I suppose you don't want to mix traffic on the two networks, so you are trying to ensure that traffic sourced from the 'lab' network interface will only use lookups from the 'lab' table, and not be routed via the other interface toward the internet. Is that correct?
I just ran across this blog post, which was helpful for me to understand why you might be doing what you're doing:
http:// blog.scottlowe. org/2013/ 05/29/a- quick-introduct ion-to- linux-policy- routing/
I suppose you don't want to mix traffic on the two networks, so you are trying to ensure that traffic sourced from the 'lab' network interface will only use lookups from the 'lab' table, and not be routed via the other interface toward the internet. Is that correct?