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What is Loopback ?

Loopback?(loop-back) refers to the routing of electronic signals, digital data streams, or flows of items back to their originating devices or facilities without intentional processing or modification. This is primarily a means of testing the transmission or transportation infrastructure.

Example applications:

  • A communication channel with only one?endpoint. Any message transmitted by such a?channel?is immediately and only received by that same channel.
  • Performing?transmission?tests of?access?lines from the serving?switching center, which usually does not require the assistance of personnel at the served terminal.
  • Testing between stations (not necessarily adjacent) where in two lines are used, with the test being done at one station and the two lines interconnected at the distant station. Commonly called?loop around?when the interconnecting circuit is accessed by dialing.
  • A?patch cable, applied manually or automatically, remotely or locally, that facilitates a loop-back test.

Checksum and CRC

Checksum and CRC

Another secure-computing need is to ensure that the data has not been corrupted during transmission or encryption. There are a couple of popular ways to do this:

Checksum– Probably one of the oldest methods of ensuring that data is correct, checksums also provide a form of authentication because an invalid checksum suggests that the data has been compromised in some fashion. A checksum is determined in one of two ways. Let’s say the checksum of a packet is 1 byte long. A byte is made up of 8 bits, and each bit can be in one of two states, leading to a total of 256 (28?) possible combinations. Since the first combination equals zero, a byte can have a maximum value of 255.

  • If the sum of the other bytes in the packet is 255 or less, then the checksum contains that exact value.
  • If the sum of the other bytes is more than 255, then the checksum is the remainder of the total value after it has been divided by 256.

Let’s look at a checksum example:

  • Bytes total 1,151
  • 1,151 / 256 = 4.496 (round to 4)
  • 4 x 256 = 1,024
  • 1,151 – 1,024 = 127 checksum

Cyclic Redundancy Check (CRC) – CRCs are similar in concept to checksums, but they use polynomial division to determine the value of the CRC, which is usually 16 or 32 bits in length. The good thing about CRC is that it is very accurate. If a single bit is incorrect, the CRC value will not match up. Both checksum and CRC are good for preventing random errors in transmission but provide little protection from an intentional attack on your data. Symmetric- and public-key encryption techniques are much more secure.

All of these various processes combine to provide you with the tools you need to ensure that the information you send or receive over the Internet is secure. In fact, sending information over a computer network is often much more secure than sending it any other way. Phones, especially cordless phones, are susceptible to eavesdropping, particularly by unscrupulous people with radio scanners. Traditional mail and other physical mediums often pass through numerous hands on the way to their destination, increasing the possibility of corruption. Understanding encryption, and simply making sure that any sensitive information you send over the Internet is secure (remember the “HTTPS” and padlock symbol), can provide you with greater peace of mind.

What is packet tracer ?

What is packet tracer ?

For Students

Welcome to the world of computer networking. Packet Tracer can be a fun, take-home, flexible piece of software to help with your CCNA studies, allowing you to experiment with network behavior, build models, and ask “what if” questions. We hope that Packet Tracer will be useful to you whatever your goals are in networking, be they further education, certification, employment, or personal fulfillment. We want to emphasize how important it is for you to also gain in-person, hands-on experience with real equipment as part of preparing to join the community of networking professionals.

For Instructors

Packet Tracer is a simulation, visualization, collaboration, and assessment tool for teaching networking. Packet Tracer allows students to construct their own model or virtual networks, obtain access to important graphical representations of those networks, animate those networks by adding their own data packets, ask questions about those networks, and finally annotate and save their creations. The term “packet tracing” describes an animated movie mode where the learner can step through simulated networking events, one at a time, to investigate the microgenesis of complex networking phenomena normally occurring at rates in the thousands and millions of events per second.

unistall software from cmd

unistall software from cmd

1. Open a command prompt.

2. Input?WMIC?and press Return. You will see a prompt that looks like this:

3. At the new prompt, execute the following command:
product get name

This will generate a list of installed applications.

4. At the prompt, execute the following command:
product where name="" call uninstall

where?application name?is the name of the program you wish to uninstall (use the exact name provided by the previously generated list).

