link cost in network designing

[Sulaiman Ejaz]

Hi,

In spanning-tree algorithem, cost field plays an important role while selecting best BPDU, i wanna ask what is its impact while designing a network ?

I mean what consideration network designer made while desinging a network ?

I'm attaching a diagram to illustrate me question, try to understand that !

regards,

Sulaiman Ejaz.

[Rolf Fischer]

Hi,

by changing the STP cost you can influence which port will become the root port (lowest path cost to the root bridge).

The result is that you influence which path the data traffic will take through the LAN.

In case of various ports with equal lowest path costs, there are additional tie-breakers: Sender bridge ID and port ID.

An interesting option in regard of design is the capability of load-sharing by implementing more than one ST-topology.

With Cisco's per-VLAN ST versions you can do that on a per-VLAN basis, with MST on a per-Instance basis.

A port can be in blocking state for one range of VLANs (or instances) and be in forwarding state for others at the same time. So you can make use of all the existing links instead of using only half of them while the other half is solely for redundancy.

Useful link: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800ae96a.shtml

Hope that helps

Rolf

[devils_advocate]

Assuming both interfaces are running at the same speed then lowering the cost of one of them will make that the active path. By default, if both interfaces are the same speed then the lowerest Interface ID becomes the active path.

If link 2 is in an STP blocking state and you manually lower the cost of that link then it should become the active path.