Na verdade, percebi que o PPTP estava desativado no MikroTik v6.25, ou seja, no endereço IP 172.16.100.2, por isso não consegui me conectar ao servidor VPN.
Depois de ativar o PPTP no MikroTik, consigo conectar-me ao servidor assim:
me@linux-box:~> sudo pppd call TUNNEL debug nodetach
using channel 15
Using interface ppp0
Connect: ppp0 <--> /dev/pts/5
sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0xa905bd9b> <pcomp> <accomp>]
sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0xa905bd9b> <pcomp> <accomp>]
sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0xa905bd9b> <pcomp> <accomp>]
sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0xa905bd9b> <pcomp> <accomp>]
sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0xa905bd9b> <pcomp> <accomp>]
sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0xa905bd9b> <pcomp> <accomp>]