Spanning tree help

Unanswered Question
Jul 6th, 2010

Hello,

We have 4 switches in a spanning tree, each switch is connected to each other (circle topology) however all trunks show as up and connected, shouldn't one of the trunk ports show as err-disabled or down as it has been blocked during the the spanning tree convergence?

I'm pretty sure I know what port should be in blicking mode how do I find out?

Thanks

I have this problem too.
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Giuseppe Larosa Tue, 07/06/2010 - 09:03

Hello Andy,

what you see is correct: STP does not put the port in error disable but on one side the port will be in STP blocking state meaning not used for user traffic.

Should a link fail the currently blocking port could move to an STP forwarding state if necessary

Edit:

use show spanning-tree commands to see this

show spanning-tree vlan van_number

show spanning-tree interface type X/Y

Hope to help

Giuseppe

Andy White Tue, 07/06/2010 - 09:36

Hi,

What makes a port err-disable?

What command will show the blocked port due to the spanning tree, I'm not near a switch, but is it one of the show trunk commands?

Thanks

Nagaraja Thanthry Tue, 07/06/2010 - 09:45

errdisable could be due to various reasons including interface flaps and security violations. If you want to verify your spanning-tree working, please issue the command "show spanning-tree vlan ". This will show you individual interface status. Hope this helps.

Regards,

NT

Giuseppe Larosa Tue, 07/06/2010 - 12:25

Hello Andy,

use show spanning-tree commands to see this

show spanning-tree vlan van_number

show spanning-tree interface type X/Y

there are some possible reasons for error disable the only one related to STP is the use of spanning-tree bpduguard on user ports that will react to the event of STP message received on the port by putting the port in error disable state.

You can recover a port in error-disable state using shut / no shut of the port

Hope to help

Giuseppe

Andy White Tue, 07/06/2010 - 12:47

Hi,

Thanks those commands worked great! I see that port 12 is blocked, what does edge P2p mean:

Interface Role Sts Cost Prio.Nbr Type

Giuseppe Larosa Tue, 07/06/2010 - 12:54

Hello Andy,

read the following

! rapid STP explanation:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SX/configuration/guide/spantree.html

show command output has been changed using terminology of newer 802.1W rapid spanning-tree even if you are using default PVST+ mode

Hope to help

Giuseppe

chireadorin Tue, 07/06/2010 - 14:49

Hi,

   Ports between trunks should not be marked as Edge. Check if there is port filtering on one end of the trunk/access link. If you said that there is a circle on that vlan, the normal behavior for STP with same speed between links, should be:

        R   rp     dp

        sw1 -----  sw2

       dp I              Idp 

       rp I              Iblocked/alternate

          sw4 dp----rp sw3

                

sw1=root bridge for that vlan/CST

Please paste the output of what you see and try to describe the network diagram, maybe this will help.

sh span sum

show span intf

show span vlan

show int status

sh run int

sh logg

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