The issue concerns one of mapping layer
3 addresses to layer 2 (DDR), which is
a multipoint (ie can dial multiple sites)
interface. When you have
ip route 0.0.0.0 0.0.0.0 172.16.40.5
Every packet will be given a next hop
destination address of 172.16.40.5; if
this points to a DDR interface, and
that DDR interface has a dialer map for
the IP address 172.16.40.5 then all is
well.
However, if you have
ip route 0.0.0.0 0.0.0.0 bri 0
Then (let's say) we receive an IP
packet with a destination address of
10.1.1.1; the routing table points us
to BRI0, and unless BRI0 has a dialer
map for 10.1.1.1 we will get an encapsulation
error.
With DDR interfaces the preferred approach
is to use a specific next hop address; if
that is a problem, and you are only dialing
to one site, use dialer profiles (which
are point to point interfaces) and the
BRI static route should work.