Understanding spanning-Tree

Discussion in 'Cisco' started by Chino, Nov 2, 2006.

  1. Chino

    Chino Guest

    Hi all.
    I would ask something about spanning-tree, as I'm not expert to it and I'm
    not sure I understood well how it works.

    Let's start with a question: talking about a switched LAN running more than
    one VLAN, where every switch partecipate in spanning tree.
    After a switch is elected as the root bridge for one VLAN, is it true that
    every packet sent by a host in that VLAN must pass through the root bridge
    before reaching the destination in the same VLAN?

    I have a LAN splitted in two different geographical locations (they are
    layer 2 linked using routers with bridging).
    I all above is true, if I have 2 communicating hosts on one side of the LAN,
    and the root of their VLAN is on the other side, packets exchanged must
    "cross" the bridge to reach the root and then came back, wasting resources
    (bandwidth between the two phisical locations), as I didn't make any tuning
    on the spanning-tree.
    Does it make sense to you?
    Chino, Nov 2, 2006
    #1
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  2. Chino

    Sam Wilson Guest

    In article <eidduh$p9b$>,
    "Chino" <> wrote:

    > Hi all.
    > I would ask something about spanning-tree, as I'm not expert to it and I'm
    > not sure I understood well how it works.
    >
    > Let's start with a question: talking about a switched LAN running more than
    > one VLAN, where every switch partecipate in spanning tree.
    > After a switch is elected as the root bridge for one VLAN, is it true that
    > every packet sent by a host in that VLAN must pass through the root bridge
    > before reaching the destination in the same VLAN?


    No, but if the path between two hosts passes through the root bridge
    then the traffic will flow that way. There can only be one path between
    any two nodes in a tree.

    > I have a LAN splitted in two different geographical locations (they are
    > layer 2 linked using routers with bridging).
    > I all above is true, if I have 2 communicating hosts on one side of the LAN,
    > and the root of their VLAN is on the other side, packets exchanged must
    > "cross" the bridge to reach the root and then came back, wasting resources
    > (bandwidth between the two phisical locations), as I didn't make any tuning
    > on the spanning-tree.


    Since the assumption about traffic passing through the root is not true
    neither is the example. Traffic between nodes on one side the LAN
    should not cross to the other side.

    > Does it make sense to you?


    I'm not sure I can answer that! :)

    Sam
    Sam Wilson, Nov 2, 2006
    #2
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  3. Chino

    BernieM Guest

    "Chino" <> wrote in message
    news:eidduh$p9b$...
    > Hi all.
    > I would ask something about spanning-tree, as I'm not expert to it and I'm
    > not sure I understood well how it works.
    >
    > Let's start with a question: talking about a switched LAN running more
    > than one VLAN, where every switch partecipate in spanning tree.
    > After a switch is elected as the root bridge for one VLAN, is it true that
    > every packet sent by a host in that VLAN must pass through the root bridge
    > before reaching the destination in the same VLAN?


    No. Spanning-tree prevents layer-2 loops from forming but once the layer-2
    topology has been decided normal frame forwarding occurs followinng the
    rules of a switch ie. mac-tables etc.
    >
    > I have a LAN splitted in two different geographical locations (they are
    > layer 2 linked using routers with bridging).
    > I all above is true, if I have 2 communicating hosts on one side of the
    > LAN, and the root of their VLAN is on the other side, packets exchanged
    > must "cross" the bridge to reach the root and then came back, wasting
    > resources (bandwidth between the two phisical locations), as I didn't make
    > any tuning on the spanning-tree.
    > Does it make sense to you?
    >
    BernieM, Nov 2, 2006
    #3
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