STP Priority Configuration with Redundant Core Switches

Unanswered Question
Sep 13th, 2007

Hi,

I just started managing network that has 2 Core Switches connecting various Distribution Layer switches etc.

The core switches are also doing Inter-VLAN Routing and configured with Active\Standby VLAN interfaces with SW-1 having ALL the Active interfaces and configured for preempt.

However in the spanning tree configuration SW-1 and SW-2 are configured with the same bridge priority (8192) for ALL VLANs. These result is that SW-2 is now the Root for majority of the VLANs.

Does this effect network performance considering that SW-1 is doing ALL the routing while SW-2 is Root for all those VLANs ?

Does it make sense to configure the Bridge priority on SW-2 to 16384 for All VLANs so that SW-1 is the Root ?

Could there be a good reason to configure same Bridge Priority on both Core switches ?

Thanks,

Naman

I have this problem too.
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Edison Ortiz Thu, 09/13/2007 - 07:51

If you have a lot of Vlans, split the load. Depends on the numbering scheme, I usually have SW1 handling the odd-numbered Vlans while SW2 handling the even-numbered Vlans.

Jon Marshall Thu, 09/13/2007 - 08:50

Hi Naman

As Edison said you can split the vlans for STP across both switches which is failry common. You may also want to then match those up with your HSRP active gateways ie.

Odd vlans

---------

Priority 8192 on sw1

Priority 16384 on sw2

HSRP active gateway on sw1

Even vlans

----------

Priority 8192 on sw2

Priority 16384 on sw1

HSRP active gateway on sw2

HTH

Jon

lgijssel Thu, 09/13/2007 - 09:15

Cisco had a theory about this:

http://www.cisco.com/en/US/netsol/ns340/ns394/ns74/ns149/networking_solutions_white_paper09186a00800a3e16.shtml

I do not know if they still stick to it but it goes like this:

In a dual-core environment like yours, you can divide the load by setting up HSRP to be active on half the valns (lets say even vlans) on one side and the odd ones on the other.

The STP root should be there where the HSRP-active interface resides, hence also on a 50-50 basis.

As a genuine cisco-adept I have in fact built several networks according to these guidelines. What happened then was that users (administrators!) found this environment complex to manage. Especially troubleshooting was found complex due to the split STP.

After giving it some thought, it was decided to modify the system in such a way that one switch has all HSRP-active nodes AND is the stp root for all vlans. In the real world this setup performs just as good as the design with load balancing. My own impression is that it is perhaps even a bit more stable and it has less traffic over the backbonelink interconnecting the switches.

Perhaps the networks that I have done so far are simply too small? ;-)

A well known person has said it all in just one sentence: Everything is relative.

regards,

Leo

mnlatif Thu, 09/13/2007 - 10:35

Thanks ALL for very informative and helpful replies.

I think that this network was also designed for simplicity due to no technical staff available, perhaps that is why they choose to have one Switch for HSRP-Active and Root as Leo said above.

However i guess it was a mistake to have the same bridge priority on both switches though. I should be able to fix that..

Thanks again

\\ Naman

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