[rbridge] STP and ISIS
touch at ISI.EDU
Wed Sep 21 16:56:21 PDT 2005
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michsmit at cisco.com wrote:
>>From: rbridge-bounces at postel.org
>>[mailto:rbridge-bounces at postel.org] On Behalf Of Joe Touch
>>Tom Sanders wrote:
>>>Going through draft-perlman-rbridge-03.txt raised a doubt
>>in my mind.
>>>Please let me know if my understanding of the matter is indeed
>>>As per the draft, spanning tree algos concentrate a lot of
>>>certain links as it marks some ports as blocked and only
>>the ones that
>>>are in the forwarding state can pass traffic. But as per my limited
>>>knowledge, wouldnt STP mark only the redundant links as
>>blocked? If we
>>>dont mark them as blocked, then wouldnt that result in transient
>>>Then the draft says that there are other issues as well because of
>>>which the group has decided to use a link state routing
>>>specifically ISIS because of the advantages it offers against OSPF
>>>(TLV encoded, runs over L2, etc).
>>>So, is the proposal to totally do away with STP et. al and run only
>>>ISIS instead? The spanning tree is then computed with the topology
>>>information provided by ISIS. Is this correct?
>>>If this is so, then we will once again mark some ports as blocked,
>>>etc. How different is this from calculating the spanning
>>tree via the
>>The difference is that the spanning tree will be used only
>>for broadcast messages. There may also be more than one
>>spanning tree, e.g., one per origin. In either case, the
>>spanning tree is NOT used for forwarding unicast traffic; the
>>ISIS-configured routing tables are.
> The statement above should be clarified as *known* unicasts do not
> follow the spanning tree. Unicast traffic for unknown destinations will
> still follow the spanning tree.
The only reason unknown unicasts would follow the tree is because they
are broadcast - though the reason they are broadcast is not because they
are sent on broadcast addresses, but rather flooded because of lack of
knowledge of attached port.
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