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New Member

Spanning-tree cost or port-priority command

I have a 6500 switch connected to a 3560.  I have the 6500 as a vtp server and the 3560 as a vtp client in the same vtp domain.  1 of the links is a gigabit  L2 trunk link with all vlans allowed.  The second link is also a gigabit non trunk link that is in only 1 particular vlan on both ends.  I have 1 server connected to the 6509, and 1 server connected to the 3560 both on the same vlan as this second link.  If I want to force the servers to communicate via this second link only, do I need to modify the spanning-tree cost, or spanning-tree port priority to make sure these servers only communicate to each other via this second gigabit link?

Thanks.

12 REPLIES
Cisco Employee

Re: Spanning-tree cost or port-priority command

Hi David,

I assume that the 6500 is the root switch for all VLANs. In that case, you can make the non-trunk link to become forwarding for the server VLAN

  • either by lowering the port priority on the 6500 switch for that particular VLAN,
  • or by lowering the port cost on the 3560 switch for that particular VLAN

If the root bridge is the 3560 then the changes would be reversed.

Best regards,

Peter

New Member

Re: Spanning-tree cost or port-priority command

Yes, the 6500 is the root bridge for all vlans.  So do I need to one make one one of the changes on either switch that you mentioned below, or do I make it on both?  For example, this config on the 6500

interface GigabitEthernet4/24
description server

no ip address
switchport
switchport access vlan 210
spanning-tree vlan 210 port-priority 16

?

thanks.

Cisco Employee

Re: Spanning-tree cost or port-priority command

Hi David,

You are correct. From both alternatives I have suggested, it is sufficient to implement only one, not both. The configuration you have suggested seems to be OK and I would say it should work nicely. Just configure the port on your 6500 as you have suggested (by lowering the port priority) and leave the 3560 at its defaults.

Best regards,

Peter

Hall of Fame Super Gold

Re: Spanning-tree cost or port-priority command

If you want servers to go through a dedicated L2 Trunk link how about configuring the 1st Trunk as allow all VLANS except the server VLANs and the 2nd Trunk link as deny all VLANs except the server VLAN?

Cisco Employee

Re: Spanning-tree cost or port-priority command

Hi Leo,

Your solution would certainly work but David would lose the redundancy if he pruned the server VLAN from the trunk link.

Best regards,

Peter

Hall of Fame Super Blue

Re: Spanning-tree cost or port-priority command

Oops, sorry Peter, i didn't see this response before i posted. Just ignore me

Cisco Employee

Re: Spanning-tree cost or port-priority command

Jon,

You are always welcome and you should never, ever, be ignored! I am glad you responded.

Best regards,

Peter

Hall of Fame Super Gold

Re: Spanning-tree cost or port-priority command

Hi Peter,

Thanks for the critique but the thread states "I want to force the servers to communicate via this second link only" thus my suggestion.

Cisco Employee

Re: Spanning-tree cost or port-priority command

Hi Leo,

Oh, yes, you're right I just snapped onto the STP issue automatically.

Best regards,

Peter

New Member

Re: Spanning-tree cost or port-priority command

I actually did consider this, but it seems a bit of a hassle on the configuration since it seems the  interface trunk command only has an allow statement, and not a deny one:

6509(config-if)#switchport trunk ?
  allowed        Set allowed VLAN characteristics when interface is in trunking
                 mode
  encapsulation  Set trunking encapsulation when interface is in trunking mode
  native         Set trunking native characteristics when interface is in
                 trunking mode
  pruning        Set pruning VLAN characteristics when interface is in trunking
                 mode

or is there an easier way?

thanks.

Hall of Fame Super Blue

Re: Spanning-tree cost or port-priority command

Hi all

Modifying the STP costs would be a better solution simply because if you use the allowed vlan command and the link fails then you can't use the other link to carry on sending traffic. So you are removing redundancy from your network which seems a bit counterintuitive as you are running STP anyway

Jon

Cisco Employee

Re: Spanning-tree cost or port-priority command

David,

The command switchport trunk allowed vlan provides also the exclusion option: switchport trunk allowed vlan except XXX where XXX is the VLAN you want to disallow on the trunk. But as Jon has pointed out, this is an inferior solution as it removes the redundancy from your network.

Best regards,

Peter

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