TCP and UDP Tutorial

September 20th, 2016 in CCNA Knowledge

The Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are the two most popular protocols in the transport layer. They ensures that messages are delivered error-free, in sequence, and with no losses or duplication. The key difference between TCP and UDP is that TCP provides a wide variety of services to applications, whereas UDP does not. At the result of this, TCP is much more complex than UDP so this tutorial is dedicated to explore TCP in detail but we still compare them.


Both TCP and UDP are protocols at the Transport layer (of both OSI and TCP/IP model) but why we need both of them? The answer is:

+ TCP is slower but reliable
+ UDP is faster but unreliable


Border Gateway Protocol BGP Tutorial

August 25th, 2016 in CCNA Knowledge

Basic understanding about BGP

We really want to show you why we need BGP first but it is very difficult to explain without understanding a bit about BGP. So we will learn some basic knowledge about BGP first.

First we need to understand about the different between Interior Gateway Protocol and Exterior Gateway Protocol. The difference between them is shown below:


Interior Gateway Protocol (IGP): A routing protocol operating within an Autonomous System (AS) like OSPF, EIGRP… Usually routers running IGP are under the same administration (of a company, corporation, individual)
Exterior Gateway Protocol (EGP): A routing protocol operating between different AS. BGP is the only EGP used nowadays


NetFlow Tutorial

May 13th, 2016 in CCNA Knowledge

Note: NetFlow is no longer a topic in CCNAv3 200-125 exam.

One of the most important tasks of a network administrator is to monitor the health of our networks, learn how our bandwidth is being used, what applications are consuming it, when it needs upgrade… Although monitoring protocols like SNMP and SPAN (port mirroring) can help us answer some questions but they are not enough to give us an insightful view of our networks. Luckily we have another amazing tool: NetFlow!

NetFlow is a networking analysis protocol that gives the ability to collect detailed information about network traffic as it flows through a router interface. NetFlow helps network administrators answers the questions of who (users), what (application), when (time of day), where (source and destination IP addresses) and how network traffic is flowing.

Let’s take an example! In the topology below, when traffic from Network 1, 2, 3… passes through the interfaces of a NetFlow enabled device, relevant information is captured and stored in the NetFlow cache. NetFlow collects IP traffic information as records and sends them to a NetFlow collector for traffic flow analysis.



Point to Point Protocol (PPP) Tutorial

March 11th, 2016 in CCNA Knowledge

Point-to-Point Protocol (PPP) is an open standard protocol that is mostly used to provide connections over point-to-point serial links. The main purpose of PPP is to transport Layer 3 packets over a Data Link layer point-to-point link. PPP can be configured on:
+ Asynchronous serial connection like Plain old telephone service (POTS) dial-up
+ Synchronous serial connection like Integrated Services for Digital Network (ISDN) or point-to-point leased lines.

PPP consists of two sub-protocols:
+ Link Control Protocol (LCP): set up and negotiate control options on the Data Link Layer (OSI Layer 2). After finishing setting up the link, it uses NCP.
+ Network control Protocol (NCP): negotiate optional configuration parameters and facilitate for the Network Layer (OSI Layer 3). In other words, it makes sure IP and other protocols can operate correctly on PPP link



WAN Tutorial

March 8th, 2016 in CCNA Knowledge

Unlike LAN which is used effectively in relatively small geographic areas, WAN services help connect networks at a broad geographic distance, from a few to thousands of kilometers. Let’s see the network below, while LANs are used inside buildings like Home, Office, Internet Service Provider (ISP)… WANs are often used to connect between them. By the way, Internet is the largest WAN nowadays.


Because of long distance connection, individuals usually do not own WAN (unlike LAN which they often own it). They do not have the rights to bury a long cable between buildings either. Therefore they hire available network service providers, such as ISPs, cable or telephone companies… in their cities instead. This helps reduce the connection cost very much.


DHCP Tutorial

June 18th, 2015 in CCNA Knowledge

In IP environment, before a computer can communicate to another one, they need to have their own IP addresses. There are two ways of configuring an IP address on a device:
+ Statically assign an IP address. This means we manually type an IP address for this computer
+ Use a protocol so that the computer can obtain its IP address automatically (dynamically). The most popular protocol nowadays to do this task is called Dynamic Host Configuration Protocol (DHCP) and we will learn about it in this tutorial.

