Hi,
The whole MAD concept is their to assist in case of a split brain, which means the stacking links are down, so there would be 2 switches on the network with the same MAC address.
MAD is just the detection mechanism of this situation, but it cannot fix your broken stack links.
Every vendor with stacking can face this scenario, but not every vendor will give you an option to detect and take action on this scenario.
To resolve the twin switch scenario, MAD will remove 1 of the 2 by disabling (shutdown) the ports on 1 of the 2 sides (side for which the master has the lowest stack member ID wins, other side will shutdown ports).
This would mean in case of your 9 switches, that in case you have unit 1,2,3 running OK (1 as master) , but there is a split so units 4,5,6,7,8,9 are isolated (with e.g. unit 4 as new master), that this second stack would shutdown all its ports.
When you do not configure MAD, you are accepting that the 2 new logical switches will stay online with the same MAC, causing all kinds of tricky network situations.
For the 5500, the default behavior when the master changes, is that after 6 minutes, the stack will assign itself a new MAC address, so then the 2 logical switches will appear as unique devices again (no config needed for this).
Altough this seems like a fix, it is not: you typically have some link-agg with LACP configured from the edge stack to the Core. Assume you have 2 uplinks from the stack, from member1 to the core and from member 5 to the core.
When all is fine, there is 1 logical switch (of 9 physical units), which operates with the same MAC, so the Core will see the same LACP ID on the link-agg to the edge.
When there is a split stack (assume 1/2/3 vs 4/5/6/7/8/9), for 6 minutes each of the 2 new logical edge switches is still using the same original MAC, so the Core will see no change. Realize however that the core will "loadbalance" traffic over the 2 links, so it could send data intended for a client which is connected to unit1 over the link to unit 5. Since the stack is broken, unit5 cannot reach unit 1 anymore, so the packet is dropped. In other words, in these 6 minutes, you have to be "lucky" for your traffic to get to the correct host.
After 6 minutes, the situation changes: since the logical switch with 4/5/6/7/8/9 will assign itself a new MAC, this new MAC will be reported over LACP.
The core will now see that there are different LACP-IDs (based on the MAC) on the link-agg, and it will block (in software) all in/out traffic of 1 of the links, ensuring the network is stable again.
The link to the switch with the lowest LACP-ID will be preserved, so it is unpredictable which one will remain connected.
The net result is that e.g. users on 1/2/3 will remain online, while users on 4/5/6/7/8/9 will have a physical link on their pc, but the uplink is blocked, so they have no access to the core.
I sometimes refer to this as implicit MAD, since it has the same result as MAD, without configuring MAD.
This config can be verified with dis irf (mac persistent option).
So with or without MAD : a broken stack cannot be automatically fixed !
Hope this helps understanding the situation,
Best regards,Peter.