STP in Cascades Stack

[dknechtenhofer]

Hello,

we are using seven 2950 in a cascade stack. On switch 1 and seven we are connected to a core-network with 1000BaseSX. The switches are not in a cluster managment. ! With default STP parameter the blocked port is now on switch 1 on Gigastack port, but I like to have the blocked port for example on switch 4 on the Gigastack port ! The root bridge is not in the cascade !

I configured also with different cost on the ports, but the blocked port is always in switch 1!

My question: Can I move the STP blocked port within a cascade stack, or is that not possible ?

Thanks David

[lgijssel · Accepted solution]

It is not possible to block a port half-way in the stack.

You can do the following:

Define a user vlan on half of the switches and a second one on the others.

Configure spanning tree so that the uplink port on switch1 forwards the first user vlan and the uplink on switch7 forwards the other one. You are then feeding only half of the switches via each side. On a failure sapnning tree will kick in as usual.

Regards,

Leo

[milan.kulik]

AFAIK it should be possible.

Check the root path cost on the core switches your switches 1 and 7 are connected to (let's call them C1 and C2).

Then set the port cost of the port you want to be blocked to be high enough, i.e. it must be higher them the difference between the C1 and C2 root path costs plus the path cost to C1 or C2.

Let's say that C1 root path cost is 16 (for Gbps hops) and C2 root path costs is 20. Then setting switch 4 port cost to 28 should be enough. (I suppose all lines to be 1 Gbps).

My calcullations might not be precise (I don't know your network topology) but if you simply set the switch 4 port cost to high enough it should be blocked.

Regards,

Milan