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Trunking and Port Channels


Basic Trunk Interface: 


!
interface GigabitEthernet2/0/24
 description Uplink: uplink to something special
 switchport access vlan 3
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 2
 switchport trunk allowed vlan 3,50-53
 switchport mode trunk
 logging event trunk-status
 spanning-tree cost 10
!

Where switchport trunk allowed specifies which vlans are allowed over the trunk, and spanning-tree cost 10 hard codes the cost of the link.  (if you have a redundant link from a different switch, set it to 20, and it will always be the blocked link.)


Port Channel Interface: 


Define the port channel interface
interface Port-channel5
 description Uplink: access-switch-1
 switchport access vlan 2
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 2
 switchport trunk allowed vlan 249,501-503,599,753,754
 switchport mode trunk
 logging event trunk-status
 spanning-tree cost 10
!

The vlans allowed MUST be the exact same for both.  When modifying, change the interfaces before the port channel, otherwise the link will fail.  

Define the uplink ports: 
interface GigabitEthernet1/0/17
 description Uplink: access-switch-1 port g0/1
 switchport access vlan 2
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 2
 switchport trunk allowed vlan 249,501-503,599,753,754
 switchport mode trunk
 logging event trunk-status
 carrier-delay msec 0
 no keepalive
 channel-group 5 mode active
!
interface GigabitEthernet2/0/17
 description Uplink: access-switch-1 port g0/2
 switchport access vlan 2
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 2
 switchport trunk allowed vlan 249,501-503,599,753,754
 switchport mode trunk
 logging event trunk-status
 carrier-delay msec 0
 no keepalive
 channel-group 5 mode active

Note that the channel-group setting can either be on or active.  if you are ever having problems setting up the channel, set this to on.  



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