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basic questions about stp

Discussion in 'Routing & Switching' started by insurin, May 4, 2009.

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  1. insurin

    insurin Bit Poster

    In a medium size network consisting of a couple of 6500 catalysts and the rest being standard 3500/2950 access switches. If there is only the default priority set for spanning tree (32768 + vlanid (I think)), does this mean that during a spanning tree election that any switch could become the root bridge, even a 2950.

    I should be setting the Priority on the 6500 cores so they should become the root bridge should I not?


    Once spanning tree has converged and ports are either blocking or forwarding, would a port that has been set to blocking mode pass layer 3 data through it and only block layer 2 data.
  2. Richie1888

    Richie1888 Byte Poster

    I think im right in saying that with default priority the lowest mac breaks the tie which means one of the older switches is liable to win the election.

    when a port is blocking its blocking layer 2 because switches work at layer 2 switches (unless multi layer) have no concept of layer 3 (IP adds)

    I think im right in saying that anyway.
    Certifications: See Sig
    WIP: Security+
  3. ramarchant

    ramarchant New Member

    good practice would be to set the stp priorities low at the core of the network, 8192?, and yes, if you were to add an old switch (ie older therefore lower mac address) to the network it could potentially become the root bridge, stp is a layer 2 mechanism, so no traffic, l2 or l3 would pass through a blocked port, i think ( not sure about routing updates though)
    Certifications: ccna,
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  4. insurin

    insurin Bit Poster

    why do you say no layer 3 would pass through. Is this because in an IP packet it will have the destination IP but also need to know the next hop mac addresses therefore still utilizing layer 2
  5. MrShed

    MrShed Bit Poster

    Think of OSI model.

    In a IP network, L3 data will always be encapsulated in a L2 frame so that it can be transported.

    Therefore, by blocking L2 data, you are automatically dropping the L3 payload within that L2 packet.
    Certifications: CCNA, ITIL Foundation, MCSA, MCDST

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