[c-nsp] Loose uRPF behaving like strict mode on 7600

[Jose]

I was wondering if someone might have an explanation as to why we 
encountered an issue with uRPF (loose mode) when we tried enabling it on 
our upstream facing links.  We have 2 x 7603s w/ SUP32 acting as our Gwy 
routers and our transit providers connect into them (one on each gwy + 
private peers).  We are fed from each of them the entire internet table 
along with a default route.

Now I know that we are multi-homed and obviously have asymmetrical 
routing occurring so I decided to implement loose uRPF on the 
interfaces:  ip verify unicast source reachable-via any

However shortly after enabling it we got calls that our customers could 
not reach parts of the internet.  Specifically destinations where the 
packets would travel over the links that had RPF enabled on them and 
were our transits.  Traffic to and from our private peers appeared fine 
though with RPF.  Pings to our internal CIDRs from external 
route-servers would fail as well so long as the path was over the 
transits.  Disabling RPF on the interfaces resolved the problem immediately.

 From my understanding of this feature, it would seem as if the RPF 
check was working in strict mode vs loose mode.  Could there have been 
something that we missed?  Should the "allow-default" be used in this 
case?  I've never had to use it before when I've implemented loose mode 
in other environments.

The 7603s are running 12.2(18)SXF11 Advanced IP Services.

Thanks for any feedback.

Jose