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

"Ketan Talaulikar (ketant)" <ketant@cisco.com> Tue, 20 August 2019 18:01 UTC

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From: "Ketan Talaulikar (ketant)" <ketant@cisco.com>
To: Nandan Saha <nandan@arista.com>, "draft-ketant-idr-rfc7752bis@ietf.org" <draft-ketant-idr-rfc7752bis@ietf.org>, "idr@ietf.org" <idr@ietf.org>
CC: Prakash Badrinarayanan <prakash@arista.com>
Thread-Topic: Possibility of empty Link Descriptors in BGP-LS?
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Date: Tue, 20 Aug 2019 18:01:47 +0000
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Subject: Re: [Idr] Possibility of empty Link Descriptors in BGP-LS?
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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?

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>; 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

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