Jul 3 2008
Lighting up a layer 2 trunk
I’ve seen this a number of times. A user lights up a trunk and doesn’t restrict which VLANs can use the trunk, so Cisco IOS obviously allows all VLANs and this causes spanning tree to send a whole load of BPDUs to the remote device and if it has also been configured in a similar way at the remote end then spanning tree will then have to reconverge. At this point you’ll probably have been kicked off the box and you’ll be sweating prefuously. Hmm, not a good way to bring a trunk live.
The right way.
When you configure the port, make sure it is shut down. Only bring it live (no shut) when you have configured both sides of the link and are sure that both sides have a compatible setup. A good rule of thumb is to disallow ALL VLANs when you bring the trunk up. Once the trunk is up, you can easily add more VLANs as and when required.
Heres a copy of a possible configuration you could use before you unshut the port.
interface GigabitEthernet1/1
switchport
switchport trunk encapsulation dot1q
switchport trunk allowed vlan none
switchport mode trunk
