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New Member

set port to 802.1q brings down switch

We're setting up a separate vlan for guest wireless access. When I configured the uplink port on a 2950 (to connect to our core 4006) as a trunk port to pass the "new" vlan traffic, it brought the switch down. I confirmed that the port on the 4006 it connects to is also set to trunk.

Can someone tell me why when I set the 2950 uplink port I lose connectivity?


Re: set port to 802.1q brings down switch

I assume from what you have posted the 2950 switch didn't actually reboot or lock up, what happened is you lost management connectivity to it (and possibly the attached workstations)? This sounds like you were configuring the switch remotely (telnet, SSH) and you lost your connection. You should be able to do this remotely but you would need to make the configuration changes in the right order. The issue was probably because the Native VLAN wasn't configured to be same as the access VLAN and this is what you were managing the switch from.

Assuming your uplink is just configured as an access port:

interface GigabitEthernet0/1

switchport access vlan 10

switchport mode access

And you want to convert this to a trunk to pass two VLANs - 10 & 20. You would need to first set the Native VLAN to 10 so the management is still available, and then configure the port as a trunk. Depending on the topology there might be a delay as STP converges but that should work. I would also prune off the VLANs that aren't needed with the 'switchport trunk allowed vlan' command.

This isn't best practise however and you shouldn't use the Native VLAN on a trunk for data.




Re: set port to 802.1q brings down switch

My guess is maybe the native vlans do not match up on both sides . If the port on the 4006 was set as access before say vlan 20 and you did not specify on the 2950 that vlan 20 was the native then you broke connectivity because by default the native would have then been vlan 1 on the 2950. Make sure the native vlan matches on both sides.

Sorry Andy guess I should read all the posts before replying , basically the same answer .

New Member

Re: set port to 802.1q brings down switch

Native vlan is 1 as well as mgmt on all switches. I wonder if Andy was correct, though, when he mentioned STP. Is there a connectivity loss when a new vlan is added or when a port is trunked?


Re: set port to 802.1q brings down switch


You have mentioned 802.1.q, I know 2950 use .1q not sure what 4006 use by default. Are You shure it is .1q .

If it's isl then the ports will come up but no traffic will flow.


New Member

Re: set port to 802.1q brings down switch

I took the 2950 off-line (removed all clients from the switch) and re-did everything. This time it came up after about 10 seconds.

As a previous poster stated, it was prolly Spanning Tree that caused the delay.

thanks everybody for comments.