12-20-2007 06:28 AM - edited 03-05-2019 08:06 PM
I am wondering what would occur if two 3750G switch stacks were connected together via two pairs of SFP ports.
The ports were configured identically as VLAN Trunks with the same vlans allowed on all trunks.
In addition if the ports are made members of a single Etherchannel group 1 what happens if one port of the four has the "channel-group 1 mode on" command missing from its config.
Would the switches see this as two links instead of the one logical etherchannel group.
Would this cause the vlans allowed on the trunk to flap. ( Loop Condition)
I encountered this scenario recently when
one swith lost power and on reboot its running cfg had not been saved this caused the one port to not have the channel mode on conmmand.
I believe this may have caused a loop condition but I am not 100% sure.
Anyone know for sure ?
Relevant config info
port-channel load-balance src-dst-mac
no file verify auto
!
spanning-tree mode pvst
spanning-tree portfast bpduguard default
no spanning-tree optimize bpdu transmission
spanning-tree extend system-id
spanning-tree uplinkfast
.......
interface Port-channel1
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 1,291,297,300-302,993
switchport mode trunk
..........
Misconfigured port######
interface GigabitEthernet1/0/25
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 1,291,297,300-302,993
switchport mode trunk
srr-queue bandwidth share 10 10 60 20
srr-queue bandwidth shape 10 0 0 0
queue-set 2
mls qos trust cos
auto qos voip trust
########
Three correctly configured ports######
interface GigabitEthernet2/0/25
switchport trunk encapsulation dot1q
switchport trunk allowed vlan 1,291,297,300-302,993
switchport mode trunk
srr-queue bandwidth share 10 10 60 20
srr-queue bandwidth shape 10 0 0 0
queue-set 2
mls qos trust cos
auto qos voip trust
channel-group 1 mode on
##################################
12-21-2007 02:29 PM
The switchports would go into err-disabled. It's the same concept as plugging a copper cable from F0/1 to F0/2 on the same switch.
The stack cable converts 2 switches into one logical hardware.
12-27-2007 05:55 AM
The wording of you reply does not make it clear which ports of the four ports involved would go err-disabled. The misconfigured port ? The misconfigured port plus the one that it is directly "fiber connected" to ? or all four.
This flies in the face of what I encountered.
The two directly connected pairs of SFP ports behaved as follows:
The properly configured directly connected pair stayed up.
The pair with one end port improperly configured did not participate in etherchannel but also stayed up.
The vlans on the trunks flapped.
I don't understand why ?
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