router(config)#ip route 0.0.0.0 0.0.0.0 192.157.1.1

You can also set default route by specifying the interface as below: Router(config)#ip route 0.0.0.0 0.0.0.0 s0

The syntax is:

ip route 0.0.0.0 0.0.0.0

iv. WAN

1. WAN (Wide Area Network) devices extend the reach of LAN (Local Area Network)

devices. WANT typically span over a wide area, such over multiple cities / countries. WANS

are connected over serial lines that operate at lower speeds than LANs. Some of the WAN

devices are:


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1. Routers: Routers are responsible for routing the packets in an internetwork.

2. Modems: Modems connect to public telephone circuits through dial-up.

3. CSU/DSU: Stands for Channel Service Unit / Data Service Unit. CSU/DSUs are used for

connecting to Central Office of a Telephone switching company and provides serial WAN

connections.

4. Communication Servers: These are used for dial in/out to remote users. Provides RASRemote Access Server) functionality.

5. Multiplexors (mux): Multiplexors combine two or more signals before transmitting on a

single channel. Multiplexing can be done by sharing "time" or "frequency".

2. WANS are connected over serial lines and typically operate at lower speeds than LANs,

and more expensive. Some of the WAN terms are:

1. Modems: Modems connect to public telephone circuits through dial-up.

2. CPE stands for Customer Premise Equipment.

3. Demarc: Demarcation point between carrier equipment and CPE.

3. Typical WAN interfaces, that one come across frequently are:

EIA 232, EIA 449, EIA 530, V.35, and X.21. The Cisco router serial interface will be a 60-

pin D-shell connector. This in turn is connected to service provider equipment, usually

through a CSU/ DSU. Therefore, appropriate conversion cable need to be used when

interfacing with a WAN device such a CSU/DSU. CSU/ DSU in turn is connected to the

service provider through the cable supplied by the service provider.

4. You need to assign a different network number for each subnet. Also, you need to set aside

one network number for each WAN connection.

v. Basic Utilities (ping, traceroute)

1. The syntax for ping command is: "ping "

The following are possible responses to "ping" command:

Ping Explanation

Response

U destination Unreachable

C Congestion experienced

! Successful receipt of echo reply

. Time out

? Packet type unknown& Packet time to live exceeded

Ping sends ECMP echo. Ping can be used with almost any type of Network layer protocols

including IPX, IP, VINES, AppleTalk etc.

2. Trace command has syntax, "trace ip ". The trace command traces

the path to the destination hop by hop. This is achieved by setting the TTL (Time To Live) to

1 for the first packet to send and increasing TTL. This causes the successive next hop devices

to send a time out message. This message is used for finding out the route that the packet

takes to reach the destination.

Important Trace responses are:

Trace ExplanationResponse


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P The Protocol was Unreachable

N The Network was Unreachable

U The port was Unreachable

* The packet timed out

!H .. Could not be forwarded due to access list violation

Tracert, Ping use ICMP as their base protocol.

3. ICMP (Internet Message Control Protocol) messages are used for basic error reporting

between host to host, or host to gateway. It is not used for error reporting between Gateways.ICMP messages are encapsulated using the IP protocol. For example, the command ping

uses ICMP protocol. In the OSI Reference model, ICMPs are generally considered part of the

IP layer.

ICMP (Internet Control Message Protocol) sends an ICMP message if the destination is

unreachable. Routers can make alternate route decisions based on ICMP messages, if

appropriate.

vi. Others

1. HTTP is the protocol used for accessing the World Wide Web services. HTTP operates

over TCP/IP. TCP/IP is the protocol, which is used by all internet applications such as

WWW, FTP, Telnet etc. IPX/SPX is proprietary protocol stack of Novell NetWare.

2. If a properly configured workstation is unable to access the network information, it is

likely that the DHCP information available with the client workstation had become obsolete.

Issuing ipconfig command with /release and /renew commands will enable the client

workstation to obtain latest changes to the DHCP server.

3. 127.0.0.1 is the local loopback address defined by the Internet Protocol and used for by a

host to ping itself to verify the IP connectivity.

4. Important Layer 2 LAN protocols types:

802.3 - Layer 2 protocol that defines Ethernet CSMA/CD protocol

802.5 - Layer 2 protocol that defines Token Ring protocol

Note that 802.3, and 802.5 are examples of layer 2 LAN protocols, whereas, HDLC, PPP are

examples of layer 2 WAN protocols.

5. A switch forms a single broadcast domain, whereas, it provides an independent collision

domain for each port. Here, three ports of the switch are used, and hence it will have threecollision domains. On the other hand, a Hub forms a single collision as well as a single

broadcast domain. Therefore, the total number of broadcast domains in this case is two, and

the total number of collision domains is four.

