Hi Jeremy,

I like to suggest, for educational purposes, that the trunks ports be moved so that the port numbers are not both ascending together on both sides.

For example, on Block B...

SW2#sh cdp ne
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone

Device ID Local Intrfce Holdtme Capability Platform Port ID
SW4 Gig 0/21 122 R S I WS-C3550- Fas 0/18
SW4 Gig 0/20 122 R S I WS-C3550- Fas 0/17
SW4 Gig 0/19 122 R S I WS-C3550- Fas 0/16

instead of this physical cabling...

gi0/19 -> fa0/16
gi0/20 -> fa0/17
gi0/21 -> fa0/18

...you could make it...

gi0/19 -> fa0/18
gi0/20 -> fa0/17
gi0/21 -> fa0/16

This would help "prove" step 4, "lowest sender port-ID", in the STP path selection process.

For example, with default settings, SW2 has gi0/19 as the Root Port because the port-ID on SW4 fa0/16 "128.16" is the lowest sender port-ID.

SW2:

Interface Role Sts Cost Prio.Nbr Type

Gi0/19 Root FWD 19 128.19 P2p
Gi0/20 Altn BLK 19 128.20 P2p
Gi0/21 Altn BLK 19 128.21 P2p

SW4:

Interface Role Sts Cost Prio.Nbr Type

Fa0/16 Desg FWD 19 128.16 P2p
Fa0/17 Desg FWD 19 128.17 P2p
Fa0/18 Desg FWD 19 128.18 P2p

It's not because "128.19" is the lowest port-ID on SW2. But this would be easier to see in the lab if the trunks were rearranged like above. As you probably know teaching people, STP path selection process is a little confusing, and best to be seen hands-on.

Anyways, just thought I'd suggest this. I'm not sure if doing this would break anything in your lab.