I need expert help with this scenario. Switch A is CAT 3560E PoE and switch B is CAT 3750 PoE. Connectivity between these swithces are 10gb fiber and is set to trunking so vlan1 can be access on both switches. I have a firewall dzm interface that i would like to bring in to switch A port # 48 and create a vlan 2 dmz there. Then on switch B, port #48, i configure it as a vlan2. Then when I connect my laptop, i could get an IP address from DHCP on the firewall dmz interface. So, looks like trunking between switch A and B carries all vlans. So, i have a Cisco CAT 3500XL that i would like to connect to port #48 on switch B so that i could add more workstations here. However, when i connect CAT 3500xl to port #48 of CAT 3570, port number #48 is disabled. Then, I set it up as trunk port but i am no longer have access to vlan2.
Here is the worse part, while trying to troubleshoot this, suddenly all VoIP phones and connectivities were down. I do not know if me adding the 3500xl would cause this problem but it seems we had a brownout while i was troubleshooting.
Please help since this is something i've never done before.
From what you describe it looks like a spanning tree issue. For how long the network was down? STP re-convergence should not last more than 50 sec otherwise it's a loop. VTP transparent should be set on _all_ the switches unless you using it for global dynamic VLAN management. If after you disconnected the "bad" switch everything went back to normal and you did not have to re-create your vlans on existing switches then it was not a VTP related issue.
On your network you should pick and hardcode the spanning tree root bridge otherwise addition of a switch could cause STP re-convergence and sub-optimal L2 paths. Before connecting your 3500xl check it's STP bridge priorities for all the VLANS and make sure it's not lower than your new root. In this case the addition of this switch should be safe.
hope that helps.
To understand if you are creating loop or not we need to know if your topology has redundant L2 connections between the switches. From what you describe it sounds to me that the switches are just daisy-chained to each other. Can you give us little more information on your network as it's not clear what do you mean you have connectivity down... is the internet connectivity down, or local within the office etc? when the connectivity was lost were you able to type on the console of the switches with no delay? If the spanning tree loop happens the CPU of the switches normally goes 100% so you should notice this at the console.
From what I see in your reply and unless it's needed the VTP mode should be set to transparent to eliminate issues with lost vlans when you plug in swicth with higher revision #. VTP transparent should be set on all of your switches.
It would be helpful if you can attach configurations of the switches to your post so we can analyze more what's going on here.
Just a few questions:-
Which of switch A or B is the spanningtree root?
Have you configured VTP?
Have you configured portfast on the port 48 trunking port on any of the switches?
Have you configured spanningtree BPDUportfast guard/filter anywhere?
I "sounds" like you are creating a switching loop when you connect the 3500XL to port 48 in the switch.
How long did the VoIP phones go down for - as this could have been due to a spanngtree recalculation.