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Re: [Lsvr] Need clarification on IGP-Metric TLV for LS Link Attributes in BGP-SPF deployments

"Acee Lindem (acee)" <acee@cisco.com> Fri, 20 March 2020 18:40 UTC

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From: "Acee Lindem (acee)" <acee@cisco.com>
To: Pushpasis Sarkar <pushpasis.ietf@gmail.com>, "draft-ietf-lsvr-bgp-spf@ietf.org" <draft-ietf-lsvr-bgp-spf@ietf.org>
CC: "lsvr@ietf.org" <lsvr@ietf.org>
Thread-Topic: Need clarification on IGP-Metric TLV for LS Link Attributes in BGP-SPF deployments
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Date: Fri, 20 Mar 2020 18:40:09 +0000
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Archived-At: <https://mailarchive.ietf.org/arch/msg/lsvr/5BfGBNV4cZoTXbvD6JJ5waesfek>
Subject: Re: [Lsvr] Need clarification on IGP-Metric TLV for LS Link Attributes in BGP-SPF deployments
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Hi Pushpasis,
I think for BGP-LS SPF we should always use 3 octet metrics. This will offer the most flexibility w/o redefining the TLV. If you agree, I will update the SPF draft to state this.
Thanks,
Acee

From: Pushpasis Sarkar <pushpasis.ietf@gmail.com>
Date: Monday, March 9, 2020 at 11:31 AM
To: "draft-ietf-lsvr-bgp-spf@ietf.org" <draft-ietf-lsvr-bgp-spf@ietf.org>
Cc: "lsvr@ietf.org" <lsvr@ietf.org>
Subject: Need clarification on IGP-Metric TLV for LS Link Attributes in BGP-SPF deployments
Resent-From: <alias-bounces@ietf.org>
Resent-To: Keyur Patel <keyur@arrcus.com>, Acee Lindem <acee@cisco.com>, Shawn Zandi <szandi@linkedin.com>, Wim Henderickx <wim.henderickx@nokia.com>
Resent-Date: Monday, March 9, 2020 at 11:31 AM

Hi Authors,

I need a small clarification on how the Link IGP-Metric TLV (type 1095) for the links originated by an BGP-SPF speaker look like. My doubt is specifically on what would be the length of the metric value. For example, following is the excerpt from RFC7752 section 3.3.2.4 which specifies the length to be 1, 2 or 3 bytes for ISIS narrow-metrics, OSPF and ISIS wide-metrics.


3.3.2.4.  IGP Metric TLV



   The IGP Metric TLV carries the metric for this link.  The length of

   this TLV is variable, depending on the metric width of the underlying

   protocol.  IS-IS small metrics have a length of 1 octet (the two most

   significant bits are ignored).  OSPF link metrics have a length of 2

   octets.  IS-IS wide metrics have a length of 3 octets.



      0                   1                   2                   3

      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     |              Type             |             Length            |

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     //      IGP Link Metric (variable length)      //

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                     Figure 21: IGP Metric TLV Format

What should be the length of the metric field when the origin is a BGP-SPF speaker?

Looking forward to your clarification on this. Also it will be appreciated a lot if a sentence or two can be added to the draft clarifying the above in the next version.

Thanks and regards,
-Pushpasis

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