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New Member

STP questions

Hi

how does spanning tree port cost and port priority work.

I think that after determinig the cost to the root then port priority comes into play. am i right.

My question if i have two ports on my switch with port 1 cost to root less than port 2 and if i set port 2 priority good than port 1 prioroty then will my swith select port 2 as root port?

5 REPLIES
Silver

Re: STP questions

New Member

Re: STP questions

The link doesnot explain about port costs or priorites. It tells abt TCN. Thanx

Silver

Re: STP questions

I am so sorry for the wrong link. Below the section "Election of the Root Bridge" for how to assign the root bridge.

http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_guide_chapter09186a008007e71a.html

*** Quote start

When you change the bridge priority value, you change the probability that the switch will be elected as the root bridge. Configuring a higher value increases the probability; a lower value decreases the probability.

*** Quote end

In response to the question. If your port 2 of your bridge is higher than the original root bridge, then the port 2 will be root bridge. Otherwise, if the priority of the port 2 is less than original root, no matter port 1 priority is lower than potr 2, the root will still be remain at the original switch port.

Hope this helps.

New Member

Re: STP questions

Hi

I was just confused please clear me

1. If I have two ports on switch say fa0/1 and fa0/2. Cost to root bridge through fa0/1 is 4 and through fa0/2 is 6 then is there any way that i can make fa0/2 as my root port by changing the port priority of fa0/2.

Silver

Re: STP questions

Referring to the link that I provided. You can set higher port priority og fa 0/2 to make it as root bridge. Search keyword "Configuring STP Port Priority " in the link.

However, you have to configure the fa0/2 priority is also higher than the remote connected port. Otherwise, the remote switch may be the root bridge due to higher priority.

Hope this helps.

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