DHCP and PPPoE are completely different protocols.
DHCP works on DORA process. Here address is assigned by DHCP server.
DHCP server is configured on MX to listen port 67 and 68. It uses AAA to assign address in case loopback address matches the pool subnet or you can assign the address via radius.
In case you don't configure the dhcp-local-server, MX will not process the DHCP discover as it won't be listen to the ports.
TEST# show system services dhcp-local-server
TEST> show system processes extensive | grep jdhcp
TEST> show system connections | match 67
TEST> show system connections | match 68
TEST# set system services dhcp-local-server group test interface ge-0/0/0
labroot@ERX-MX960-NS-1> show system processes extensive | grep jdhcp
12067 root 2 39 0 545M 96108K select 3 0:00 0.78% jdhcpd
TEST> show system connections | grep 67
udp4 0 0 *.67 *.*
TEST> show system connections | grep 68
udp4 0 0 *.68 *.*
PPPoE has two stages. Discovery and PPP session . DHCP server doesn't play any role here.
PPPoE is a rather simple layer 2 protocol which allows to tunnel PPP frames accross an ethernet segment.
The protocol uses 4 way handshake where the subscriber side tries to discover the Access Concentrator (the MX in our case) available to offer service, this is the discovery phase which uses ethertype 8863. The subscriber select an offer from the AC available which answers with session id.
When successful PPPoE negotiation completes the peers have an session_ID and each other MAC addresses which is uniquely identifying a PPP session. Then start the session phase which uses ethertype 8864.
NCP [IP negotiation]
The Link Control Protocol is used to establish, configure, monitor and terminate the link, it is always the first protocol negotiated. Most common options negotiated during the LCP phase are MRU, authentication protocol, if options are not present default values of the parameter are assumed (i.e. MRU=1500).
PPP is a symetric peer to peer protocol and LCP phase is said to be finished only when both peers have sent and received configuration acknowlegdment to confirm alignment on the LCP parameters negotiated. NCP packets received during this phase should be silently discarded.
NCP is used by PPP to negotiate network protocols parameters. I will concentrate on IPCP since this is the most used one and corresponds to IP network protocol, in BRAS evironment Service Provider use the protocol to transport IP datagram accrros the link, to assign IP addresses and DNS address to subscribers among the most use parameters.
Accept this as solution if it resolved your issue.
Kudos would be appreciated too.