Oracle Clusterware lower vs upper stack

 Lower stack(has) processes are :- gpnpd,GIPC,mDNS,GNS

 Upper stack(crs) processes are :- CRS,CSS,CTSS,EVM,ONS,orarootagent,oraagent

The lower and upper stack terms have come up since 11.2 with the introduction of OHASD.

In the versions prior to 11.2, there used to be the script, init.crsd which would be read with the initialization of INIT process. It would start CRSD, EVMD and CSDD daemons and thus that would constitute the cluster stack for the node. As there was just one process-CRSD, there was no concept of any other stack, lower, upper whatever.

Now, from 112, the previous script is replaced by INIT.OHASD. Now, with the INIT process getting started, instead of CRSD being up, it's the OHASD daemon that comes up. The start of this daemon is now considered as the start of the lower stack or low stack(as I prefer to call it). OHASD starts further, CSSDAGENT, CSSDMONITOR, ORAROOT, ORAAGENT . You can see that now the CRSD does NOT get started with the node getting up. Instead, ORAROOTAGENT would start CRSD afterwards. The startup of CRSD would be considered as the Upper stack(or High stack, as I prefer to call it). I won't list here the entire process startup stack but I believe that should clear to you what happens now from 112 and that is a significant change from the previous versions.

Example perspective with out the GNS the clusterware doesn't knew which nodes are part of cluster since its needs get handshake with DNS. So its at lower stack of Clusterware components before CSS comes to picture (sync)