Dani,
You can add the new FEX - FI links at anytime, but they will not get used until you re-acknowledge the Chassis.Chassis re-ack s a disruptive operation as traffic will get re-pinned/balanced across the links.
Going from 4 -> 2 links is also disruptive to any service profile that has NICs pinned to those links. Associated profiles would require a re-acknoledge (or a Chassis re-acknoledge) to re-pin the traffic to one of the 2 remaining fabric links.
Rule - When adding or removing FEX links, expect disruption and schedule a maintenance window accordingly. This shouldnt' be an operation you need to do often to a production system.
Here's how the pinning of servers to FEX uplinks works.
You can also view your current HIF -> NIF pinning from the CLI under the NXOS context:
UCS-250-A(nxos)# show interface fex-fabric
Fabric Fabric Fex FEX
Fex Port Port State Uplink Model Serial
---------------------------------------------------------------
1 Eth1/1 Active 1 N20-C6508 QCI1424A5AP
1 Eth1/2 Active 2 N20-C6508 QCI1424A5AP
1 Eth1/3 Active 3 N20-C6508 QCI1424A5AP
1 Eth1/4 Active 4 N20-C6508 QCI1424A5AP
3 Eth1/9 Active 3 N20-C6508 QCI1422A690
3 Eth1/10 Active 2 N20-C6508 QCI1422A690
3 Eth1/11 Active 1 N20-C6508 QCI1422A690
3 Eth1/12 Active 4 N20-C6508 QCI1422A690
4 Eth1/13 Active 1 N20-C6508 QCI1440A4AN
4 Eth1/14 Active 2 N20-C6508 QCI1440A4AN
4 Eth1/15 Active 3 N20-C6508 QCI1440A4AN
4 Eth1/16 Active 4 N20-C6508 QCI1440A4AN
Regards,
Robert
