Dual management modules in a VSF stack
The behavior of dual MMs is different in a standalone 5400R chassis versus VSF stack. On a standalone 5400R chassis, when the primary MM fails, there is immediate takeover of that chassis by the secondary MM leading to a short duration of control plane outage.
However, in a VSF stack if both chassis have dual MMs, when the primary MM fails on either chassis, the entire chassis reboots causing traffic loss to devices that are connected to the blades on that chassis. If the downstream devices are dual homed, this traffic loss can be avoided. On reboot, the second MM on that chassis takes over and joins the VSF stack.
This setup of having all four MMs plugged into a VSF stack offers hardware-level redundancy, but not the same level of availability provided by a standalone 5400R. For high availability use cases where the 5400R is being used as a core switch for a small network, it is recommended to use Distributed Trunking instead of VSF.
Have console access for all four MMs.
Set up a VSF stack on standalone chassis with already available dual MMs. This action ensures that the VSF configuration pushed is synchronized to both MMs before the secondary MM becomes inactive after it moves to VSF.