a question about STP 802.1d

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.

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).


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