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nibauramos
Beginner

Port-channel and STP problem

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

22 REPLIES 22

Glad its sorted for you.  I replicated a similar issue today all be it on some old test kit (3550 switches).  Did you do a show spanning-tree?  As I got the %ETHCNTR-3-LOOP_BACK_DETECTED when I turned spaning tree off on the native VLAN with some ports in LACP mode.


Cheers