MPC10E-15C MRate card has three Packet Forwarding Engines, each providing a maximum bandwidth of 500 Gbps.
When you configure PIC 1 (PFE1) as a 400G port, that points out the possibility to turn on another 100G QSFP on Port 0 if you do not explicitly configure it. 

By default, all ports run at 100Gb. If you configure a 400g port, you can only configure another 100Gb. This helps to prevent a possible oversubscription. Refer to the documentation on Supported Active Physical Ports on MPC10E-15C-MRATE to Prevent Oversubscription

A single PIC is capable of up to 500G bandwidth on the MPC10E line cards. However, there are corner cases where some port configurations are invalid when 400GE is used.

Also on MPC10E FPC's, 400g is vaild only on port 4 of any PIC.

Only the following port configurations are valid:

400GE + 4x10GE 400GE + 4x25GE 400GE + 40GE 400GE + 100GE

Configure the speed on port 3 and insert the 100Gb there. After doing the configuration change, users must offline/online the PIC

Example:

#set chassis fpc 2 pic 1 port 3 speed 100g
#set chassis fpc 2 pic 1 port 4 speed 400g

#commit

>request chassis pic pic-slot 1 fpc-slot 2 offline  

>request chassis pic pic-slot 1 fpc-slot 2 online  

2023-10-19: Published

2024-10-22: KB modified; PIC restart will be required

2026-02-25 : KB modified to reflect that 400g is only valid on port 4

Supported Active Physical Ports on MPC10E-15C-MRATE to Prevent Oversubscription

https://www.juniper.net/documentation/us/en/software/junos/interfaces-ethernet/topics/topic-map/port-speed-mx-routers.html#id-port-speed-on-mpc10e10cmrate-overview