ifIndex in .rrd file name

[saman]

i had planned to migrate mrtg to cacti.
1. convert from logfiles [mrtg] to rrd files [cacti]----> [DONE].
[exampe graphed host is switch.mydomain.com] ------> name of files export result is switch.mydomain.com_1.rrd,switch.mydomain.com_2.rrd,switch.mydomain.com_3.rrd ..............
2. create graph on cacti for host switch.mydomain.com -----> DONE
rrd files name is switch.mydomain.com_ds0_100.rrd, switch.mydomain.com_ds0_101.rrd,switch.mydomain.com_ds0_102.rrd.....
3. Rename files on step 1 (one) became switch.mydomain.com_ds0_1.rrd, switch.mydomain.com_ds0_2.rrd, switch.mydomain.com_ds0_3.rrd.......... [done using bash to rename]
4. i had problem why cacti using numbering from 100 [switch.mydomain.com_ds0_100.rrd] or other names exampe router.mydomain.com_ds0_222.rrd for first interface in my switch or my router ???

can it setting manually ?? so numbering name became switch.mydomain.com_ds0_1.rrd, switch.mydomain.com_ds0_2.rrd ......
why cacti using random number ???

i will override new cacti graph with my rrd file [export from mrtg] so i will not miss old graph from mrtg, (continue by cacti).

Thanks beforhand,
Sorry for my bad english