a question about STP 802.1d

Archived from groups: comp.dcom.lans.ethernet (More info?)

If we I have port A and port B ocnnected from two bridges, is it
mandatory to set port cost the same on two bridges? What if not?

can not find why it is necessary. However it's very possible the cost
has be changed by mistake. I think if it happens each bridge can not
compute path cost consisently, and result in differently views of path
cost, then trouble will surface. Just wonder if the specification
explictly state measures or guide lines to prevent that happen, or my
thought is just not right.
Thanks
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  1. Archived from groups: comp.dcom.lans.ethernet (More info?)

    ask8y@yahoo.com wrote:
    > If we I have port A and port B ocnnected from two bridges, is it
    > mandatory to set port cost the same on two bridges? What if not?
    >
    > can not find why it is necessary. However it's very possible the cost
    > has be changed by mistake. I think if it happens each bridge can not
    > compute path cost consisently, and result in differently views of path
    > cost, then trouble will surface. Just wonder if the specification
    > explictly state measures or guide lines to prevent that happen, or my
    > thought is just not right.
    > Thanks
    >

    OK, the ports SHOULD be the same cost. Don't HAVE to be, but SHOULD be.
    In fact, the cost SHOULD comply with the 802.1d reccommended costs for
    different port types.

    LinkSpeed RecommendedCost RecommendedCostRange
    4Mbps 250 100 to 1000
    10Mbps 100 50 to 600
    16Mbps 62 40 to 400
    100Mbps 19 10 to 60
    1Gbps 4 3 to 10
    10Gbps 2 1 to 5

    I'm no expert on spanning tree, and I don't know the nitty gritty
    details of how it eliminates loops, but given that the lowest cost path
    is what is used, I'd say your idea that not setting the port costs the
    same on each side would interfere with the computation of the least cost
    path, and yes, two switches will see the path differently. I don't THINK
    this will cause a loop, tho, since the Bridge Protocol Data Units are
    exchanged between the switches, and all decisions hing off being able to
    get to the root bridge.

    But i'd say it's best practice (if nothing else) to have the costs be
    the same.
  2. Archived from groups: comp.dcom.lans.ethernet (More info?)

    In article <10rc885sqah0qc9@news.supernews.com>,
    "T. Sean Weintz" <strap@hanh-ct.org> wrote:

    > ask8y@yahoo.com wrote:
    > > If we I have port A and port B ocnnected from two bridges, is it
    > > mandatory to set port cost the same on two bridges? What if not?
    > >
    > > can not find why it is necessary. However it's very possible the cost
    > > has be changed by mistake. I think if it happens each bridge can not
    > > compute path cost consisently, and result in differently views of path
    > > cost, then trouble will surface. Just wonder if the specification
    > > explictly state measures or guide lines to prevent that happen, or my
    > > thought is just not right.
    > > Thanks
    > >
    >
    > OK, the ports SHOULD be the same cost. Don't HAVE to be, but SHOULD be.
    >

    Actually, there is no particular reason why the same link must be
    assigned the same cost as seen from both ends. Remember, only ONE of
    those bridges will ever be the Designated Bridge for that particular
    link. The path cost is computed individually by each bridge contending
    to be the Designated Bridge for a given link; it has no knowledge of
    what other bridges may be using for the cost of that same link. The tree
    will resolve properly regardless of whether all bridges use the same
    cost for that link.

    That said, there is no particular reason why one would go out of their
    way to do such a thing; it might make troubleshooting difficult. (The
    network administrator would have to keep track of the link cost assigned
    at every bridge, rather than just assuming that the link had a
    "universal" cost.) While you can "force" particular topologies by
    adjusting link costs, it is easier to do this by adjusting bridge
    priorities (indeed, this is the reason why bridge priorities exist).


    --
    Rich Seifert Networks and Communications Consulting
    21885 Bear Creek Way
    (408) 395-5700 Los Gatos, CA 95033
    (408) 228-0803 FAX

    Send replies to: usenet at richseifert dot com
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