[c-nsp] Why do I have to specify "allow-default" uRPF option on 4500-E?

[Sebastian Wiesinger]

Hello,

I have a Cisco 4500-E / SUP6-E switch on which I want to configure
uRPF.

I tried to enable it and got the following message:

re1-new(config-if)#ip verify unicast source reachable-via rx 
% ip verify configuration not supported on interface Vl13
 - must specify allow-default


With the allow-default option no problem:

re1-new(config-if)#ip verify unicast source reachable-via rx allow-default 
re1-new(config-if)#

Any idea why I have to enable allow-default? In the configuration
guide for the 4500-E the command is printed with the allow-default
option but without any explanation why it has to be specified.


And what *exactly* does the allow-default option do? In the Cisco
paper it says:

The allow-default option may be used with either the rx or any option
to include IP addresses not specifically contained in the routing
table.

Am I right that this would only affect uRPF in the case that I point a
default 0.0.0.0/0 towards the interface?


Regards,

Sebastian


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