Hi Reload,
There is a solved query regarding this on our forum :
"Redundancy in Fabric depends on mode at which SCBE3-MX cards are operating at, there are 2 modes [increased-bandwidth | redundant] and this is configurable
https://www.juniper.net/documentation/en_US/junos/topics/reference/configuration-statement/redundanc...
- MX960 can have up to 3 SCB cards, in redundant mode 2 fabric cards are active and 1 card is standby, while in non-redundant mode / increased-bandwidth all 3 SCB cards are active.
- when all 3 SCB cards are active you get 1.5 Tbps fabric capacity per slot and when only 2 cards are active you get 1Tbps fabric capacity per slot and this is the default mode for SCB3-MX"
Query link:
https://forums.juniper.net/t5/Junos/Question-about-SCBE3-MX/td-p/480680
For details about SCB: https://www.juniper.net/documentation/en_US/release-independent/junos/topics/concept/SCB-description.html
No, it is not mandatory to have 3 SCBs installed. Your MX device will work just fine with two. The third one is for redundancy purposes. ( standby )
HTH
Mark "Accept As Solution" in case this helps.