Thank for replying me!
I tried follow your ideas. My if-down script was not working.
However i find out this issue regrading xl2tpd. In/Makefile/ of xl2tpd source code v 1.3.6 :
// # The cost of not trusting pppd to die (and shoot it down hard), is that your//
// # pppd’s //*ip-down scripts*//will not have a chance to run/.”
So i just enable /-DTRUST_PPPD_TO_DIE /and rebuild xl2tpd. The problem solved. My if-down script ‘s working well now!