The csnp-interval knob does not work on point-to-point ISIS interfaces.

If you configure the csnp-interval on a point-to-point ISIS interface the output of show isis interface extensive does not reflect the configured csnp-interval.

The csnp-interval knob applies only to LAN interfaces. Configuring it on point-to-point interfaces has no effect.

The csnp-interval for point-to-point interfaces is determined by multiplying the number of point-to-point ISIS interfaces configured by 5. i.e If you have 3 point-to-point interfaces the csnp-interval of all point-to-point interfaces is 15.