For example, if I were wanting to uninstall Adobe Reader 9, my command would look like this:
product where name="Adobe Reader 9" call uninstall

vtp modes

VLAN Trunking Protocol (VTP) Server Mode

VLAN Trunking Protocol (VTP) server mode is the default VTP mode for all Catalyst switches. At least one server is required in a VTP domain to propagate VLAN information within the domain. We can create, add, or delete VLANs on a VTP domain VTP Server and change VLAN information in a VTP Server. The changes made in a switch in server mode are advertised to the entire VTP domain.

VLAN Trunking Protocol (VTP) Client Mode

VLAN Trunking Protocol (VTP) client mode switches listen to VTP advertisements from other switches and modify their VLAN configurations accordingly. A network switch in VTP client mode requires a server switch to inform it about the VLAN changes. We CANNOT create, add, or delete VLANs in a VTP client.

VLAN Trunking Protocol (VTP) Transparent Mode

VLAN Trunking Protocol (VTP) transparent mode switches do not participate in the VTP domain, but VTP transparent mode switches can receive and forward VTP advertisements through the configured trunk links.

AAA Lab in Packet Tracer

AAA Lab in Packet Tracer

Lab Objective : any one try to Telnet the router must be authenticated through AAA server First and in case AAA server is down , routers will use his local user accounts database.

Configuration on the router:

=setting telnet=

Router (config) #enable secret 1111

Router (config) #line vty 0 4

Router (config-line) #login authentication default ?( to apply an on Telnet lines )?

Router (config-line) #login

Router (config-line) #exit

Router (config) #username JKR password 3333

=AAA commands=

Enable AAA on the router :

Router (config) #aaa new-model

Set authentication for login using two methods , method 1 uses AAA server through Tacacs+ protocol , method 2 using local router user accounts:

Router (config) #aaa authentication login default group tacacs+ local

Tell the router what is the IP address for Tacas+ server and key (password) to connect to:

Router (config) #tacacs-server host key 8888

Configuration on AAA server


User account :

Username : JKR

Password: 4444

tacas+ client :

Key : 8888

Now here is few show commands we can use plus one command to unlock any user account reach max failed attempts to logon:

Router#show AAA user all

Router#show AAA sessions

Router#show a local user lockout

Router#clear a local user lockout username all

In best practice try to Telnet the router with local username Yasser password 3333 and it will not work then try to use? the ACS server user name we wrote above : audio password 4444 and it will work fine .

Now disconnect the ACS server or just remove the cable and try to Telnet the router using Yasser and it will work fine .

Remember method 1 fail , you will not go to method 2

But if method 1 is not available then you can go to method 2 and use it.

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Career in networking


Job Titles

Several types of positions exist in networking, each with different average salaries and long-term potential, and one should possess a clear understanding of these. Unfortunately, job titles in networking, and in Information Technology (IT) generally, often lead to confusion among beginners and experienced folks alike. Bland, vague or overly bombastic titles often fail to describe the actual work assignments of a person in this field.

The basic job titles one sees for computer networking and networking-related positions include

  • Network Administrator
  • Network (Systems) Engineer
  • Network (Service) Technician
  • Network Programmer/Analyst
  • Network/Information Systems Manager
The Network Administrator

In general, network administrators configure and manage?LANs?and sometimes WANs. The job descriptions for administrators can be detailed and sometimes downright intimidating! Consider the following description that, although fictitious, represents a fairly typical posting:

“Candidate will be responsible for analysis, installation and configuration of company networks. Daily activities include monitoring network performance, troubleshooting problems and maintaining network security. Other activities include assisting customers with operating systems and network adapters, configuring routers, switches, and firewalls, and evaluating third-party tools.”

Needless to say, a person early in their career often lacks experience in a majority of these categories. Most employers do not expect candidates to possess in-depth knowledge of all areas listed in the job posting, though, so a person should remain undeterred by the long, sweeping job descriptions they will inevitably encounter.

Comparing Roles and Responsibilities

The job function of a?Network Engineer?differs little from that of a Network Administrator. Company A may use one title while Company B uses the other to refer to essentially the same position. Some companies even use the two titles interchangeably. Firms making a distinction between the two often stipulate that administrators focus on the day-to-day management of networks, whereas network engineers focus primarily on system upgrades, evaluating vendor products, security testing, and so on.