A big advantage of using DHCP is the ability to join a network without knowing detail about it. For example you go to a coffee shop, with DHCP enabled on your computer, you can go online without doing anything. Next day you go online at your school and you don’t have to configure anything either even though the networks of the coffee shop and your school are different (for example, the network of the coffee shop is while that of your company is Really nice, right? Without DHCP, you have to ask someone who knows about the networks at your location then manually choosing an IP address in that range. In bad situation, your chosen IP can be same as someone else who is also using that network and an address conflict may occur. So how can DHCP obtain an suitable IP address for you automatically? Let’s find out.



Simple Network Management Protocol SNMP Tutorial

June 16th, 2014 in CCNA Knowledge

Building a working network is important but monitoring its health is as important as building it. Luckily we have tools to make administrator’s life easier and SNMP is one among of them. SNMP presents in most of the network regardless of the size of that network. And understanding how SNMP works is really important and that what we will learn in this tutorial.

Understand SNMP

SNMP consists of 3 items:

+ SNMP Manager (sometimes called Network Management System – NMS): a software runs on the device of the network administrator (in most case, a computer) to monitor the network.
+ SNMP Agent: a software runs on network devices that we want to monitor (router, switch, server…)
+ Management Information Base (MIB): is the collection of managed objects. This components makes sure that the data exchange between the manager and the agent remains structured. In other words, MIB contains a set of questions that the SNMP Manager can ask the Agent (and the Agent can understand them). MIB is commonly shared between the Agent and Manager.



Syslog Tutorial

May 22nd, 2014 in CCNA Knowledge

As an administrator of a network, you have just completed all the configuration and they are working nicely. Now maybe the next thing you want to do is to set up something that can alert you when something goes wrong or down in your network. Syslog is an excellent tool for system monitoring and is almost always included in your distribution.

Places to store and display syslog messages

There are some places we can send syslog messages to:

Place to store syslog messages Command to use
Internal buffer (inside a switch or router) logging buffered [size]
Syslog server logging
Flash memory logging file flash:filename
Nonconsole terminal (VTY connection…) terminal monitor
Console line logging console

Note: If sent to a syslog server, messages are sent on UDP port 514.

By default, Cisco routers and switches send log messages to the console. We should use a syslog server to contain our logging messages with the logging command. Syslog server is the most popular place to store logging messages and administrators can easily monitor the wealth of their networks based on the received information.


Gateway Load Balancing Protocol GLBP Tutorial

May 4th, 2014 in CCNA Knowledge

The main disadvantage of HSRP and VRRP is that only one gateway is elected to be the active gateway and used to forward traffic whilst the rest are unused until the active one fails. Gateway Load Balancing Protocol (GLBP) is a Cisco proprietary protocol and performs the similar function to HSRP and VRRP but it supports load balancing among members in a GLBP group. In this tutorial, we will learn how GLBP works.

Note: Although we can partially configure load balancing via HSRP or VRRP using multiple groups but we have to assign different default gateways on the hosts. If one group fails, we must reconfigure the default gateways on the hosts, which results in extra administrative burden.

GLBP Election

When the routers are configured to a GLBP group, they first elect one gateway to be the Active Virtual Gateway (AVG) for that group. The election is based on the priority of each gateway (highest priority wins). If all of them have the same priority then the gateway with the highest real IP address becomes the AVG. The AVG, in turn, assigns a virtual MAC address to each member of the GLBP group. Each gateway which is assigned a virtual MAC address is called Active Virtual Forwarder (AVF). A GLBP group only has a maximum of four AVFs. If there are more than 4 gateways in a GLBP group then the rest will become Standby Virtual Forwarder (SVF) which will take the place of a AVF in case of failure. The virtual MAC address in GLBP is 0007.b400.xxyy where xx is the GLBP group number and yy is the different number of each gateway (01, 02, 03…).

+ In this tutorial, the words “gateway” and “router” are use interchangeable. In fact, GLBP can run on both router and switch so the word “gateway”, which can represent for both router and switch, is better to describe GLBP.
+ For switch, GLBP is supported only on Cisco 4500 and 6500 series.

The gateway with the highest priority among the remaining ones is elected the Standby AVG (SVG) which will take the role of the AVG in the case it is down.



