For example:

1st line (basic): Take a call/ticket from user. Find out if it's an easy fix, and fix it. If unable to fix it, write all known information and troubleshooting steps taken and route to 2nd line. (e.g. ping, traceroute)

2nd line (advanced): Receive ticket from 1st line and use advanced troubleshooting methods and advanced knowledge to solve the problem. If unable to do so, write everything down and route problem to 3rd line (e.g. RSTP, MST, EIGRP, OSPF, BGP, QoS, MPLS)

3rd line (expert): Receive ticket from 2nd line and use vast expert knowledge and experience to solve the problem. This line cannot fail. If problem is unsolvable, find the wrong system behavior and solve the problem with manufacturer. (e.g. debug, memory values, complex system states)

Here it is explained in another manner: http://www.northharriscollege.com/23169/

Hope this helps.

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