Basic stp question

[sarg]

Hi, how are you all?

I'm doing a little homework, and I've come across a situation and I don't what it means, if anyone could enlighten me I'd be grateful.

Here's the gist.

 I have this little part of a network on a simple stp exercise.

The priority values are sw1(RB)<sw2<sw3<sw4

The exercise made me turn off stp in switch4.

My question is why is that on sw3 the Fa port is the designated fwd one and the Gi is the altn blocked one.

The dpc on Gig0/2 should be the same as Fa0/2, and it makes sense that it blocks on the sw3 side because sw2 has priority on it's side.

Is it just because the Fast ethernet ports have a lower mac address than the Gi ones? This way we're not taking full advantage of the network right?

Thanks.

[dbeattie]

Hi,

Once the root election is complete, bridge priority is no longer used. This means that once SW1 has been selected as root, only port cost (default is taken from port speed) and port ID(including port priority) matter. In normal running, if we look at SW3 it is a cost of 4 away from root via G0/1 and 27 via G0/2 so it selects G0/1 as its root port. SW2 sees 19 via F0/1 and 12 via G0/1, so it would select G0/1. Similarly SW4 would select G0/2.

Now, the big question is what happens when we turn off STP on SW4. For STP this becomes a single shared segment (effectively a hub). I can't see that this would have any effect on the STP topology, other than shortening the cost across that switch from 8 to 4. This would not affect the topology decision SW2 or SW3.

Hope this helps,

Dave

[paul driver]

Hello

That switch 3 has negotiated to be the shortest path toward the root switch so its designated port would then be defined due the stp port costs of those links between switch 4, So in theory an gig interface will have a lower stp cost then a fastethernet however is all depends on you stp topology.

Do you have different roots for various vlans?
Are you manually using port priority/cost on those links or using stp feature like rootguard uplinkfast ?