A?Network Technician?tends to focus more on the setup, troubleshooting, and repair of specific hardware and software products. Service Technicians in particular often must travel to remote customer sites to perform “field” upgrades and support. Again, though, some firms blur the line between technicians and engineers or administrators.

Network Programmer/Analysts?generally write software programs or scripts that aid in network analysis, such as diagnostics or monitoring utilities. They also specialize in evaluating third-party products and integrating new software technologies into an existing network environment or to build a new environment.

Managers?supervise the work of administrators, engineers, technicians, and/or programmers. Network / Information Systems Managers also focus on longer-range planning and strategy considerations.


CCNAX online Training

We provide ?ccnax online Training

Interconnecting Cisco Networking Devices Part 1 (ICND1) v1.1

Our Course content for

  • How networks function, network components, and the OSI reference model
  • Binary, decimal, and hexadecimal numbering
  • Switching operations and theory
  • Host-to-Host packet delivery process through a switch and through arouter
  • TCP/IP network addressing and routing
  • IP subnetting
  • Providing Local Area Network (LAN), Wide Area Network (WAN), and remote access services
  • How Wireless LANs (WLANs) work, including the latest IEEE 802.11nstandard
  • Introduction to Cisco Internet Operating System (IOS)
  • Configuration of Cisco Routers and Catalyst Switches
  • Using your router as a Dynamic Host Configuration Protocol (DHCP) server and/or client and to provide Port Address Translation (PAT) functionality
  • Network discovery and management using Cisco Discovery Protocol (CDP), Link Layer Discovery Protocol (LLDP), Telnet, secure shell (SSH), and TrivialFTP (TFTP)
  • Expand a switched network from a small to medium network environment
  • Issues with redundant switching
  • Spanning Tree Protocol (STP)
  • Concepts of VLANs and trunking and routing between VLANs
  • Implementing VLSM
  • Configure, verify, and troubleshoot OSPF and EIGRP
  • Determine when to use access control lists (ACLs) and how to configure,verify, and troubleshoot them
  • Configure NAT and PAT
  • Configure IPv6 addressing and Routing Information Protocol new generation(RIPng)
  • VPN solutions
  • Configure PPP, CHAP, and PAP
  • Frame Relay operation and troubleshooting

CCIE R&S Books list

CCIE R&S Reading List

This page lists books on topics appearing on the CCIE Written and Lab Exam.? These books are not required study resources, however, they can be used to build knowledge in certain areas.

Many of the Cisco Press books are available to certified individuals and Cisco customers at prices discounted up to 30% off. To check for discounts, visit the?Cisco Marketplace, click on (Cisco Press) Bookstore, and login with your Cisco CCO ID. Search for the titles using the ISBN number indicated.

  • CCIE Routing and Switching Exam Certification Guide
  • CCIE Routing and Switching Exam Quick Reference: Exam 350-001 v3.1 (Digital Short Cut)
  • CCIE Routing and Switching Practice Labs (Digital Short Cut)
  • CCIE Routing and Switching v4.0 Troubleshooting Practice Labs
  • Routing TCP/IP, Volume 1
  • Routing TCP/IP, Volume II (CCIE Professional Development)
  • Troubleshooting IP Routing Protocols
  • Inside Cisco IOS Software Architecture (CCIE Professional Development Series)
  • Cisco LAN Switching (CCIE Professional Development series)
  • Cisco OSPF Command and Configuration Handbook (paperback)
  • Cisco BGP-4 Command and Configuration Handbook
  • Cisco Field Manual: Router Configuration
  • Cisco Field Manual: Catalyst Switch Configuration
  • Developing IP Multicast Networks, Volume I
  • Internet Routing Architectures
  • MPLS and VPN Architectures
  • MPLS and VPN Architectures, Volume II
  • Cisco Catalyst QoS: Quality of Service in Campus Networks (paperback)
  • End-to-End QoS Network Design: Quality of Service in LANs, WANs, and VPNs
  • Deploying IPv6 Networks
  • Network Security Technologies and Solutions (CCIE Professional Development Series)

The following titles are no longer for sale in print format, but are available for free online view at the InformIT Reference Library:

1. CCIE Practical Studies, Volume I

2. CCIE Practical Studies, Volume II

3. Troubleshooting Remote Access Networks

4. Troubleshooting VPNs