Two SCB are capable to handle the full capacity of lin cards, third SCB is for redundancy purpose, if any of the first two SCBs goes down, then the third standby will take over.
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
- 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"
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 )
Mark "Accept As Solution" in case this helps.
Yes, the third SCB is to maintain 2+1 redundancy. Which comes into play when there is an issue with one of the active ( primary and backup ) SCBs.