6. An administrative distance of 0 represents highest trustworthiness of the route.

An administrative distance of 255 represents the lowest trustworthiness of the route.

7. Anytime that you hook up two end-devices like routers, PCs, print

servers, etc. to one another, you'll need a crossover cable. A switch or hub already all have

their ports crossed over, so hooking up an end-device to a switch uses a straight-through

cable. When hooking up two switch ports together, you use a cross-over cable again.


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8. BooTP (Bootstrap Protocol): When a diskless workstation is powered on, it broadcasts a

BootP request on the network. A BooTP server responds with its IP address, Default

gateway, etc.

9. Debugging output takes priority over other network traffic. Also, the debug all command

generates more output than any other debug command, and it can severely affect the router'sperformance. In practically all cases, it is best to use more specific debug commands.

Extended Service Set (ESS) is a set of two or more Basic Service Sets (BSSes) working

together to form a single wireless network. Stations are able to move between BSS within asingle ESS yet remain connected to the fixed network and so continue to receive emails etc.

As a Station moves into a new BSS, it will carry out a re-association procedure with the new

AP (Access Point).

10. For accessing a switch over IP network, you need to do some minimum configuration

tasks using Console port on the switch. In the given question, the default gateway needs to be

configured on the switch for accessing the management interface (VLAN1) from HostA.

11. For communicating between two hosts directly, you need a cross cable. Also, the subnet

mask 255.255.255.192 cannot accommodate the given IP addresses. Therefore, you need to

change either the host IP addresses so that they are within a given subnet, or change the

subnet address itself so that both the hosts are within the subnet.

12. For using full duplex Ethernet transmission, a switch is required. A Hub cannot support

full duplex transmission. In full duplex mode, there will not be any frame collisions.

13. Frame tagging is a technique, where a unique identifier is tagged to each frame as it is

forwarded through the switch fabric.

14. IANA (Internet Assigned Numbers Authority) is the organization under the Internet

Architecture Board (IAB) of the Internet Society that oversees the allocation of Internet

Protocol addresses to Internet service providers (ISPs). ICANN (a non governmental

organization) has now assumed responsibility for the tasks formerly performed by IANA.

ISPs in turn allot IP addresses to small companies, and businesses.

15. In "ad-hoc" or Independent Basic Service Set (IBSS) configuration there is no backbone

infrastructure. An IBSS is one BSS or Basic Service Set. Mobiles can talk to each other

without the use of an Access Point (AP). In the Extended Service Set (ESS) configuration,

there will be two or more Access Points (APs), and users can freely roam between the AccessPoints without any disconnection or reconfiguration. BSS will use only one Access Point.

16. In Carrier Sense Multiple Access with Collision Detection (CSMA/CD), a node transmits

only after listening to the media, and ensuring that no transmission is taking place. Further,

sending nodes detect when a collision occurs and stop transmitting immediately, backing off

for a random amount of time before trying again. This results in efficient use of the media

since the bandwidth of transmitting the entire frame is not wasted.

17. In Cisco routers, the interfaces are administratively down by default. You need to

enable an interface to use it.


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18. Internet connectivity is normally achieved using a WAN connection. Therefore, you need

a serial port for connecting to the Internet. On the other hand, a switch normally requires an

Ethernet port for connectivity.

19. IP is the protocol that works at Internet layer. It forwards the packets based on the

network address (IP address) in the packets and the routing tables.IP Packets are transported from source network to the destination network by what is known

as routing. Hop-by-hop routing model is used by the Internet for delivery of packets. At each

hop, the destination IP address is examined, the best next hop is determined by the routing

protocol (such as RIP, OSPF or BGP) and the packet is forwarded by one more hop through

this route. The same process takes place at the next hop. During this process, the logical

addresses remain same. In an IP network, the logical addresses are IP addresses. The

hardware interface addresses, such as MAC address change with each hop.

20. IP RIP based networks send the complete routing table during update. The default update

interval is 30 seconds.

ISL, 802.1Q are the VLAN trunking protocols associated with Fast Ethernet. The VLAN

trunking protocol defined by 802.10 is associated with FDDI. LANE (LAN Emulation) is

associated with ATM.

21. It is important to know the difference between a collision domain and a broadcast

domain. When you use Hubs, all the nodes connected to the hub will be in the same collision

domain. However, when you use switches and implement VLANs, each VLAN will be in a

separate broadcast domain. The packet forwarding between VLANs is achieved through the

use of routing.

22. Network layer is responsible for end to end delivery of packets. IP and IPX are examples

of two routing protocols that work at the Network layer of the OSI reference model. Network

layer addresses remain constant as a packet travels from source to destination network,

whereas physical (such as MAC address) addresses change with each hop that a packet takes.

