EtherChannel – Configuring Port-Channel Interface with LACP

I recently went through a similar migration that eradicated proprietary protocols (i.e., PVST+, HSRP, etc.); our transition was focused on posturing us for a 'vendor neutral' network. Since you're running Cisco devices, you're likely running PVST+.

Cisco covers this in more detail, but the crux of it is fairly simple: establish a hierarchy and, contrary to what common sense would tell you, start from the core and move outward.

Set your priorities in a manner that cascades down. Base this hierarchy on what is closest to a central node. All nodes participating in this STP setup should be on a common structure, so for simplicities sake, manually set one root bridge with the spanning-tree vlan xx,xx root primary.

Once you're ready for the migration to begin, start with the core. The reason for this is because MST is backwards compatible with PVST, not the other way around. A PVST core won't be able to communicate with BPDUs from MST nodes; those trunk links will drop. If everything is going right, you'll see a small drop while the new STP protocol comes online and figures out the current lay of the land.

Then just trickle these protocol changes down all the way to your furthest nodes.

End-users might not even notice the small blip while MST elects a root bridge.

1. Yes, if I am reading things correctly it appears your understanding is correct.
2. Yes, there are implementations that will allow you to do link aggregation between a host and two switches. Switch stacking will allow a stack of individual switches to be managed as one device. Typically one of switches in the stack becomes the master for the stack allowing it to manage link aggregation across multiple switches. A second option is virtual switching which also allows this functionality across multiple switches even if they are not stacked. This typically requires higher end hardware, specific software versions and additional requirements in order to implement. Examples are virtual switching system (VSS)/multichassis EtherChannel (MEC)/virtual port channel (VPC) from Cisco or virtual chassis from Juniper.
3. No. One of the hard invariants (i.e. absolute requirements) for L2 networking is the sequential delivery of frames. In link aggregation this is enforced by requiring a flow to traverse only one link in the group. If there is any sort of delay on that link, this invariant can still be maintained. If a flow were traversing two links and one of the links were to experience a delay (even a very short one), this could result in frames being delivered out of order violating this invariant.

Ultimately, if you are running into a need to exceed the speed of a link for a single flow, you would need to upgrade your interfaces to the next available speed technology (i.e. 1G to 10G, 10G to 40G, etc). Cisco is also spearheading a push for *multigigabit" providing speeds of 2.5G or 5G across Cat5e/6 cabling at distances up to 100 meters.