load balancing with f5s

I was wondering how f5s configured as active-active do load balancing. That is, for the nodes or real servers that they load balance, how do they a) accept packets from a source, without configuring two separate default routes on the router sending the packets, and have a different IP on each f5 (having static default routes would be problematic if one of the f5s failed, so it would have to be able to detect this), and b) how do they accept traffic from the servers they are load balance equally, as the servers only have one default route.

I poked around the web and found that the Ultramonkey project, using Saru, can do this, among others. I think the way it might work is to have a common MAC address shared between two f5s, so that the packet goes to both f5s, and then use some mechanism whereby the f5s compare the packets to make sure that they are the same, and one of them forwards the packet, and one of them drops it. In a way, this scenario is not really load balancing, as both machines still receive the packet and process it to some degree. It would only really be worthwhile if very little processing was done on each packet to decide which machine was to forward it. Otherwise, you get no real gain from having two machines active. Perhaps this could be a hash lookup, which, if the hashes match, then some very simple algorithm could then be used to decide which unit will forward and which unit will drop the packet. Maybe a lot more of the CPU and system resources would be dedicated to NATing the packet and applying various other rules to it, so this scheme would work. Anyway, I don't really know if that is how it does it. I'll have to research a bit more.