For this reason, a Network layer address is a logical address (not hardcoded like MAC

address).

23. Originating source port numbers are dynamically assigned by source host, usually greater

than 1023. The following are the recommended port numbers:

Numbers 0 - 255 are used for public applications

Numbers 255 - 1023 are assigned to companies so that they can use these port numbers intheir applications.

Numbers above 1023 are used by upper layers to set up sessions with other hosts and by

TCP to use as source and destination addresses.

24. PAP uses 2-way handshaking. Passwords are sent in clear text across the link. Therefore,

PAP is to be used only when it not possible to use CHAP.

CHAP uses 3-way handshaking. CHAP uses Challenge/ Response method which provides

protection against the password capture while authenticating the user. One should use CHAP

whenever it is possible.


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25. PPP and Cisco HDLC support multiple upper layer protocols. SLIP and LAPB does not

support multiple upper layer protocols. PPP supports Password Authentication Protocol

(PAP) and, Challenge Handshake Authentication Protocol (CHAP).

26. service password-encryption command:

Directs the router to encrypt passwords that are saved in the configuration file. Servicepassword encryption is applied to all passwords, including username passwords,

authentication key passwords, the privileged command password, console and virtual

terminal line access passwords. This command is primarily useful for keeping unauthorized

individuals from viewing your password in your configuration file.

27. The command "ipx routing" selects IPX as a routing protocol and starts the routing

process. Some other important commands are:

1. ipx maximum-paths 2: This command allows load sharing over parallel metric paths to the

destination. The parallel paths that can be used is limited to 2.

2. Interface ethernet 0.1: Indicates the first sub interface on interface e0.

28. The following are true about HDLC encapsulation over serial links:

a. HDLC encapsulation used on Cisco routers is proprietary (uses a Type field), and may not

be compatible with routers manufactured by other vendors

b. It is recommended to use other encapsulation schemes such as PPP when connecting

between a Cisco router and a non-Cisco router instead of HDLC encapsulation.

29. The following precautions may be taken to harden network infrastructure:

a. Use physical barriers such as room lock so that un-authorized persons do not have access

to the network devices.

b. Use firewall so that outsiders cannot access network devices from outside the network

c. Enable SSH so that passwords are transmitted in encrypted form

30. The four layers of DoD model from Top to Bottom are:

1. Process Layer: Responsible for implementation of user-level applications such as mail

delivery, file transfer, and remote login.

2. Host-to-Host Layer: Responsible for flow control, and retransmission of lost packets. TCP,

and UDP protocols work at this layer.3. Internet Layer: Responsible for routing data across a network of networks. Routed

protocols such as IP work at this layer. Routing protocols are also associated with this layer.

4. Network Access Layer: Responsible for the delivery of data segments over a given

particular hardware media. Different protocols such as CSMA/CD work at this layer.

31. MAC sub-layer:

The Mac sublayer specification 802.3z describes gigabit Ethernet over fiber or shielded

copper.

The Mac sublayer specification 802.3ab (1000BaseT) describes gigabit Ethernet over

Category 5 UTP.


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32. The MAC address contains 48 bits expressed as 12 hexadecimal digits. Note that 2

hexadecimal digits represent a byte. Therefore, a MAC address contains 6 bytes. The first 3

bytes represent the manufacturer identification code. The next 3 bytes represent the interface

number unique to a given manufacturer. MAC address represents the hardware address and is

usually burned into the ROM. A typical MAC address looks like:

xx-xx-xx-xx-xx-xx, where x represents a hexadecimal digit.Ex: 00-00-0c-12-14-33

Here, 00-00-0c represents the vendor address, and 12-14-33 typically represents the interface

serial number. The last 6 hex digits are administered by respective vendors, and unique to a

given vendor.

Traditionally, the router interface on a serial link is a 60-pin D-shell connector. The

CSU/DSU may have EIA/TIA 232 interface to the router. The CSEU/DSU interface to the

service provider could be RJ-11, RJ-45 etc.

33. Transport Layer is responsible for multiplexing the packets received from upper layers. It

also makes the network transparent to the upper layers (Session, Presentation, and

Application layers).

34. When designing an ESS WLAN, APs with overlapping coverage areas should be

configured to use non-overlapping channels. This is true whether the APs are using DSSS

(Direct Sequence Spread Spectrum) or FHSS (Frequency Hopping Spread Spectrum)

technology.

35. While a packet travels through an Internetwork, it usually involves multiple hops. It is

important to know that the logical address (IP address) of source (that created the packet) and

destination (final intended destination) remain constant, the hardware (Interface) addresses

change with each hop.

36. Windowing, Buffering, and Congestion avoidance are three different types of flow

controls used frequently.

You can use "show controllers serial [port number]" to see if the interface detects a DCE or

DTE cable.

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