You should configure the ports first, otherwise you'd have spanning tree will down a port or you forgot about spanning tree and you have a loop.
Or down the ports, cable them, configure, then enable them.
I don't know if the 6450 has any automatic stacking; the 7450's I deployed at work automatically stacked using the 40G ports, but I connected them in a linear stack first, waited for it to configure, then closed the loop.
Here're some config snippets from my 7450 at home using both 40G ports as a LACP bundle to my Arista core:
Code:
lag 40G-Core dynamic id 4
ports ethe 1/3/1 ethe 1/4/1
port-name 40G-Core:E35 ethernet 1/3/1
port-name 40G-Core:E36 ethernet 1/4/1
!
...
interface lag 4
spanning-tree 802-1w admin-pt2pt-mac
!
VLANs are assigned to it the same as any other port, referencing the LAG port (in this case lg4 for me) rather than the physical ports.
Here's the Arista side, which is more or less Cisco syntax:
Code:
interface Port-Channel1
description 1G Core LAG
switchport trunk allowed vlan <range>
switchport mode trunk
port-channel min-links 1
!
...
interface Ethernet35
description To Core:1/3/1
switchport mode trunk
channel-group 1 mode active
!
interface Ethernet36
description To Core:1/4/1
switchport mode trunk
channel-group 1 mode active
!