External Transparent Bridge between two VLANs

Good Morning,

I have recently run into a problem when trying to recable our network to utilize a managed catalyst switch as a distribution switch. Our current network looks like this:

WAN Link -> Catalyst (VLAN 2) -> Router -> Catalyst (VLAN 3) ->

Transparent Bridge with Firewall -> Dumb Switch -> Rest of switches

Basically The WAN and External Router Card are on one VLAN on the switch, the Internal Router Card and External Transparent Bridge port are on another, and then the Internal port of the bridge goes to a 24 port unmanaged switch that then feeds the rest of our access switches.

The plan is to use another VLAN on the catalyst to distribute to the rest of the switches. The final goal would have the Internal port on the Bridge going to the catalyst on VLAN 4, and the rest of the switches all linked to over VLAN 4 as well.

So I tried that, and this is what happened... The external interface of the bridge was in VLAN 3 on port 8, the internal interface was in VLAN 4 on port 15. For approximately 1 minute the network stayed up with no problem, then suddenly we lost external access, but communication was still working between racks. Looking on the switch it showed the two firewall ports to be up, but they had dropped out of their VLANs. Running "sh vlan brief" showed that they did not exist in any VLAN at all. Im guessing that the switch assumed there was a network loop, I need to know how to tell it to ignore that and continue on as usual...

Any ideas? Has anyone gotten something like this to work?

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