It is important to note that hash tables only have average access time of
O(1). This means a particular operation could be much worse. Additionally, there are several requirements for properly formed hash trees:
- Mostly empty - few hash algorithms perform well beyond 70% usage, and most recommend 50% usage.
- Collision handling is complex - either having to use a list as the "stored value" or accepting a worst case linear search time
- Hashing cost - you have to perform a hashing operation, which may or may not involve a lot of work
It sounds like the course mentions that in many routers these issues have been solved so they use a hash table for storage. However there are several advantages to splay trees
- No overhead - if you allocate 10 MB to storage you get 10 MB worth of data without adjusting your run-times
- Recent access preference - recent data is more easily accessed, and routers access more recent data frequently (new network nodes are likely to talk, and most networks are not used evenly)
- Dups are fine - can be useful depending on what you are storing
- Simple implementation - routers are typically run on fairly weak processors and are incredibly difficult to update, so simplicity is nice
Unfortunately this doesn't answer your question directly, but hopefully it sheds some light on why that is the case.
BTW, here are some notes on why routers may have moved away (these line up with advantages):
- Storage sizes have gone up, due to the cost of RAM going down, ability to keep things in memory is easier
- Hash tables when properly maintained have constant access to all elements
- Hash tables can handle dupes if you need them to
- Router manufacturers are typically long lived and can overcome complexity issues with time