EtherChannel Tutorial

January 17th, 2014 in CCNA Knowledge

EtherChannel is the technology which is used to combine several physical links between switches or routers into one logical connection and treat them as a single link. Let’s take an example to see the benefits of this technology: Suppose your company has two switches connecting with each other via a FastEthernet link (100Mbps): Switch_single_link.jpg Your company is growing and you need to transfer more than 100 Mbps between these switches. If you only connect other links between the two switches it will not work because Spanning-tree protocol (STP) will block redundant links to prevent a loop: Switch_STP_block.jpg To extend the capacity of the link you have two ways:
+ Buy two 1000Mbps (1Gbps) interfaces
+ Use EtherChannel technology to bundle them into a bigger link READ MORE…

Hot Standby Router Protocol HSRP Tutorial

November 7th, 2013 in CCNA Knowledge

In this tutorial we will learn what is HSRP and the need of HSRP in a network.

Most of the company in the world has a connection to the Internet. The picture below shows a most simple topology of such a company:


To make above topology work we need to:

+ Configure IP addresses on two interfaces of the Router. Suppose the IP address of Fa0/0 interface (the interface connecting to the switch) is
+ Assign the IP addresses, default gateways and DNS servers on all PCs. In this case we have to set the default gateways to Fa0/0 interface (with the IP address of the router. This can be done manually or automatically via DHCP.


InterVLAN Routing Tutorial

February 22nd, 2012 in CCNA Knowledge

In the previous VLAN tutorial we learned how to use VLAN to segment the network and create “logical” broadcast domains. In this tutorial we will learn about InterVLAN Routing.

What is InterVLAN routing?

As we learned, devices within a VLAN can communicate with each other without the need of Layer 3 routing. But devices in separate VLANs require a Layer 3 routing device to communicate with one another. For example, in the topology below host A and B can communicate with each other without a router in the same VLAN 10; host C and D can communicate in the same VLAN 20. But host A can’t communicate with host C or D because they are in different VLANs.


To allow hosts in different VLANs communicate with each other, we need a Layer 3 device (like a router) for routing:


The routing traffic from one VLAN to another VLAN is called InterVLAN routing.


Cisco Command Line Interface CLI

December 20th, 2011 in CCNA Knowledge

In the previous tutorial we learned about the boot sequence of a Cisco router/switch. After that, the router will allow us to type commands but in different modes we can only used specific commands. So in this tutorial we will learn about the Command Line Interface (CLI) and different modes in a Cisco router/switch.

Below lists popular modes in Cisco switch/router:

Router> User mode
Router# Privileged mode
Router(config)# Configuration mode
Router(config-if)# Interface level (within configuration mode)
Router(config-router)# Routing engine level (within configuration mode)
Router(config-line)# Line level (vty, tty, async) within configuration mode

Now let’s discuss each mode in more detail


Cisco Router Boot Sequence Tutorial

December 16th, 2011 in CCNA Knowledge

In this article we will learn about the main components of a Cisco router and how the boot process takes place.

Types of memory

Generally Cisco routers (and switches) contain four types of memory:

Read-Only Memory (ROM): ROM stores the router’s bootstrap startup program, operating system software, and power-on diagnostic test programs (POST).

Flash Memory: Generally referred to simply as “flash”, the IOS images are held here. Flash is erasable and reprogrammable ROM. Flash memory content is retained by the router on reload.

Random-Access Memory (RAM): Stores operational information such as routing tables and the running configuration file. RAM contents are lost when the router is powered down or reloaded.

Non-volatile RAM (NVRAM): NVRAM holds the router’s startup configuration file. NVRAM contents are not lost when the router is powered down or reloaded.


OSI Model Tutorial

November 2nd, 2011 in CCNA Knowledge

Welcome to the most basic tutorial for networker! Understanding about OSI model is one of the most important tools to help you grasp how networking devices like router, switch, PC… work.

Let’s take an example in our real life to demonstrate the OSI model. Maybe you have ever sent a mail to your friend, right? To do it, you have to follow these steps:

1. Write your letter
2. Insert it into an envelope
3. Write information about sender and receiver on that envelope
4. Stamp it
5. Go to the post office and drop it into a mail inbox

From the example above, I want to imply we have to go through some steps in a specific order to complete a task. It is also applied for two PCs to communicate with each other. They have to use a predefined model, named OSI, to complete each step. There are 7 steps in this model as listed below:


This is also the well-known table of the OSI model so you must take time to learn by heart. A popular way to remember this table is to create a fun sentence with the first letters of each layer. For example: All People Seem To Need Data Processing or a more funny sentence sorted from layer 1 to layer 7: Please Do Not Throw Sausage Pizza Away.