STP Port Priority

Unanswered Question
Oct 4th, 2008

In a situation where you have two equal cost links going from a noneroot switch to the root switch, the port priority setting on the root switch determines the root port on the noneroot switch. I've tested this by setting the port priority on my Root switches DPs to 112 and then back to 128 while watching the root port change on my nonroot switch.

I wanted to point this out because the ICND2 book seems to say otherwise. Page 92.

I have this problem too.
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Giuseppe Larosa Sun, 10/05/2008 - 12:18

Hello Yuri,

your understanding is correct

All STP

decisions are based on the following sequence of four conditions:

1. Lowest Root Bridge ID

2.Lowest Root Path Cost to Root Bridge

3.Lowest Sender Bridge ID

4.Lowest Sender Port ID

in your case the sender = root bridge and by lowering the STP port priority you change the 4 point in the list, the first 3 match.

If you change the STP port cost on the non-root switch port you are able to change point 2.

This is the more common way to influence the root port election and it can be done on a per vlan basis.

You can get some form of load balancing on two trunk links for different Vlans

Hope to help

Giuseppe

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