STP question

When 2 switches have redundant links to each other that are not part of an etherchannel, what is the process that causes only one of them to forward traffic? I understand the process behind choosing a DP for each segment, but the text doesn't explain how it comes to that decision when more than one segment lead to the same switch...

Does my confusion make sense or am I thinking about this wrong?

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notgoing2fail

xenodamus wrote: »

When 2 switches have redundant links to each other that are not part of an etherchannel, what is the process that causes only one of them to forward traffic? I understand the process behind choosing a DP for each segment, but the text doesn't explain how it comes to that decision when more than one segment lead to the same switch...

Does my confusion make sense or am I thinking about this wrong?

It's all about the lowest number.

The switch with the lowest bridge ID becomes the root. Then if you have multiple redundant links, you then look at the ports, the lowest port because the DP/RP and the other port is blocked...

xenodamus

I think the lightbulb went off while I was typing a confused response. I was thinking DP=traffic moving over that link. Even if both ports on one switch were DP...the other end of one would be blocking because it's NOT a DP

abefroman

Can the root switch have DP's?

networker050184

All of the roots ports that are connected to other switches would be designated ports.

ConstantlyLearning

notgoing2fail wrote: »

It's all about the lowest number.

The switch with the lowest bridge ID becomes the root. Then if you have multiple redundant links, you then look at the ports, the lowest port because the DP/RP and the other port is blocked...

Well that's in a two switch topology with equal path costs and port priorities.

Full process for choosing the DP/RP on the non-root switch would be:

Lowest root bridge ID
Lowest path cost to the root bridge
Lowest sender bridge ID
Lowest port priority
Lowest port ID

abefroman

What is the difference between a root port and designated port?

xenodamus

abefroman wrote: »

What is the difference between a root port and designated port?

The root port is the 1 port on each switch that hears the best BPDU. It's the port on each switch with the best path to the root.

The designated port is the port that advertises the best BPDU on each segment, which could include ports from many different switches if a hub is used.

All the ports that connect to PC's or other devices qualify as DPs because they are advertising the ONLY (and therefore BEST) BPDU on that segment.