I have a 3560 8 port switch. Int gi0/9 is trunked to another switch downstream. When I try to configure int gi0/10 to trunk to a switch upstream the interface on the switch goes down and I have to either reboot the switch or plug directly into the switch and telnet into it to turn off trunking on the interface. When I configure trunking on the interface on the upstream switch that connects to this interface the same happens on that switch. The upstream switch is a 3750 with 12 sfp ports. Several interfaces are trunking to other switches from this switch. Spanning tree is not configured on the 3750 at all , and is not configured on either gi0/10 or gi0/9 on the 3560. I was consoled into the 3560 during a reboot after the interface went down, a message came up that said something like "Spanning Tree returning gigabit ethernet 10 to constant state" Why would I get this message if spanning tree is not enabled on the gig ports on either end of the trunk? There is no loop to require spanning tree to shut down an interface. I have several other 3560's configured as I would like to configure this switch and they are trunking without issue.
When I hit enter after the switchport mode trunk command the link goes down and stays down until i reboot the switch or plug directly into the switch, telnet in and do a no switchport mode trunk to bring the link back up, the console port is no longer accessible when the switch is in this state, this happens on the interface on both ends of the link.
I am not going to put spanning tree on a trunk port. On all the other switches I have configured the link is back up in about 30 seconds after changing to a trunk port. These two ports just don't want to play nice.
I would open a case with Cisco TAC but am still waiting for my VAR to provide me the the info I need to get support.
How are the trunk ports setup now ? I don't see any trunking parameters in your post . If they are setup as "switchport mode dynamic desirable" then the vtp domain names must match on each end or the trunk will go down . If using anything other than vlan 1 as the native make sure they match on each end . Can you post your port configs for each end ?
At the moment we don't have a VTP domain configured. The ports are currently access ports. I configured trunking on about 30 switches in my environment and this is the only link that has been a problem.
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