IF I have 2 web front ends in a farm, both hosting websites and running central admin and all other services, on which farm should the search be running, if not on both, and what urls should it have in the content crawl, that of both web front ends? The point is, I Want the search to keep working even if one web front end shuts down..
if you want redundancy, you should enable Search Services on both WFE servers. You probably have a Load Balancer for the traffic. You need crawl web applications urls and they must pointing to the NLB so if one server will go down, it will be redirected to another one.
You should have two applications servers too where one is for Central Administration and rest of services are running on both servers.
Hope it helps!