BCRAN - Building Cisco® Remote Access Networks exam

BCRAN - Building Cisco® Remote Access Networks exam is a requirement towards obtaining CCNP certification. Skills measured are: Building Cisco® Remote Access Networks exam include topics on Designing and implementing remote access networks using Dial-up, ISDN BRI, PRI; Authentication schemes, Frame Relay, X.25 network configurations. Valid CCNA certification is a pre-requisite for obtaining CCNP certification. The exam also counts toward CCDP Certification.

To be CCNP certified, the following exams need to be successfully completed:

Exam	Exam Code	Study material covering exam objectives
BSCI	642-801	Building Scalable Cisco Internetworks. Note that BSCI replaces Routing Exam (640-603).
Switching Exam	642-811	Building Cisco Multi-layer Switched Network or BCMSN
Remote Access Exam	642-821	Building Cisco Remote Access Networks
Support Exam	642-831	Cisco Internetwork Troubleshooting

Alternatively, one can take the following exams to obtain CCNP certification:

Exam	Exam Code	Study material covering exam objectives
Foundations Exam	642-891 Composite	Building Scalable Cisco Networks,
		Building Cisco Multi-layer Switched Network.
Remote Access	642-821	Building Cisco Remote Access Networks
Support Exam	642-831	Cisco Internetwork Troubleshooting.

Exam cram 0 1 2 3 4 5

1.0 Central site equipment:

Cisco^® recommends any of the following router equipment for central site or Corporate site:

1. Cisco 3600 series: Several models are available including 3620, 3640, and 3660. 36XX series of routers/access servers support variety of connections and protocols. The supported network interfaces include the following:
Ethernet, Fast Ethernet, Token Ring, Asynchronous, Synchronous serial, High Speed Serial Interface, ISDN BRI, Channelized T1/ISDN PRI (with and without CSU) , Digital Modems, Analog Modems, Voice, among others.

2. Cisco 4000 series: Cisco 4000 series include Cisco 4500 and Cisco 4700. Each of these have three network module slots. The presence of network module slots allows us to use variety of network modules and protocols. WAN Interface Card (WIC) can also be plugged in.
Cisco AS5X00: Cisco AS5X00 family of access servers support both analog modem and ISDN dial services. It is ideal where large number of remote users want to access the central site using dial up or ISDN services.

3. Cisco 7000 series: These are high end routers that supports combination of Ethernet, Fast Ethernet, Token Ring, FDDI, ATM, serial, ISDN etc.

The selection of equipment depends on the number of computers situated at the central site, type of applications that are being used, the number of remote connections from branch offices/home offices/telecommuter sites, and the bandwidth requirements.

1.1 Cisco recommends the following router equipment for branch office:

Series 1600: Cisco 1600 series includes Cisco 1601 through Cisco 1605. Each router has one empty network module slot.

Series 1700: Cisco 1700 series include Cisco 1720. It has one 10/100 Ethernet interface and two WAN slots.

Series 2500: Cisco 2500 series include Cisco 2501,2502. These routers are typically fixed configuration with any combination of the Ethernet, Token Ring, synchronous serial, ISDN BRI, and Hub (at least two of these interfaces are present).

Series 2600: Cisco 2600 series includes Cisco 2610, and 2620 routers. Each router has two empty WAN slots and one empty network module slot.

Note that Cisco series 1600, 1700, 2500, and 2600 are recommended for branch office applications.

1.2 The following are recommended by Cisco for Telecommuter site:

Series 700 are easy to configure multiprotocol ISDN access routers.
Series 800 are fixed configuration routers with ISDN BRI and Ethernet interfaces. Cisco recommends these models for home office/telecommuter facility.

Note that Series 700/Series 800/ Series 1000 routers are recommended for home office/telecommuter facility.

2.0 Some important routers series and their configurations:

Series 800 are fixed configuration routers with one Ethernet interface and most models (Only Cisco 805 does not have ISDN BRI interface) have ISDN BRI interface. Cisco805 has Serial interface. Cisco803, 804, and 813 can also accommodate 2 POTS in addition to Ethernet and ISDN BRI.

