Hi Saptarshis,
Have a look at this documents
http://www.cisco.com/warp/public/535/3.html
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/snmp.htm
These 2 are the very basic documents for SNMp which will cover in detail with header information
ifIndex —A unique number (greater than zero) that identifies each interface for SNMP identification of that interface.
Read this link for more details about IFindex persistence
Cisco IOS Release adds support for an ifIndex value that can persist across reboots. The Interface Index Persistence feature allows for greater accuracy when it collects and processes network management data by uniquely identifying input and output interfaces for traffic flows and SNMP statistics
http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121newft/121t/121t5/dt5ifidx.htm#20001
HTH, if yes please rate the post.
Ankur
