I am connected to two tier 1 ISP's and take full routes from both. I have a T3 to one and dual T1's to the other. The dual T1's are intended to be a backup to the T3. I have local-pref set to 200 for all routes learned via the T3 ISP. I also am prepending my AS# several times to the T1 ISP. For 99% of routes this has the desired affect of routing traffic via the T3. For some reason the T1 ISP is learning more specific routes for a small number of IP blocks. This causes a small number of /24 blocks to use the T1's.
I want to ensure all traffic utilizes the T3 because the asymmetric paths seem to fail.
I was thinking of modifying BGP to only look at /16 or larger IP blocks.