Hello Jaipal,
>> what exactly IGP used for in an ISP network ?
to provide routing information for the network infrastructure.
Information about loopback(s) and internal links has to be propagated everywhere is needed.
Info about loopbacks is used by BGP, LDP, RSVP to build their relationships.
The modern approach is a "divide and conquer" approach.
Thanks to MPLS now an internal node can forward packets to destinations that are not in its routing table.
This also has relaxed in some ways the scalability issues of IGPs ( I remember your OSPF versus IS-IS enquiry).
Most of IP routes are carried by BGP, infrastructure routes are few in comparison and they are handled by the IGP(s) that are more reactive (provide more detailed info) but far less scalable then BGP.
Hope to help
Giuseppe
