[c-nsp] Inter ISP routing question

[Roy]

Chris Smith wrote:

>Here's the setup:
>
>T-1 connection to ISP A, /24 from ISP A
>
>2xT-1 connection to ISP B
>
>If I point the default route to ISP B, most networks
>appear to stop respondiing.  I guess that I'm tripping
>over some anti-spoofing configurations.  Is there a
>way to make this work without going BGP?
>
>--Chris
>
>  
>
You are a victim of a security feature called RPF.  Basically you can 
not send a packet out a path that is does not have a reverse path back 
to that IP address over the same interface. 

Some info is on the web page
http://www.cisco.com/en/US/products/sw/iosswrel/ps1820/products_feature_guide09186a00800f489a.html

You can make it work by having provider B add static routing table 
entries for the IP space.  This isn't recommended since you will now 
have inconsistent AS numbers on your routes and that may introduce more 
problems.  The correct answer is to add BGP to keep all the routes 
consistent.

Roy