Re: [Pce] A New Internet-Draft on Advertising of inter-AS TE links

[Zhang Renhai <zhangrenhai@huawei.com>]

Hi, Peng



>I agreed with the following:
> 
> "... we need to look more closely at the scenarios. I don't
>>> think we have given enough thought to the nested domains case (i.e.
>>> areas in ASes) given that both pd-path and brpc (largely) treat the
>>> nested case as simply a flat sequence.
>>
>> Looking at the nested case, what would be the point of domain-scope
>> since all TE-related info for the intra-area links have an area scope ?"
> 
> I guess following the thinking line of this draft, we might need
> another extension so as to advertising "inter-area" TE links into
> non-backbone areas, or, just treat one area as a "domain"?
Not sure if there really are inter-area TE links. Apart from the inter-AS links,
all physical links exist exactly in one area for sure.

> 
> And in Renhai's "[ZRH]I try to give an answer.", last line, how to
> perform this inter-area computation? per-area? or Backward Recursive
> Path Computation in a multi-area AS?
Currently, there is no limitation of computation method, I think this is not what
is intended to be discussed in this draft. However, this draft does provide the 
necessary info no matter which method is deployed.

Thanks,
Zhang Renhai

> 
> Regards,
> Peng
> 
> On 2/6/07, Zhang Renhai <zhangrenhai@huawei.com> wrote:
>> Hi, JP
>>
>> See in-line please.
>>
>> > Hi Adrian,
>> >
>> > On Feb 5, 2007, at 7:11 PM, Adrian Farrel wrote:
>> >
>> >> Hi Greg,
>> >>
>> >>> - would you agree that usually ASBRs are eBGP speakers
>> >>> and are fully mesh connected. In that case eBGP can be
>> >>> viable solution for the inter-AS TE links computation by
>> >>> ASBR.
>> >>
>> >> So the question has to be:
>> >> Do other nodes apart from ASBRs need this information?
>> >>
>> >> What about an ingress LSR trying to compute a path out of the AS?
>> >>
>> >> If we require that the ingress LSR always consults an external PCE
>> >> that is a BGP speaker, then I guess this is fine, but most LSRs
>> >> today are capable of path computation and could handle this case
>> >> (for example, for the pd-path scenario) without needing to consult
>> >> an external PCE.
>> >>
>> >>> - I'm concerned with scaling aspect of flooding inter-AS TE
>> >>> information throughout both AS and an area
>> >>
>> >> I have this concern, too, but I wonder how many TE links we are
>> >> talking about, and how this compares with the number of TE links
>> >> within an area.
>> >>
>> >
>> > It is probably negligible ... Note that by contrast with the approach
>> > proposing to flooding Inter-ASBR TE LSP, we're only looking at
>> > flooding the TE information of the inter-ASBR *links*.
>> >
>> >>> and I see that  you're concerned as well (SHOULD for Type
>> >>> 10 and MAY for Type 11). I think that it would be
>> >>> helpful if use of both Type 10 and Type 11 for inter-AS
>> >>> TE Link advertisement be illustrated by scenarios. I think
>> >>> that use of area scope makes these OSPF extensions less
>> >>> applicable to inter-AS path computation by the head-end
>> >>> LSR/LER.
>> >>
>> >> Yes, that would be the case.
>> >
>> >> I agree that we need to look more closely at the scenarios. I don't
>> >> think we have given enough thought to the nested domains case (i.e.
>> >> areas in ASes) given that both pd-path and brpc (largely) treat the
>> >> nested case as simply a flat sequence.
>> >
>> > Looking at the nested case, what would be the point of domain-scope
>> > since all TE-related info for the intra-area links have an area scope ?
>> [ZRH]I try to give an answer.
>> With a new sub-tlv(remote AS number)and a new link type
>> (inter-AS link type) are specified, in a multi-areas AS, the entry ASBR
>> when receving a path mesg can get the exit ASBR(in another area) with
>> this AS-scope advertisement and the path mesg(downstream AS number
>> is given in ERO). then, the inter-area computation can be performed.
>>
>> Regards,
>> Zhang Renhai
>>
>> >
>> >>
>> >>> - Could you please illustrate which links are excluded by the
>> >>> following:
>> >>> "   Routers or PCEs that are capable of processing advertisements of
>> >>>   inter-AS TE links SHOULD NOT use such links to compute paths that
>> >>>   exit an AS to a remote ASBR and then immediately re-enter the AS.
>> >>>   Such paths would constitute extremely rare occurrences and MUST
>> >>> only
>> >>>   be allowed as the result of specific policy configuration at the
>> >>>   router or PCE computing the path."
>> >>> Are there two links that interconnect a pair of ASBRs that belong
>> >>> to two
>> >>> different neighboring ASes?
>> >>
>> >> Renhai can comment, but I assumed that this meant that two ASes are
>> >> linked by more than two TE links. The LSP should not under normal
>> >> circumstances leave AS1 to AS2 through TE link 1 and return to AS1
>> >> from AS2 through TE link 2.
>> >>
>> >> The example you give (ASBR1 in AS1 connects to ASBR2 in AS2 with
>> >> two links, the LSP goes out on one and back on the other) would be
>> >> detected as a loop in RSVP-TE, and would not offer any benefit anyway.
>> >
>> > Thanks.
>> >
>> > JP.
>> >
>> >>
>> >> Regards,
>> >> Adrian
>> >
>> >
>>
>>
>