Why my MX480 have 6 FPC slots, however the document say only support 3?

Question

In Juniper PFC document:

The maximum number of supported FPCs varies per router:

• MX480 router—3 FPCs

but in my MX480, why there have 6 FPC Slots.

admin@MX480-RE0> show chassis fpc 
                     Temp  CPU Utilization (%)   CPU Utilization (%)  Memory    Utilization (%)
Slot State            (C)  Total  Interrupt      1min   5min   15min  DRAM (MB) Heap     Buffer
  0  Online            29     18          4       17     17     17    2048       43         15
  1  Online            30     14          0       12     12     12    2048       43         15
  2  Empty           
  3  Online            47     10          0       10     10     10    3200       20         32
  4  Empty           
  5  Empty  

Answer 1

The document you reference isn't entirely accurate (anymore). The limitation isn't for physical space, but for fabric bandwidth, the redundancy/subscription capacity for that bandwidth, temperature, and power.

In short, you can physically populate 6 MPCs, just make sure you plan other aspects of the chassis like high capacity power supplies, fan trays, etc. accordingly.

EDIT: To address your question in the comment.

There's a couple of reasons it you'd be seeing slots 0, 1, and 3 being populated.

1. Either you have an old-school FPC that is a two slot module (these are why the original physical space limitation used to exist). These cards have long been end of life.

2. You have cards that are simply slotted that way. Ignore the DPC nomenclature here, but it's a good reference.

Answer 2

A Flexible PIC Concentrator (FPC) occupies two DPC slots on an MX Series router. The router has six dedicated DPC slots that are numbered 0 through 5, bottom to top. Up to three FPCs can be installed horizontally on the front of the router. The interface corresponds to the lowest numbered DPC slot for which the FPC is installed.

https://www.juniper.net/documentation/en_US/release-independent/junos/topics/topic-map/mx480-interface-modules-fpcs.html