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

Nandan Saha <> Mon, 16 September 2019 04:41 UTC

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From: Nandan Saha <>
Date: Mon, 16 Sep 2019 10:11:26 +0530
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To: "Ketan Talaulikar (ketant)" <>
Cc: "" <>, "" <>, Prakash Badrinarayanan <>
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Subject: Re: [Idr] Possibility of empty Link Descriptors in BGP-LS?
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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) <>

> 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
> 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 <>
> Sent: 20 August 2019 21:48
> To:;
> Cc: Prakash Badrinarayanan <>; Ketan Talaulikar
> (ketant) <>
> Subject: Possibility of empty Link Descriptors in BGP-LS?
> Hi folks,
> I'm wondering whether some text needs to be added to
> 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, and
> 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