Error trying to create gre tunnel

[D'Arcy Cain <darcy%NetBSD.org@localhost>]

I made this work on one NetBSD system but two machines on the local side 
fail.  One is a 7.1 server and the other is current.

ifconfig gre0 create
ifconfig gre0 10.255.255.1 10.255.255.2 netmask 255.255.255.252
ifconfig gre0 tunnel 207.35.13.14 98.158.139.79

The last line gives this error.

ifconfig: SIOCSLIFPHYADDR: Can't assign requested address

Both of those addresses are live, public addresses.  I can ping either 
one from the machine I am trying to use as the tunnel endpoint.  Why 
would this work on the remote side but not the local side?

Even after the error I am able to add a route to the remote network but, 
of course, it doesn't work.

--
D'Arcy J.M. Cain <darcy%NetBSD.org@localhost>
http://www.NetBSD.org/ IM:darcy%Vex.Net@localhost