Spanning-Tree issue

[pcristina]

Hi,

I’m having an issue with spanning tree protocol. All my network is configured with 3750 switches. I have,

. Two core switches

. Seven stacks as access switches

All stacks are connected with two fiber cables to the main core switches. One fiber to primary core and another to secondary.

What I want to do is configure spanning tree between access switches to core. But not using STP over vlan (spanning-tree vlan vlan_no). I want to configure it as, for example, if a fiber cable from one stack fails to automatically use the other.

Which commands should I use in Core switches, edge switches, uplinks ?

I have attached uplinks config.

Thank you !

[Jon Marshall]

Pablo

Are you 2 core switches running as a stack or separate switches ?

If they are also stacked then why not simply make the 2 links from each access switch an etherchannel trunk link then STP will see it as one link and use both. In fact if the core switches are stacked they are seen as one logical switch so i'm not sure you could favour one link over another although others may be able to add to this.

If the core switches are separate swirtches then simply make one core switch STP root for all vlans and the other secondary. You could also consider using MST.

Jon

[pcristina]

Hi Jon,

Thank you very much for your reply. Core switches are connected via portchannel.

Switch#sh run int po1

Building configuration...

Current configuration : 182 bytes

!

interface Port-channel1

description EtherChannel to Secondary-Core

switchport trunk encapsulation dot1q

switchport trunk pruning vlan none

switchport mode trunk

end

Should I set one as root? Can you send me an example?

Thank you

[Beetlejuice01]

Look at this documen about flex link, it seem's to be what you want, but if your core is one the root and the other is secondary the link on the acces one si designated and the other is in altn/block, so one is working, and when the link fail it use the other link. From global config on core 1 use :spanning-tree vlan vlan_id root primary. Substitute vlan_id with the number of vlan or range 1-4095.

[Jon Marshall]

Pablo

you can use either the command Fabio sent ie.

spanning-tree vlan root primary  on the root switch and

spanning-tree vlan root secondary on the secondry switch ie. the other core switch

or

spanning-tree vlan priority 8192 on root switch and

spanning-tree vlan priority 16384 on the the secondary switch

both will achieve the same thing providing all switches currently have default STP values, the only difference being the first set of commands are a one off calculation ie. if you happened to add another switch with a lower STP priority by mistake then it would not be recalculated.

As Fabio also mentioned one link will block.

An alternative would be to make the 2 core switches a stack and then run etherchannel from the access links to the core switches, spreading the 2 links across both switches. This way STP would not be an issue (although you should still run it and set STP prorities) and you would get the full bandwidth of both links.

Jon

[pcristina]

Hi Fabio, Jon,

Thank you both for the quick reply ! ••J

Fabio, I think you forgot to attach the document.

Jon, you say that if I add those commands to root and secondary switch STP will automatically work? Should I apply any config on access switches?

Isn’t there a way to make redundancy between access and core not using the “root” thing? I just want to, if access switch primary fiber cable fails to use the other one.

Thank you !

[Jon Marshall]

Pablo

You should be fine with the default config on the access switches although i would recommend running rapid pvst+ which you may need to enable on all switches.

Not sure why you are reluctant to use the root commands, they are simply 2 lines of config and thats it, you have redundant links. Be aware though that you need to do this out of hours or schedule an outage. It will be a very short outage but STP will very probably reconverge and this will disrupt traffic. A 10 minute outage should be fine.

Alternatively have a look at the flexlink doc posted by Fabio if you want a different solution.

Jon

[Beetlejuice01]

Hi Jon, don't you think that can be usefull to set priority to 0 on the first, to avoid someone put a switch with a priority lower than 8192 or a different vendor switch with a default priority value that can be lower?

To Pablo... you are right :-) see this :

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

[Jon Marshall]

Yes, you could indeed set the STP priority to 0.

Jon

[pcristina]

Jon, Fabio, again, thanks a lot for your help.

I will go for the STP over vlan testing this Friday and use flex link as backup.

On core switches I will use,

spanning-tree vlan priority 8192 on root switch and

spanning-tree vlan priority 16384 on the the secondary switch

What about uplinks in Core swithes? Should I leave them as they are?

And in access switches? Should I paste some command? And in uplinks to core?

Thank you !

[Jon Marshall]

Pablo

You only need those comands, there is nothing you need to do with the uplinks or the access switches.

Jon

[pcristina]

Jon, Fabio,

Thank you both for your replies

I have scheduled a Request for change and will test this on Friday.

I will let you know the results.

Thanks again

[Beetlejuice01]

Hi Pablo,

what about the change? It's worked?

[pcristina]

Hi Jon, Fabio,

I applied the changes on Friday and It Worked !!!!!!!

Thank you very much both for your help on this !!!