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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

But can anyone explain to me if the no keepalive isn't a problem? Shouldn't I have this enabled?

https://supportforums.cisco.com/thread/22739

From post on internet: "Keepalives are used on the routers interfaces as hello mechanism to check the end to end connectivity to the other end.Routers interface used this mechanism to check the interface status.If you have no keepalive command its means that inerface status check mechansim in disabled and router will not transmit any keepalive packet on the link."

I have never had a need for that command, so some others might weigh in. Curious if it really went down on a port channel how the port channel would react because the IOS thinks its up/up....I am smelling weekend lab

pjmonline
Beginner

Are these switches stacked? One switch has interfaces gi 1/0/22 and the other switch has gi 2/0/23. If they are stacked then there is no need for uplinks cause the switch is already connected via the stacking cable.

Sent from Cisco Technical Support iPhone App

Hello, yes it is true that it says 2/0/X and in the other 1/0/X but they are not stacket.... switch A was once in the past stacked and at the time that I made the copy paste to this forum that switch still thought it was switch number 2, however when I upgraded the IOS's I renumbered the switch to correct this issue.

Bottom line, no stack between them

I confess I don'tfeel confortable with having to remove the keep alives.... I believe it will bring problems sooner or later right? I'm going to try and re-enable them to see if the same behavior happens again.

pjmonline
Beginner

took a look at my switch config and I have channel-group 1 mode active on both interfaces in my port channel. If I do a sh int po1 then the bandwidth for the port channel is 2gig. I have not disabled keep alive and have no stp issues.

The switches are 3750.

Sent from Cisco Technical Support iPhone App

Hello, since I wasn't convinced of the keep alives problem...I wen't back this morning and enabled the keepalives again...so far everything is working fine.... the only thing I changed at the same time I disabled the keepalives was to change the spanningtree from RPVST to PVST...could this be what solved the problem? I'm going to leave this like this until monday....on monday I'm going to enable in SwitchA RPVST again to see if the problem returns...

I would like to isolate this problem...don't feel very confortable having it just solved and not understanding it...

Hello, I wasn't convinced about the keepalives... I believe they are important... I left this working with the keepalives on all weekend and part of today and no problems happened, so I decided to do a last test.... lets put the SwitchA in portfast again.... in lesse than 5 minutes all problems returned....

So...the problem is having one switch in port-fast and the other in normal STP right? I confess I don't know exactly why...but cisco also never recommends having port-fast in uplinks so that was something wrong on my side.

Thank you all for you help