[c-nsp] Purposed of uRPF's "allow-default" Option?

[Devon True]

All:

I am curious what the purpose of uRPF's "allow-default" option is? Based
on Cisco's page explaining the command, I interpret that it allows uRPF
to match on a default route... but doesn't that defeat the purpose of uRPF?

My best guess is that it allows you to set static routes for networks
whose source IPs you want to drop (using the null interface) while
allowing everything else.

e.g.

interface Vlan100
 ip verify unicast source reachable-via any allow-default
!
ip route 192.168.0.0 255.255.255.0 null0
ip route 0.0.0.0 0.0.0.0 x.x.x.x

uRPF would allow Vlan100 to use any source IP address except
192.168.0.0/24. Is that correct?

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/guide/secure.html

Thanks!

--
Devon