Jul 3 2008
Etherchannel configurations – woops added the VLAN to the physical port.
Adding a VLAN to an Etherchannel
How many times have you heard that someone added a VLAN to a phyiscal layer2 interface instead of the logical layer2 port-channel and subsequently brought the etherchannel down. If your lucky you might get away with it. I’ve seen engineers take down entire data centers as well as call centers by issuing this command on the wrong interface.
For those of you who havent made this mistake, here is how to avoid it:-
Task: add vlan 20 to etherchannel 1.
The right way
conf t
interface port-channel 1
switchport trunk allowed vlan add 20
end
The wrong way
Do not add the VLAN to a port which is a member of the etherchannel.
ie
The command below will show the ports which are members of the etherchannel.
show etherchannel 1 summary
!
!
text removed
!
group port-channel protocol ports
————————————–
1 po1(SU) - Gi5/3(P) Gi5/4(P)
If you then look at the physical port, you can see it is a member of port-channel 1.
show run interface gi5/3
switchport
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 10-19
switchport mode trunk
no ip address
channel-group 1 mode on
The last line ie channel-group 1 mode on indicates that this port is part of etherchannel 1. If you try to add a VLAN to the etherchannel by adding the VLAN to the physical port, this port will be removed from the etherchannel and this will cause spanning tree to throw a wobbly.
Be warned:-)
