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

[Zhang Renhai <zhangrenhai@huawei.com>]

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
> 
>