In the end I had to jump on a plane to solve this one!
It turns out that the gigstack modules get very messy when it comes to STP, especially so if the gigastacks have not been cabled properly as the specs for them are very specific - easy for somebody to adjust this and get it wrong.
The most common type of configuration in the cascaded redundent stack which employs a half duplex shared link. This is the reason why you see multiple neighbors (more than 2 sometimes up to nine) on the sh cdp nei command. This only adds to the troubleshooting confusion.
Even when spaning tree blocks one of the ports on the gigastack, this does not show in the sh spanning tree blocked command, again adding to confusion. The only way to tell is to look at the LED on the gigastack (bear in mind the physical is up if there are two ports used on the gstack module) or to issue the show interface command which details gigastack link status and redundant link.
This makes me wonder if it uses standard STP or some proprietary cisco gstack modified version for half duplex gigastack. Either way, Cisco dropped this for a reason, its a mine field in my eyes!
Even after tralling through the many forums many experts do not understand the gigastack stp operations and simply state in depth detail about how stp works on standard links which is no help (after easy points).
Obviously this is not covered on the ccie lab as very little information out there. I realise this has been dropped for sfp these days but in the real world, engineers (like me) have to still deal with legacy tech and fly miles to fix issues like this!
Here's my ouput below to help anybody in the future who has gigastack stp issues
swixl-4#sh int gi0/1
GigabitEthernet0/1 is up, line protocol is up (connected)
Hardware is Gigabit Ethernet, address is 000c.3070.7031 (bia 000c.3070.7031)
MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Half-duplex, 1000Mb/s, media type is CX
input flow-control is off, output flow-control is off
ARP type: ARPA, ARP Timeout 04:00:00
GigaStack module(0.2) in GBIC slot. link1 is up, link2 is redundant link
Last input 00:00:00, output 00:00:01, output hang never
Last clearing of "show interface" counters never
Input queue: 1/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 2142000 bits/sec, 494 packets/sec
5 minute output rate 74000 bits/sec, 26 packets/sec
195820137 packets input, 3059874038 bytes, 0 no buffer
Received 18755551 broadcasts (0 multicast)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 12245420 multicast, 0 pause inputswixl-4#sh cdp nei
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
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Gig 0/1 123 S I WS-C2950G-Gig 0/2
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Gig 0/1 177 S I WS-C2950G-Gig 0/2
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Gig 0/1 144 S I WS-C2950G-Gig 0/1
swixl-3.sagem.ads.sagem
Gig 0/1 121 S I WS-C2950G-Gig 0/2
swixl-6 Gig 0/1 120 S I WS-C2950G-Gig 0/2
swixl-4#
swixl-4#sh spanning-tree blockedports
Name Blocked Interfaces List
-------------------- ------------------------------------
Number of blocked ports (segments) in the system : 0
swixl-4#
0 input packets with dribble condition detected
85301608 packets output, 2416013666 bytes, 0 underruns
0 output errors, 117243 collisions, 2 interface resets
0 babbles, 0 late collision, 228810 deferred
0 lost carrier, 0 no carrier, 0 PAUSE output
0 output buffer failures, 0 output buffers swapped out
