Modifying Spanning Tree Costs

[JohnTylerPearce]

I just wanted to clarify a few things.

When you have Switch A and Switch B connected together via a trunk. Let's this link is a 100mbps link, so it has a STP cost of 19,

and I want it to have a cost of 10 on Switch B. As far as I know, all you do is run a 'spanning-tree vlan 1 port cost 10' command for

instance on the port going from B to A. That way when A sends out its bpdu with a cost of 0, it hits Switch B's port and then adds

0+10 = 10 instead of 0+(19 default) = 19.

Also, is it correct that when you modify port priority, for instance I would need to modify the priority on the port from Switch A going to

Switch B?

[jimmysands73_2 · Accepted solution]

Cost is done on the recv'g interface config.

Port priority is done on the opposite interface.

For a great video explaining, go to this link:

http://www.youtube.com/watch?v=8CZiVd2K9CI&feature=player_embedded

Its by Keith Barker, great video about both concepts.

Message was edited by: Jimmy Sands

[Mohamed Sobair]

Hi John,

STP uses some criteria to determine the (root port) , the root port in a designated bridge is a port that recieves the BPDU from the root bridge as well as sends TCN BPDUs towards the root.

STP choses the root port (RP) based on the following:

1-  The Lowest port cost on the designated bridge is elected first.

2-  If there is a tie, the Lowest port priority.

3-  If there is a tie, the lowest port-id.

Now, lets consider we have the following example:

SWA (F0/0) ------------------------------------------ SWB (F0/0)

SWA (F0/1) ------------------------------------------ SWB (F0/1)

Now lets assume SWA is the root bridge for whatever Vlanx, SWB determines its root port based on the lowest cost of its ports F0/0  OR  F0/1. if both ports have the Same Cost, then SWB choses the port with lowest port priority, if there is still a tie, SWB choses F0/0 as its root port since it has the Lowest Port-ID.

And yes, you are correct about your describtion.

Regards,

Mohamed

[JohnTylerPearce]

Thanks for the claification.