a question about STP 802.1d

- A
- ask8y
  
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posted
19 years ago

Tue, Dec 7, 2004 8:03 PM

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

- R
- Rich Seifert
  
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posted
19 years ago

Tue, Dec 7, 2004 11:01 PM

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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- T
- T. Sean Weintz
  
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posted
19 years ago

Wed, Dec 8, 2004 12:30 AM

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.