I referred to the link
and found that ids ranging from 1000 to 1599 are reserved for internal usage of Infinispan. However there is note saying "RANGES to be finalised yet!"
Can you clarify what is the recommended range of ids to be used for user-defined externalizers (advanced externalizers) in Infinispan 5.1.1
Those ranges are good know ranges. They were not finalised waiting for other projects to add their own. Seems like quite a few have already done so.
Anything up to 16383 will consume only two bytes, so you can be safe by using between 3000 and 16000, you decide