There are traded offs in the decision. The TCAMs generically are rows of "key," "mask" and "result." A bank of TCAM can, at great power expense, answer "which i

There are traded offs in the decision. The TCAMs generically are rows of "key," "mask" and "result." A bank of TCAM can, at great power expense, answer "which is the result from the first row where my input anded with the row's mask equals the row's key." Some chips let you swap banks between different functions. Internet carriers tend to need a lot of l3 route lookups. Enterprise customers tend to want more access control list entries. From the outside this looks like the initial firmware aimed to be a middle ground, and carriers needed space shifted around. However, at every nice round power of two we are going to be having some hardware hit limits in what can be done even if the only priority is l3 routing. Often there are fixed function banks, and even if they were all flexible, at least some ACL features are needed on most (if not all) routers.