Stacked 3500XL's and Spanning-Tree

[dmayne]

We have attempted to setup redundancy in our network by using two 6509's at the core, with 1 MSFC2 each. Each switch has one GBIC connected to each of the 6509's, with a few exceptions. We have a few network districts that require multiple switches in the closet. In this case, we are using Gigastack modules to connect 2 switches.

Switch#1 Gi0/1 - GBIC SX connected to 6509 #1

Gi0/2 - Gigastack connected to Switch#2

Switch#2 Gi0/1 - Gigastack connected to Switch#1

Gi0/2 - GBIC SX connected to 6509 #2

If 6509 #2 is the root bridge, then I would expect that STP would be forwarding on Gi0/2 on Switch #2 and blocking on Gi0/1 Switch #1.

For some reason this is not the case...both GBIC SX ports are in forwarding states and one of the Gigastack ports is in Blocking mode?

When performing a "show log" on these switches, I see a number of add/delete addresses and we experience intermittent network performance issues. Is it possible that this current configuration is causing STP or bridging loops?

Any help would be greatly appreciated.

[milan.kulik]

Are you using Uplinkfast on your 3500s?

If yes, it increases the port cost by 3000 and thats why the Gigastack port is blocking.

Even if not, I suppose there is at least one Gigabit connection between your 6509s, isn't it? If you didn't change any port cost, there are two equal cost paths to root from Switch#1: one through Switch#2 and second through 6509#1.

"The STP path cost default value is derived from the media speed of an interface. In the event of a loop, STP considers cost when selecting an interface to put in the forwarding state. You can assign lower cost values to interfaces that you want selected first and higher cost values that you want selected last. If all interfaces have the same cost value, STP puts the interface with the lowest interface number in the forwarding state and blocks other interfaces."

see http://www.cisco.com/univercd/cc/td/doc/product/lan/cat2950/1216ea2b/scg/swgstp.htm#93510

HTH,

Milan

[dmayne]

Yes we are using Uplinkfast on the 3500's and there are 2 gigabit connections between the 6509's (ports 1/1-2 on the supervisor card)