Choice to ROOT SWITCH.

[Soon Kwan Kwon]

Hello all,

I'm here to ask you guys about root switch on STP environment.

The thing I know is that Root switch is chosen if I turn off the device after typing the priority 0 or spanning-tree primary.

However, I think it is not real because I set the devices such as 4500X, 3560X, SG300.

That is,the swithes is chooing root switch automatically on real situation without turning off the devices.

Which is the true?

I'm so confused.

Regards,

[InayathUlla Sharieff]

Soon,

Thats expected.

With STP, the key is for all the switches in the network to elect a root bridge that becomes the focal point in the network. All other decisions in the network, such as which port to block and which port to put in forwarding mode, are made from the perspective of this root bridge. A switched environment, which is different from a bridge environment, most likely deals with multiple VLANs. When you implement a root bridge in a switching network, you usually refer to the root bridge as the root switch. Each VLAN must have its own root bridge because each VLAN is a separate broadcast domain. The roots for the different VLANs can all reside in a single switch or in various switches.

Note: The selection of the root switch for a particular VLAN is very important. You can choose the root switch, or you can let the switches decide, which is risky. If you do not control the root selection process, there can be suboptimal paths in your network.

All the switches exchange information for use in the root switch selection and for subsequent configuration of the network. Bridge protocol data units (BPDUs) carry this information. Each switch compares the parameters in the BPDU that the switch sends to a neighbor with the parameters in the BPDU that the switch receives from the neighbor.

In the STP root selection process, less is better. If Switch A advertises a root ID that is a lower number than the root ID that Switch B advertises, the information from Switch A is better. Switch B stops the advertisement of its root ID, and accepts the root ID of Switch A.

HTH

Regards

Inayath

[Soon Kwan Kwon]

Thank you expert,

But I still have a question..

Then, you opinion is that all the devices need no to be turned off?

For example, A, B switches connect to each other.

In this topology, supposing that I connect switch C to A and B ( switch C is the lower ID than A,B)

Switch C is automatically changed root switch? without turning off of Switch A,B..?

Regards,.

[milan.kulik · Accepted solution]

Hi,

you don't need to turn off anything.

The term "STP root election" used in many documents is sometimes misleading.

There is no "election" in fact!

When you power any switch on, it "believes" to be an STP root until it receives an STP BPDU frame from his neighbor which has got a better (lower) STP priority (or the same priority and lower MAC address).

At that moment the switch stops to advertise itself as an STP root and is just forwarding the BPDUs received from the root neighbor (with some fields modified) to other neighbor switches.

See

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2960/software/release/12-2_53_se/configuration/guide/2960scg/swstp.html

for details.

Best regards,

Milan

[Soon Kwan Kwon]

Excellent solving ^^!

Thank you InayathUlla, milan.kulik again!!

Ah... then from now on, I've misunderstood its theory..

I originally figured out that I should turn off the device in order to elect Root switch.

Thanks to you guys, I understand clearly.

Regards,