Re: [OSPF] Poll for WG adoption of draft-hegde-ospf-node-admin-tag

Peter Psenak <ppsenak@cisco.com> Wed, 03 September 2014 08:14 UTC

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Date: Wed, 03 Sep 2014 10:13:17 +0200
From: Peter Psenak <ppsenak@cisco.com>
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To: Rob Shakir <rjs@rob.sh>
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Subject: Re: [OSPF] Poll for WG adoption of draft-hegde-ospf-node-admin-tag
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Hi Rob,


On 9/3/14 10:08 , Rob Shakir wrote:
> Hi Peter.
>
> On 26 Aug 2014, at 16:43, Peter Psenak <ppsenak@cisco.com> wrote:
>
>>
>> On 8/26/14 17:32 , Hannes Gredler wrote:
>>>
>>> operators want to assign node-tags as per router function (ABR, PE, core) and then
>>> the LFA-selection becomes much easier to specify. - e.g.
>>> - only pick a LFA that does not cross another PE router.
>>>
>>> similarily it is desirable for "LFA tunnel termination"
>>> to put out a constraint which says
>>> - only pick a PQ neighbor which has node tag 'X'
>>
>> my point is that with the above approach you have to:
>> 1. On candidate PQ nodes configure the tag X
>> 2. on all other nodes configure "only pick a PQ neighbor which has node tag 'X'"
>>
>> It's (2) which makes me feel uncomfortable, as it's a config to be applied to many nodes.
>
> I’m unclear on how one would solve this — the key thing is that there are number of scenarios where it is *operator* preference rather than node capabilities that mean that we want to select a particular node for some certain application. This preference may be on a per-calculating node basis. If this is the case, then a single capability that says that a particular target node is capable of acting in a particular role is not sufficient.
>
> (Consider this scenario:
> 	- rtr-A is in country 1.
> 	- rtr-B is in country 2.
> 	- Both rtr-A and rtf-B are capable of acting as PQ nodes,and need to act as such for ‘local’ nodes (i.e., those in the same country as them).
> 	- rtr-A should never select rtr-B as a PQ.
> In this case, we need some tag that specifies country, as well as some tag that specifies that it is a valid PQ node. We then need specific policy on rtr-A and rtr-B to implement this policy.)
>
> It is very typical that where we have such policy implementations, then we need to configure the behaviour on a per-node basis. This is especially true where policies must consider characteristics of the topology.
>
>
>>>
>>> i found it always strange that we for TE (as an example for
>>> constraining paths) we have got ways to tag links, but
>>> not way to tag nodes - that draft aims to fix that.
>>
>> I'm not against tagging nodes as such. What worries me if we end up using node tags for signalling capabilities of node.
>
> As per the above, I do not think that this mechanism replaces any capability, it just gives an operator a means to be more granular than the binary “supported”/“not supported” view that a flag indicating capabilities does.

I understand. My point was that admin tags should not be used in cases 
where only a binary capability is signaled.

thanks,
Peter

>
> I, of course, support the adoption of this draft as a co-author.
>
> Cheers,
> r.
>
>
> .
>