Series 1000: These are also fixed configuration routers. Cisco1003, Cisco1004 have 1 ISDN BRI + 1 Ethernet interface. Cisco 1005 has one Ethernet + 1 serial (sync/async) interface.
Note that Cisco 700 series, 800 series, and 1000 series are recommended for telecommuter or home office.

Series 1600: Cisco 1600 series includes Cisco 1601 through Cisco 1605. Each router has one WAN slot. Models 1601,15602,1603,1604 have one Ethernet interface, whereas model 1605 has 2 Ethernet interfaces. Support is also provided for ISDN BRI (1603,1604), and serial interface (1601). Cisco1602 has 56/64K CSU/DSU interface.

Series 2500: Cisco 2500 series include Cisco 2501,2502 and several other models. These routers are of fixed configuration (except Cisco2524) with any combination of the Ethernet, Token Ring, synchronous serial, ISDN BRI, and Hub (at least two of these interfaces are present).

Series 2600: Cisco 2600 series includes Cisco 2610, 2611 and 2620, 2621 routers. Each router has two empty WAN slots and one empty network module slot. 2610 has 1 Ethernet interface, whereas 2611 has two Ethernet interfaces. 2620 has 1 Fast Ethernet interface, whereas 2621 has two Fast Ethernet interfaces. The major difference between 2610 and 2620 is that the later provides support for Fast Ethernet.

It is recommended that you practice configuration of routers using ConfigMaker, a free utility available at Cisco Website and free to download and use.

3.0 Router LEDs:

By observing the status of various LEDs on a Cisco router, the health of a router can be found quickly. However, if the LED status is OK, and the problem still remains, you need to consult the manufacturer documentation for troubleshooting. Other options include Cisco Website, CCO Website etc.

Important LEDs on a Cisco 1600 router are as follows:
1. System PWR: Green indicates that the DC power to the router is OK.
2. System OK: Green indicates that the router has booted properly.
3. WIC CD: Green indicates an active connection on the serial port of WAN interface card.
4. WIC ACT: Activity indicates an active connection on the serial port of WAN interface card.
5. LAN ACT: This indicates that data is being sent or received.
6. LAN COL: Frame collisions are indicated by a flashing yellow LED.

4.0 EIA-232 / RS 232:

In EIA/TIA-232 standard, signals can be grouped as below (Pin numbers are given assuming 25 pin connector cable):
A. Data Transfer Group:
TxD (pin 2 on DTE): Transmit Data - This represents data transmit from DTE to DCE (Note the reference is DTE)
RxD (pin 3 on DTE): Receive Data - This represents data received from DCE to DTE (Again note the reference is DTE)
GRD (pin 7 on DTE)
B. Flow Control Group:
RTS (pin 4): Request To Send - Represents that the DTE has buffer space available to receive data from the DCE.
CTS (pin 5): Clear To Send - Represents that the DCE has buffer space available to receive data from DTE.
C. Modem Control Group:
DTR: Data Terminal Ready - Represents that the DTE is ready to receive data.
CD (pin8) - Carrier Detect - Indicates that the DCE has detected carrier from remote DCE.
DSR (pin6): Data Set Ready - It indicates that the DCE is ready to use.
If you are using a 25 pin connector on either end, and the cable is straight through, there is one to one correspondence in the pin connections. That is pin 2 of DTE is connected to pin 2 of DCE, pin 3 of DTE is connected to pin 3 of DCE etc. However, if you are using 25 pin connector at one end and 9 pin connector at the other end, the pin numbers change.

5.0 Null Modem:

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 the communication need to happen between two DTEs or two DCEs (Null Modem), 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.

Cram Notes 5:Cisco Certified Network Associate CCNA

51.Routing metrics used by IGRP:

Bandwidth, MTU, Reliability, Delay, and Load.

1. Bandwidth: This is represents the maximum throughput of a link.

2. MTU (Maximum Transmission Unit): This is the maximum message length that is acceptable to all links on the path. The larger MTU means faster transmission of packets.

3. Reliability: This is a measurement of reliability of a network link. It is assigned by the administrator or can be calculated by using protocol statistics.

