As far as cipe itself is concerned, only the scheduled key exchange. You
can set that from the options file. Otherwise: nothing at all.
Do note that other apps can kill you: samba, dns, or things that are on
other networks connected to the box. I've run open cipe tunnels over
isdn autodial connections for years. A ping to some remote machine would
just dial out and you would not loose one packet. Key exchange was set
to 24 hours. Think the 1.5 gig will not be hurt by the more regular default.