Sorry, but some general STP questions

Unanswered Question
May 28th, 2008

We have two core 6509 switches at our main site.

All users and servers (with the exception of a training room that has only one path to only one of the core switches) uplink directly to either one switch or the other.

The two switches have a trunk between them on the SUP.

We are implementing some redundant paths on the servers via teamed NICs, but there are no access switchs to deal with.

I have some general questions about STP:

In the situation above, is STP really an issue?

I saw in an earlier post that the layer 2 and layer 3 path should be the same.

How can I determine my traffic path for Layer 2?

We have PVST configured, and switch 1 has the below in it's config:

spanning-tree vlan 1-2,4-8,10-12,14-15,26,28-30,35,40,50,55,70 priority 16384

But I see no priority configured in switch 2

When I do a "sh spanning-tree root" on switch 2, I see the the root port as the trunk (to the vlans shown above) as switch 1, which makes sense, but I am not sure what it is accomplishing.

I have this problem too.
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Jon Marshall Wed, 05/28/2008 - 21:40

Richard

STP is not a major issue in your scenario. The teamed servers will only have one NIC forwarding and receiving traffic at any one time. But as you are not adding any switches with redundant paths then you have no loops in your topology.

It is good pratice to explicitly set the STP root and secondary switches even if at the moment you only have 2 switches because you might want to add further switches at a later date. What i normally do is to give the root switch a priority of 8192 and the secondary and priority of 16384.

And i make switch1 root for odd vlans and secondary for even vlans and switch 2 root for even vlans and secondary for odd but in your case this may be of limited use - see below.

If there is no priority on switch 2 then it will be 32768 which means switch 1 is the root for the vlans in the list you give above. So for those vlans you would want to make the sure the active HSRP address is on switch 1 as well. This is what is meant by layer 2 and layer 3 paths should be the same. Note that it will still work if they aren't, just that your trunk link between your switches will have to move more traffic, but then in your case if all users and servers are distributed across both switches you may have a lot of traffic traversing the trunk anyway.

Jon

Not to hijack a thread...but my design is almost exactly the same (two cores with servers plugged in directly to the cores). I've set up some vlans to be root on core1 and other vlans to be root on core2, can you please explain this out put to me?

core 1:

spanning-tree vlan 15 priority 16384

spanning-tree vlan 16,206 priority 8192

core2:

spanning-tree vlan 16,206 priority 16384

spanning-tree vlan 15 priority 8192

When I go to core 1 and do show spanning-tree vlan 15 I get this:

VLAN0206

Spanning tree enabled protocol ieee

Root ID Priority 8192

Address 0019.0761.44ce

This bridge is the root

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 8192

Address 0019.0761.44ce

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 300

core2:

VLAN0206

Spanning tree enabled protocol ieee

Root ID Priority 8192

Address 0019.0761.44ce

Cost 3

Port 1667 (Port-channel6)

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 16384

Address 0019.0780.34ce

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Aging Time 300

Shouldn't the bridge on core 1 have a priority of 16384?

Jon Marshall Thu, 05/29/2008 - 17:38

Mike

I am a little confused as you talk about vlan 15 but your output is for vlan 206 and that output is correct ie. core1 has priority of 8192 and core2 has priority of 16384.

Am i missing something ?

Jon

Jon Marshall Thu, 05/29/2008 - 17:53

No problem

Root ID = the mac-address of the root bridge

Bridge ID = the mac-address of the switch you are on.

So if the the root id = the bridge ID you know that this switch is the STP root for that particular vlan.

Jon

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