4. Delay: This is affected by the band width and queuing delay.

5. Load: Load is based among many things, CPU usage, packets processed per sec.

52. The metric limit for link-state protocols is 65,533

53. Following are the possible solutions for preventing routing loops.

1. Split Horizon - based on the principle that it is not useful to send the information about a route back in the direction from which the information originally came.

2. Poison Reverse - A router that discovers an inaccessible route sets a table entry consistent state (infinite metric) while the network converges.

3. Hold-down Timers - Hold down timers prevent regular update messages from reinstating a route that has gone bad. Here, if a route fails, the router waits a certain amount of time before accepting any other routing information about that route.

4. Triggered Updates - Normally, new routing tables are sent to neighboring routers at regular intervals (IP RIP every 30 sec / and IPX RIP every 60 sec). A triggered update is an update sent immediately in response to some change in the routing table. Triggered updates along with Hold-down timers can be used effectively to counter routing loops.

54. IP RIP based networks send the complete routing table during update. The default update interval is 30 seconds. IGRP update packet is sent every 90 seconds by default.

55. For IGRP routing, you need to provide the AS (Autonomous System) number in the command. Routers need AS number to exchange routing information. Routers belonging to same AS exchange routing information. OSPF, and IGRP use AS numbers.

56. CDP stands for Cisco Discovery Protocol. This protocol is proprietary of Cisco. CDP runs SNAP (Sub network Access Protocol) at the Data Link Layer. Two Cisco devices running two different Network layer protocol can still communicate and learn about each other.

57. 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 of seconds due for next update (this is fraction of update timer). This command also gives the network number for which IP RIP is enabled, Gateway, and the default metric.

1. 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 connected

I : IGRP

R : RIP

2. 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.

3. 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.

58. Cisco router boot configuration commands:

1. 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.

2. boot system rom - Loads IOS from ROM.

3. boot system flash - Loads the first file from flash memory.

4. boot system tftp <> - Loads IOS with a filename from a TFTP server.

Cram Notes 4:Cisco Certified Network Associate CCNA

40. 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 up when you configure the router from the console. There can be five distinct passwords 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.

41. Internal memory components of a Cisco router:
. 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.

2. RAM/DRAM : stores the running configuration, routing tables, and packet buffers. Some routers, such as the 2500 series, run IOS from Flash, not RAM.

3. NVRAM (Non-Volatile Ram): Memory that does not lose information when power is lost. Stores the system’s configuration file and the configuration register. NVRAM uses a battery to maintain the data when power is turned off.

4. 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 level of the operating system in every router in your network and then, to upgrade the whole network to that version at an appropriate time.

42. While a packet travels through an Internetwork, it usually involves multiple hops. Note 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.

43. Default administrative distances some important routing protocols are as below:

Route Source Default Distance

Directly connect Interface 0

Static Route 1

IGRP 100

RIP 120

Unknown 255

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

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

Routed and Routing protocols:

- 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.

44. 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 165.44.34.0 255.255.255.0 165.44.56.5

Here, 165.44.34.0 is the destination network or subnet

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.

The default gateway is set to 165.44.56.5

3. Dynamic Routes - In dynamic routing, the routing tables are automatically updated. Dynamic routing uses broadcasts and multicasts to communicate with other routers.

The commands to enable rip are:

router rip

network .

45. 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.

46. 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. Distance vector protocols listen to second hand information to learn routing tables whereas, Link state protocols build routing tables from first hand information. Routers with distance vector protocols 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 generates the exact topology of the entire network for route computation, by listening to the first hand information. Link State protocols take bandwidth into account using a cost metric. Link State protocols only send updates when a change occurs, which makes them more efficient for larger networks. Bandwidth and delay are the most widely used metrics when using Link-State protocols. EX: OSPF and NLSP.

Benefits of Link State protocols:

1. Allows for a larger scalable network

2. Reduces convergence time

3. Allows “supernetting”

3. Balanced Hybrid - Balanced Hybrid combines some aspects of Link State and Distance Vector routing protocols. Balanced Hybrid uses distance vectors with more accurate metrics to determine the best paths to destination networks. EX: EIGRP.

47. 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 single hop through a 64KBPS line, RIP assumes that the link through 64KBPS is the best path!

