[rbridge] Default Link Costs in base protocol -13

[Donald Eastlake]

The rule specified in IEEE 802.1D-2004 in Section 17.14 is twenty
trillion (20,000,000,000,000) divided by the port speed in bits per
second. The actual values given there are for the 32-bit cost field in
RSTP BPDUs but for use in the 16-bit cost field of STP BPDUs, it says
to just clip larger values at the maximum that will fit of 65,535.

So it sounds like we should go with this formula but clipping larger
values at the 16.777,214 value (2**24 - 2) that will fit into the
24-bit cost field of the extended IS-IS metric. (2**24 - 1 is a magic
value in IS-IS which prohibits use of the link in SPF calculations.)
This causes the cost to saturate for ports slower than about 1.2
million bps. This all sounds about right for 802.3 links, which is
what we are aiming for, and is reasonably forward looking, although it
would be inappropriate for wireless links, for example.

Thanks,
Donald
=============================
 Donald E. Eastlake 3rd   +1-508-634-2066 (home)
 155 Beaver Street
 Milford, MA 01757 USA
 d3e3e3 at gmail.com

On Thu, Jul 30, 2009 at 12:22 PM, mike shand<mshand at cisco.com> wrote:
> Dinesh G Dutt wrote:
>>
>> Radia Perlman wrote:
>>
>>>
>>> No objection, though perhaps if there are only a few (say less than 10)
>>> different link speeds in common use,
>>> it might be useful to enumerate the default costs for each of those
>>> values.
>>>
>>
>> Doesn't IS-IS specify something like this already ?
>
> No it doesn't.
>>
>>  If it didn't, I'd probably stay with the formula used by IEEE in
>> computing the link cost in spanning tree protocol unless there is a strong
>> reason to change it.
>>
>
> That sounds reasonable
>
>   Mike
>
>> Radia, it is not possible to enumerate primarily because of link
>> aggregation because of which you can get various speeds.
>>
>>>
>>> And, is it always obvious to a zero configuration Rbridge what speed each
>>> of its ports is?
>>>
>>
>> Yes.
>>
>> Dinesh
>>
>>>
>>> Radia
>>>
>>> Donald Eastlake wrote:
>>>
>>>>
>>>> Hi,
>>>>
>>>> I'm thinking that something like this should be added to item 1 in
>>>> section 4.2.4.4 of the base protocol draft version -13:
>>>>
>>>>
>>>> To facilitate efficient zero configuration operation, RBridges SHOULD,
>>>> by default, set the cost of a link to one trillion (1,000,000,000,000)
>>>> diviided by the RBridge port's bit rate rounded to the nearest value
>>>> that will fit in the wide metric; for example, the cost for a
>>>> link accessed by a 1Gbps port would default to 1,000. However, the
>>>> cost MAY, by default, be increased if the link appears to be a bridged
>>>> LAN.
>>>>
>>>>
>>>> Thanks,
>>>> Donald
>>>> =============================
>>>>  Donald E. Eastlake 3rd   +1-508-634-2066 (home)
>>>>  155 Beaver Street
>>>>  Milford, MA 01757 USA
>>>>  d3e3e3 at gmail.com
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>>>
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>>
>>
>
>