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Multi Router Traffic Grapher (MRTG) is a tool used to monitor the traffic load on internet links. It provides html pages containing graphs (images) with live representation of traffic. Read below the step by step installation guide for MRTG in a Linux system.

Requirements:
Operating System: Linux
Packages: mrtg, snmp, snmp-utils
SNMP – Simple Network Management Protocol is used to manage IP network devices such as routers, switches etc.

Make sure snmp is installed.
# rpm –qa | grep snmp

If installed, avoid next step. Otherwise install it using the command given below.

Install SNMP

# yum install net-snmp-utils net-snmp
Start the snmp service

# service snmpd start

Make the service run in startup
# chkconfig snmpd on

Now use the snmpwalk utility for getting the details about your network.
# snmpwalk -v 1 -c public localhost IP-MIB::ipAdEntIfIndex

You will get an output something similar as given below.

ip.ipAddrTable.ipAddrEntry.ipAdEntIfIndex.127.0.0.1 = 1
ip.ipAddrTable.ipAddrEntry.ipAdEntIfIndex.192.168.1.5 = 2

If you can see your IP address, you can avoid next steps or configure snmp as given below.

Configure SNMP

# vi /etc/snmp/snmpd.conf

Change/Modify line(s) as follows:

Find following Blue Lines and replace with the next Green Lines:

com2sec notConfigUser default public
com2sec local localhost public
com2sec mynetwork 192.168.1.0/24 public

(Make sure you give your IP range in the above line.)

group notConfigGroup v1 notConfigUser
group notConfigGroup v2c notConfigUser
group MyRWGroup v1 local
group MyRWGroup v2c local
group MyRWGroup usm local
group MyROGroup v1 mynetwork
group MyROGroup v2c mynetwork
group MyROGroup usm mynetwork

view systemview included system
view all included .1 80

access notConfigGroup "" any noauth exact systemview none none
access MyROGroup "" any noauth exact all none none
access MyRWGroup "" any noauth exact all all none

syslocation Unknown (edit /etc/snmp/snmpd.conf)
syscontact Root (configure /etc/snmp/snmp.local.conf)
syslocation Linux (RH3_UP2), Home Linux Router.
syscontact Name <name@domain.com>

Start your snmp server and test it:
# service snmpd start

Now test your snmp server:
# snmpwalk -v 1 -c public localhost IP-MIB::ipAdEntIfIndex

Install MRTG

Check whether it’s already installed.
# rpm –qa | grep mrtg

If not done, run the below command.
# yum install mrtg

Configuring MRTG

Create document root to store mrtg graphs/html pages:
# mkdir -p /var/www/html/mrtg/

Create mrtg configuration file using cfgmaker command.
# cfgmaker --global 'WorkDir: /var/www/html/mrtg' --output /etc/mrtg/mrtg.cfg public@localhost

Create default index page for the MRTG configuration:
# indexmaker --output=/var/www/html/mrtg/index.html /etc/mrtg/mrtg.cfg

Copy all png files to the mrtg path:
# cp -av /var/www/html/mrtg/*.png /var/www/html/mrtg/

Run mrtg command from command line with your configuration file:
# mrtg /etc/mrtg/mymrtg.cfg

(You may get few warning message for first time; ignore them.)

Open a web browser and type url http://www.your-server-name/mrtg/ or http://your-ip/mrtg/

You can see the mrtg graphs of your network.

You can generate the graph in each 5 (for example) minutes using a cron job.

Login as a root and configure the cron job:

# crontab -e

Append following line to the crontab file:

* 5 * * * * /usr/bin/mrtg /etc/mrtg/mymrtg.cfg --logging /var/log/mrtg.log

Save and exit

For detailed document, please check out the link (MRTG).