Hello, I'm having a problem with some of my switchs, I'll give an example with only two switchs but this is happening in some other switchs also:
I have switchA:
#show version
Cisco IOS Software, C3750 Software (C3750-IPBASE-M), Version 12.2(35)SE5, RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2007 by Cisco Systems, Inc.
Compiled Thu 19-Jul-07 19:15 by nachen
Image text-base: 0x00003000, data-base: 0x01080000
and SwitchB:
#show version
Cisco IOS Software, C3750 Software (C3750-IPBASE-M), Version 12.2(35)SE5, RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2007 by Cisco Systems, Inc.
Compiled Thu 19-Jul-07 19:15 by nachen
Image text-base: 0x00003000, data-base: 0x01080000
These two switchs are connected with two gigabit ethernet links, like this
SwitchA:Gi2/0/21<->SwitchB:Gi1/0/23 and SwitchA:Gi2/0/22<->SwitchB:GI1/0/24
the configurations of the ports os switchA:
SwitchA#show running-config interface po3
interface Port-channel3
description uplink
switchport trunk encapsulation dot1q
switchport mode trunk
end
SwitchA#show running-config int gi 2/0/21
interface GigabitEthernet2/0/21
description uplink
switchport trunk encapsulation dot1q
switchport mode trunk
channel-group 3 mode desirable
end
SwitchA#show running-config int gi 2/0/22
interface GigabitEthernet2/0/22
description uplink
switchport trunk encapsulation dot1q
switchport mode trunk
channel-group 3 mode desirable
end
Configuration of ports in SwitchB:
SwitchB#show running-config interface po3
interface Port-channel3
description uplink
switchport trunk encapsulation dot1q
switchport mode trunk
end
SwitchB#show running-config interface gi 1/0/23
interface GigabitEthernet1/0/23
description uplink
switchport trunk encapsulation dot1q
switchport mode trunk
channel-group 3 mode desirable
end
SwitchB#show running-config interface gi 1/0/24
interface GigabitEthernet1/0/24
description uplink
switchport trunk encapsulation dot1q
switchport mode trunk
channel-group 3 mode desirable
end
I believe this is a very "simple" configuration, everything is correct with my port-channel configuration right? Am I doing something wrong?
The problem that is happening is that sometimes STP kicks in and says it detects a loop in one of the ports of the port-channel... and disables that port... I thought STP would work at the channel level and not port-level in this case...
Another thing that puzzels me is....in switchA I have several vlans, and links to other switchs, but in SwitchB, at the moment I have nothing but the two links to SwitchA, no other cables are physically connected to that switch...how is it possible to detect a loop?
Here is an example of an STP blocking message:
#show logging
1w1d: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan900, changed state to up
1w1d: %ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on GigabitEthernet1/0/23.
1w1d: %PM-4-ERR_DISABLE: loopback error detected on Gi1/0/23, putting Gi1/0/23 in err-disable state
1w1d: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/23, changed state to down
1w1d: %LINK-3-UPDOWN: Interface GigabitEthernet1/0/23, changed state to down
Why is this happening??
Thank you for your help!
just for reference I did these commands:
SwitchA#show run | i spanning
spanning-tree mode rapid-pvst
spanning-tree loopguard default
spanning-tree extend system-id
SwitchB#show run | i spanning
spanning-tree mode pvst
spanning-tree extend system-id
