STP question

xenodamusxenodamus Member Posts: 758
in CCNA & CCENT
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?
CISSP | CCNA:R&S/Security | MCSA 2003 | A+ S+ | VCP6-DTM | CCA-V CCP-V
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  • notgoing2failnotgoing2fail Member Posts: 1,138
    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...
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  • xenodamusxenodamus Member Posts: 758
    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
    CISSP | CCNA:R&S/Security | MCSA 2003 | A+ S+ | VCP6-DTM | CCA-V CCP-V
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  • abefromanabefroman Banned Posts: 278
    Can the root switch have DP's?
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  • networker050184networker050184 Mod Posts: 11,962 Mod
    All of the roots ports that are connected to other switches would be designated ports.
    An expert is a man who has made all the mistakes which can be made.
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  • ConstantlyLearningConstantlyLearning Member Posts: 445
    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
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  • abefromanabefroman Banned Posts: 278
    What is the difference between a root port and designated port?
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  • xenodamusxenodamus Member Posts: 758
    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.
    CISSP | CCNA:R&S/Security | MCSA 2003 | A+ S+ | VCP6-DTM | CCA-V CCP-V
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