48. RIP (and IGRP) always summarizes routing information by major network numbers. This is called classfull routing.

49. Convergence is the term used to describe the state at which all the internetworking devices, running any specific routing protocol, are having identical information about the internetwork in their routing tables. The time it takes to arrive at identical information of the internetwork is called Convergence Time.

50. 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

Cram Notes 3:Cisco Certified Network Associate CCNA

31. Ethernet_II has a type field to identify the upper-layer protocol. 802.3 has only a length field and can't identify the upper-layer protocol.

32.To find valid hosts given an IP address (say 156.16.3.52) and a subnet mask (sat a 12-bit subnet). The valid hosts are determined as below:

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 here by a factor of 16.

33. The following are some important commands that can be used to edit and review command history buffer on a Cisco router. 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.

34. The following are some frequently used COPY commands:

1. 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

2. COPY STARTUP RUNNING - This command merges configuration from NVRAM to RAM.

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

4. 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.

35. The banner is displayed whenever anyone logs in to your Cisco router. The syntax 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 the message to be displayed. You need to enter "#" symbol at the end of the message, signifying that the message has ended.

36. Router modes of operation:

1. User EXEC mode (Prompt: Router>):- 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 (Prompt: Router#):- 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. The default 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.

3. Global Configuration mode (Prompt: Router (Config)#):-

Global configuration mode allows you to perform tasks that affect the entire router, such as naming the router, configuration of banner messages, enabling routed protocols, and generally anything that affects the operation of the entire router.

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 Cisco router.

37. You can use "tab" to complete the command that you are typing.

38. 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 detecting keep-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 router’s 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.

39. 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.

40. 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 up when you configure the router from the console. There can be five distinct passwords 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.

Cram Notes:Cisco Certified Network Associate CCNA

Cram Notes:

11. Deleted (old objective)

12. Telnet, FTP, and TFTP:

1. Telnet is used for terminal emulation that runs programs remotely. Telnet uses TCP/IP protocol.

2. Telnet requires a username and password to access.

3. FTP (File Transfer Protocol) is a connection oriented protocol. It uses TCP/IP for file transfer. Compare this with TFTP (Trivial File Transfer Protocol) that uses UDP (Connectionless protocol). SNMP uses UDP over IP. Tracert, Ping use ICMP as their base protocol. FTP is used to transfer files.

13. Maximum hop count supported by RIP is 15.

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

I. FTP : Port #21

II. Telnet: Port #23

III. SMTP: Port #25

IV. SNMP: Port #161

It is important to know that FTP, Telnet, SMTP use TCP; whereas TFTP, SNMP use UDP.

15. SNMP is part of TCP/IP protocol suite. It allows you to monitor and manage a network from a centralized place by using SNMP Manager software. The systems or devices that provide the responses are called agents (or MIBs). An SNMP agent is any computer running SNMP agent software.

MIB stands for Management Information Base. It is part of SNMP agent database. A MIB records and stores information abut the host it is running on. An SNMP manager can request and collect information from an agent's MIB. Routers are typical MIB agents. SNMP agent generates "trap" messages that are then sent to an SNMP management console, which is a trap destination.

17. Address Resolution Protocol (ARP) is used to resolve or map a known IP address to a MAC sub layer address to allow communication on a multi-access medium such as Ethernet. Reverse ARP (RARP) is used to obtain an IP address using an RARP broadcast. RARP can be used to boot diskless workstations over a network.

18. The 7 layers of OSI model are:

1. The Application Layer: Application layer is responsible for identifying and establishing the availability of desired 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 is responsible for flow control, to maintain data integrity.

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 routing 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 signals 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: EIA/TIA-232, EIA/TIA-449,V.24,V.35,X.21,G.703,HSSI (High Speed Serial Interface).

19. Repeaters, Bridges, and Routers:

I. Repeaters work at Physical layer (Layer 1),

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

III. Routers work at Network Layer (Layer 3) of ISO Reference Model. 20. CSU / DSU is an acronym for Channel Service Unit / Data Service Unit. CSU/DSU is part of Customer Premise Equipment (CPE). CSU / DSU connect to a Central Office (CO), a telephone switching company located nearer to the customer.