Not sure what You are looking for, here are some points to get You started:
1/ there are 2 fxp interfaces in the router: fxp0 and fxp1
2/ fxp1 is NOT for management, do not configure anything on fxp1
3/ fxp0 is for out-of-band router management
4/ fxp0 does not support VLAN tagging
5/ fxp0 is not supported inside routing instance
6/ fxp0 is not supported in Logical System
7/ all router management protocols are supported through fxp0 (SNMP, NTP, SSH, Telnet, HTTP, HTTPS, etc).
8/ transit traffic through fxp0 is not supported
- for instance, the above means You cannot directly telnet/SSH from Your PC through Router1 fxp0 to Router2 lo0.0. You have to telnet/SSH to Router1 fxp0.0 and then initiate another telnet/SSH session from Router1 to Router2 (if Your company rules&policies permit that, of course).
9/ firewall filter assigned to lo0.0 also filters traffic entering fxp0.0
As per my understanding fxp0 is virtual interface.
fxp0 is 10/100 or 10/100/1000 Ethernet port on Routing Engine and it is based on INTEL PRO/100 or INTEL PRO/1000 NIC card.
You can see where fxp0 is physically located by examining techdocs pictures for each Juniper router product. For instance, on MX5...MX80 routers this port is located on front panel and is labeled "Ethernet"
So lets say if router is isolated from wan and lan then how we use fxp0 in that case . Does it means we need to map fxp0 to any physical interface if yes then how
Generally speaking, You have to build a separate Out Of Band management network to be able to use fxp0 in this case. There also are several creative ways to avoid investment in OOB network, like when deploying routers in pairs, Router1 fxp0 connects with crossover cable to Router2 port ge-1/0/0, and Router2 fxp0 connects via crossover cable to Router1 port ge-1/0/0.