Really neat paper: "Hash, Don't Cache: fast packet forwarding for enterprise edge routers". The paper and abstract are both available on line. The authors are Minlan Yu, and Jennifer Rexford - both of Princeton.
The authors share a lament of mine - caching forwarding decisions is attractive but no longer realistic as an implementation. The diversity of addresses (including those generated randomly by attackers) pollutes the cache and sends way too much traffic to slow fallback processing paths. So we end up with routers with one class of memory and no caches. Generally their memory is made totally from the really expensive fast stuff.
But the suggestion in this paper of using hashed bloom filters instead of caches is a really cool one. Essentially maintain one filter per interface in fast memory (sram probably) and evaluate those at forwarding time in parallel if you can. You probably just get a single hit and can forward the packet onwards.. you do this with only needing enough fast ram for the hash filters (i.e. not much).. whereas all the traditional trie data and routing update code can live in slow and cheap dram.
of course, due to the false positive nature of bloomies it is possible that you'll get more than one match that is going to require some kind of (probably slower) fallback plan for that packet. But the problem is nowhere near as dire as it is with caches.. with caches the misses force all the valid entries out of the cache and then all traffic is slowed down as the cache rate drops - with bloomies only the packets with the false positives are impacted, almost all of the traffic goes through the fast path unimpacted. The paper puts the false positive rate somewhere on the order of 1 in tens of thousands (depending on a bunch of factors - but that gives a feel for it.) Furthermore a cache can be intentionally attacked with a diversity of addresses in order to flood the cache and impact service for everyone - where under bloom filters such an attack is no more or less likely to impact service than any other kind of packet.
What can't you apply a bloom filter to? It's all pretty cool. The paper has a number of other details on how to minimize false positives and efficiently process route updates.
There is a related work from the same authors on a "buffalo" architecture which, after just a quick glance, appears to apply similar principles to LAN switching.
J Rexford is also an author of Web Protocols and Practice: HTTP/1.1, Networking Protocols, Caching, and Traffic Measurement which is probably the most practical description of the major web protocol I've ever seen.