explained very well under the section "Periodic packet management daemon"
https://www.oreilly.com/library/view/juniper-mx-series/9781491932711/ch01.html
there is actual slight difference between distributed and inline bfd session. We can safely say that inline is enhanced version of distributed mode.
Distributed Mode:-
For several years, ppmd
has not been a single process hosted on the Routing Engine and now it has been developed to work in a distributed manner. Actually, ppmd
runs on the Routing Engine but also on each Trio line card, on the line card’s CPU, where it is called PPM Manager, also known as ppm man. The following PFE command shows the ppm man thread on a line card CPU:
NPC11(R1 vty)# show threads
[...]
54 M asleep PPM Manager 4664/8200 0/0/2441 ms 0%
The motivation for the delegation of some control processing to the line card CPU originated with the emergence of subsecond protocols like BFD.
Inline Mode:- (it is only not limited to Trio chipset but also Available in latest Broadcom Trident chipsets)
Recently, the Trio line card offers a third enhanced version of ppm, driven also by the BFD protocol in scaled environments, which is called inline ppm. In this case, the Junos OS has pushed the session management out to the packet forwarding engines themselves
we can disable distributed and inline ppmd via
set routing-options ppm no-delegate-processing
set routing-options ppm no-inline-processin