When the line is up and you ping the 3800
from the 3800, what is really happening is
that the ICMP echo requests are still going
across the line to the 3600, which sends them
back. This is expected. In general, it doesn't
make much sense to ping yourself, so pinging
the remote end is a sufficient test. If you can't
ping the 3600 from the 3800, or vice versa,
when the ISDN link is up, you may want to check a
few things:
sh dialer map - ensure that each end has a dialer
map for each other with the correct name.
- debug ip icmp - see if either end is receiving the
ICMP packets
- sh interface - ensure that there are no drops or errors.
You should try to ping beyond the routers, too, to
see what is and is not successful. Telnet (or other
data besided ICMP) would be useful, too.