[Asterisk-Users] Internal/External IP

[Robbie Hughes]

I have an asterisk installation on an external IP, along with all the
clients on external IPs, all routed through the same router on a /29
subnet which is then connected to an ADSL connection through another
routed external /31 subnet. ie - I have 2 externally routed subnets, one
for my connectivity, and one for my LAN.
External lines are provided by Chan_Capi and 2 ISDN lines.
Now the problem I'm having is that when the ADSL connection drops, the
phones do not ring - the ISDN rings, but asterisk cannot appear to route
the calls to the phones to make them ring.

There are no DNS records used anywhere, purely IPs so external DNS
lookups should not be necessary. All the /29 subnet IPs are in the arp
cache of the router so there should be no need to go hunting outside of
the LAN to route calls.

Is this standard behaviour?
Would i be better off natting all my phones and disabling can_reinvite
and enabling use_nat

I know asterisk can be used on internal IPs, but am a little confused as
to why this setup fails when the ADSL is down.

Any networking gurus out there who can advise?