Technology Forum

Recently I got 'bitten' during a supposedly common configuration/setup with our Cisco Catalyst C2950. We fixed the problem but did not know what's going on, so we don't know how to avoid it in the future.Background Story:Cisco Catalyst C2950 running with some VLANs already set, VTP as Server, Trunk to another C2950. Everything running as it should (VLAN and all).Planned to connect a Trunk line to Cisco Catalyst Express 500.Cable connection between CE500 to C2950 is set, CE500 port was put as SwitchUplink (to accept dot1q Trunk). C2950 port is still at "switchport mode access", so Trunking is not running yet. All LED are green.The problem:At C2950, for the specified port, I typed "switchport mode trunk".Some seconds later (maybe 10-30 seconds?) all ports went amber, and all connections went dead. Thinking that I caused the problem, I tried to reverse what I did "switchport mode access".After waiting a long time, the LEDs are still amber. Because this is a production Switch and we need active connection ASAP, we disconnected the connection between C2950 and CE500, and restart the C2950.After restart, the LEDs are still amber.Checking "show running-config" and comparing with the documented configuration showed that everything is exactly the same. However, checking "show vlan" showed that all ports were not assigned to any VLAN.The emergency fix:Let's say I am on port 3, which supposed to be in VLAN 2.The "show running-config" for port fa0/3 showed that it is indeed "switchport mode access" and "switchport access vlan 2".But "show VLAN" shows that there is no "vlan 2"So, I typed "switchport access vlan 2" (just copying what already there in "show running-config") and the Console returned with "Access VLAN does not exists. Creating vlan 2". With this done, the LED on port 3 (and others belonging to VLAN 2) changed to green.Repeated for the rest of unregistered VLAN fixed the problem, everything run ok as before.My questionoes anyone know why the VLAN database went missing?We discussed this with our local 'expert', and he also as stumped as everyone else.Thank you for any insight to the problem.