According to a few documentations, including BRKDCT-2121 from this year's Networkers, there's no restriction in terms of having to reserve all ports in same port group, but it's highly recommended to reserve the entire block of 12 ports on N7K-M148GS-11.
"*Note – The M1 48 port line cards have 4 port groups of 12 ports.
Recommendation is to have all members of a port group in the same VDC"
Can someone please tell me why it's recommended that way?
What benefits are gained by reserving much more than what I need?
What negative impact is there, if I only allocate two ports in a port group for the new VDC, and leave the other 10 in the default VDC?
I believe it has to do with the fact that the ports share the same R2D2 ASIC and the Virtual Output Queues (VOQ) are applied on a per ASIC basis. So if you have two ports that share the same ASIC in two different VDCs, they also share the same VOQ, which could lead to head-of-line blocking during heavy traffic on one of the ports. Also since these ports belong to the same ASIC but different VDCs, they fate share, so events like reloading one of the VDCs could cause a few seconds of traffic loss on the ports sharing the same ASIC in another VDC. The bug id CSCtf19467documents this behavior.