11-07-2011 11:15 AM - edited 03-07-2019 03:15 AM
Hi experts,
I had an incident yesterday that some guy mistakenly removed a port from the etherchannel. However the err-disable didn't catch it and had both ports running which caused a loop and broadcast storm... I want to know how exactly switch detects the channel-misconfig. I assume that it utilizes SPT? Here is my config:
Core switch:
interface range GigabitEthernet2/0/2,GigabitEthernet1/0/26
switchport trunk encapsulation dot1q
switchport mode trunk
channel-group 12 mode on
interface Port-channel12
switchport trunk encapsulation dot1q
switchport mode trunk
Access switch:
interface GigabitEthernet1/0/1
switchport trunk encapsulation dot1q
switchport mode trunk
no channel-group 12 mode on (deliberately removed)
interface GigabitEthernet1/0/2
switchport trunk encapsulation dot1q
switchport mode trunk
channel-group 12 mode on
interface Port-channel12
switchport trunk encapsulation dot1q
switchport mode trunk
This is actually my test enviroment. I have err-disable enabled on both switches but no auto-recovery. Once I removed the G1/0/1 from the channel-group the errdisable did work and have all the ports blocked. However I just don't know why it didn't work at our client site...
BTW is it recommended to use PAGP/LACP for the L2 etherchannels between the switches or set them to statically "on"?
Thanks!
Difan
11-09-2011 08:47 PM
Hi,
Yeap STP should take place in this case. This is how it works in short:
Not also sure what happened in your client's scenario - possibility that STP loop was very fust and strong and e.g. affected CPU thus err-disable mechanism did not trigger. But it is just a guess.
Nik
11-10-2011 02:34 PM
Thank you Nik! Your explaination makes sense. I think it is good enough to explain to the customer.
So do you always recommend using pagp or lacp and avoid hard coded on at all times?
thanks!
difan
11-10-2011 09:40 PM
Hi
I would say PAGP or LACP give you more features to control the etherchanel negotiation and operation. SO those can be more reliable. However all should be considered based on the support for those feature on the switches yourun etherchannel between.
In this case LACP or PAGP can be more effective though anyway should be tested first on that environment.
Nik
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