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Spanning tree issue with trunks

Occasional Contributor

Spanning tree issue with trunks

Hi all,

I have a problem in our netwerk with recently installed Procurve switches and was looking for some help.

The situation:
Switch A (5406 ZL)
Switch B (5406 ZL)
Switch C (2610)

Switch A and B are directly connected to eachother with a trunk of 4. Total 4Gb

Switch C is connected to A and B with a 1Gb per link. When switch C was connected spanning tree blocked the link between A and B. This link has a Cost of 10.000 and a priority of 64.

The links from A to C and from B to C have a cost of 20.000 and a priority of 128.

In a test setup with 3 switches (2610) no matter what I do, the trunk will be the active link or become the active link when it was interupted and returns.

What to do/configure?

Re: Spanning tree issue with trunks

Which one is you STP root switch? I guess it is the "Switch C". You should be able to check it with "sh spanning-tree".

The solution is pretty simple: make sure one of your 5406 becomes the root, you should be able to do this with "spanning-tree priority 0"
Tijl van der Steeg
Valued Contributor

Re: Spanning tree issue with trunks

I agree. Costs are only relevant when you have multiple links on one switch, not to set priorities between switches

"sh span" should help you
Occasional Contributor

Re: Spanning tree issue with trunks

I looked at the spanning-tree config on switch C but did not see if it the root.

This the result:
Multiple Spanning Tree (MST) Information

STP Enabled : Yes
Force Version : MSTP-operation
IST Mapped VLANs : 1-4094
Switch MAC Address : 0024a8-07d6c0
Switch Priority : 32768
Max Age : 20
Max Hops : 20
Forward Delay : 15

Topology Change Count : 125
Time Since Last Change : 5 days

CST Root MAC Address : 0008a2-01c8b0
CST Root Priority : 32768
CST Root Path Cost : 200000
CST Root Port : 2

IST Regional Root MAC Address : 0024a8-07d6c0
IST Regional Root Priority : 32768
IST Regional Root Path Cost : 0
IST Remaining Hops : 20

Root Guard Ports :
TCN Guard Ports :
Protected Ports :
Filtered Ports :

Re: Spanning tree issue with trunks

No, it seems that it is not a root. If it is, you will see:

"CST Root Port : This switch is root"

instead of:

"CST Root Port : 2"

But wait. AFAIK port #2 is only capable of 10/100, so it seems that you root switch is connected by this port, so your topology looks probably like this:

(port #2)
/ \
/ \
5406 5406

What device do you have there on that port?
Respected Contributor

Re: Spanning tree issue with trunks


Just to unblock the port between A and B, do as they said.

use this commands in A or B

#spannint-tree instance 0 priority 1

this will make that switch the ROOT and will unblock the trunk link between the 2 switches.

and just for clarification, would you please use #show lldp info remote-device
in switch C, just to check the ROOT MAC address belongs to which device in your network.

Islam Hassan
Occasional Contributor

Re: Spanning tree issue with trunks

I'm looking into what device is on switch C behind port 2. I can't resolve it physically (large factory) so i'm trying to resolve the IP based on the mac. (not yet succesfull)

The "show lldp info remote-device" command returns 2 entries, the link to switch A and the link to switch B. It does not talk about the STP root (port 2).

Re: Spanning tree issue with trunks

OK, if it is an independent network all you need to do (and definitely should!) is to split spanning trees by enabling bpdu filtering on that port:

spanning-tree 2 bpdu-filter

If your switch supports pvst-filter then use this:

spanning-tree 2 bpdu-filter pvst-filter

BTW: you should also consider enabling bpdu-protection/pvst-protection on ports you don't use to connect your switches together.