Re: [Idr] Possibility of empty Link Descriptors in BGP-LS?

Nandan Saha <nandan@arista.com> Wed, 09 October 2019 15:51 UTC

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From: Nandan Saha <nandan@arista.com>
Date: Wed, 9 Oct 2019 21:21:07 +0530
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To: "Ketan Talaulikar (ketant)" <ketant@cisco.com>
Cc: "draft-ketant-idr-rfc7752bis@ietf.org" <draft-ketant-idr-rfc7752bis@ietf.org>, "idr@ietf.org" <idr@ietf.org>, Prakash Badrinarayanan <prakash@arista.com>
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Subject: Re: [Idr] Possibility of empty Link Descriptors in BGP-LS?
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Hi Ketan,

On Wed, Oct 9, 2019 at 7:52 PM Ketan Talaulikar (ketant) <ketant@cisco.com>
wrote:

> Hi Nandan,
>
>
>
> How about adding the following text in Sec 4.2.2?
>
>
>
> The TLVs/sub-TLVs corresponding to the interface addresses and/or the
> local/remote identfiers may not always be signaled in the IGPs unless their
> advertisement is enabled specifically. In such cases, a BGP-LS Producer may
> not be able to generate valid Link NLRIs for such link advertisements from
> the IGPs.
>
Sounds good!

>
>
> Thanks,
>
> Ketan
>
>
>
> *From:* Nandan Saha <nandan@arista.com>
> *Sent:* 16 September 2019 10:11
> *To:* Ketan Talaulikar (ketant) <ketant@cisco.com>
> *Cc:* draft-ketant-idr-rfc7752bis@ietf.org; idr@ietf.org; Prakash
> Badrinarayanan <prakash@arista.com>
> *Subject:* Re: Possibility of empty Link Descriptors in BGP-LS?
>
>
>
> Hi Ketan,
>
>     Thank you for your patience. I thought I had replied to this, but
> somehow hadn't.
>
> On Tue, Aug 20, 2019 at 11:31 PM Ketan Talaulikar (ketant) <
> ketant@cisco.com> wrote:
>
> Hi Nandan,
>
> You are correct that an ISIS implementation (in non-TE/SR) environment
> will not advertise the necessary link descriptors without which it will not
> be possible to describe links completely. It is not just about empty link
> descriptors, but when there are parallel links, they will end up
> overwriting each other's attributes in BGP-LS as those NLRIs would get
> mixed up. IMHO it does not serve much purpose advertising such links via
> BGP-LS.
>
> What text would you suggest we add in the BGP-LS specification for this?
>
> Something along the lines that a BGP-LS producer may not be able to
> generate valid link NLRIs in the absence of the required sub-TLVs in the
> IGP. I'm mostly thinking from the point of view of guiding operators that
> the IGP needs to be configured appropriately.
>
> I'm not fully in agreement about leaving out such links. In the case
> there's only one p2p link between devices we'll simply be losing
> information in BGP-LS, since it's possible to identify such a link
> uniquely. (Though it's an academic discussion since most deployments are
> likely to use bgp-ls for TE)
>
> In anycase, I don't feel too strong about adding new text in case you feel
> it's obvious from the rest of the text.
>
>
> Thanks,
> Ketan
>
> -----Original Message-----
> From: Nandan Saha <nandan@arista.com>
> Sent: 20 August 2019 21:48
> To: draft-ketant-idr-rfc7752bis@ietf.org; idr@ietf.org
> Cc: Prakash Badrinarayanan <prakash@arista.com>om>; Ketan Talaulikar
> (ketant) <ketant@cisco.com>
> Subject: Possibility of empty Link Descriptors in BGP-LS?
>
> Hi folks,
>
> I'm wondering whether some text needs to be added to
> https://tools.ietf.org/html/draft-ketant-idr-rfc7752bis-01#section-4.2.2
> for the case where neither the ipv4/6 address sub tlvs nor link/remote
> identifiers are present in the IGP's LSP/LSA.
> For IS-IS specifically, it seems to me that an IS-IS implementation (in a
> non-TE) scenario is free to leave out the ipv4/6 interface/neighbor address
> sub-tlvs based on https://tools.ietf.org/html/rfc5305#section-3.2, and
> https://tools.ietf.org/html/rfc5305#section-3.3. The link/remote link
> identifiers also appear optional.
>
> In such a case, the link descriptor in the link nlri will be empty which
> is problematic in the case where there are multiple links between 2 nodes
> as there's no way to distinguish between the different links.
>
> Thanks,
> Nandan
>
>