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Re: [Idr] Few questions about Segment Routing extensions for BGP LS (draft-ietf-idr-bgp-ls-segment-routing-ext)

Robert Raszuk <robert@raszuk.net> Wed, 21 July 2021 11:20 UTC

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From: Robert Raszuk <robert@raszuk.net>
Date: Wed, 21 Jul 2021 13:19:43 +0200
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To: Boris Hassanov <bhassanov=40yahoo.com@dmarc.ietf.org>
Cc: IDR List <idr@ietf.org>
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Subject: Re: [Idr] Few questions about Segment Routing extensions for BGP LS (draft-ietf-idr-bgp-ls-segment-routing-ext)
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Boris,

IMO Anycast SID is a Prefix SID. Just like anycast address is an IPv4 or
IPv6 address. What makes it an anycast is the fact that it is advertised
from more then one node. So anycast in IP or SR is a deployment construct.

In other words node advertising it may have no clue that it is anycast. And
the moment other nodes from the anycast group stop advertising it - is it
no longer anycast.

To your other question it is my understanding that you use SID/Label TLV
when you have SR-MPLS, and for SRv6 you use Prefix SID TLV.

Best,
R.

On Wed, Jul 21, 2021 at 11:57 AM Boris Hassanov <bhassanov=
40yahoo.com@dmarc.ietf.org> wrote:

> Hi all,
>
> 1) Need an advice, I might be missing something but I see that Anycasi SID
> (3333 in the example below)  is in BGP-LS update as regular Prefix SID TLV:
>
>  Path Attribute - BGP-LS Attribute
>             Flags: 0x80, Optional, Non-transitive, Complete
>                 1... .... = Optional: Set
>                 .0.. .... = Transitive: Not set
>                 ..0. .... = Partial: Not set
>                 ...0 .... = Extended-Length: Not set
>                 .... 0000 = Unused: 0x0
>             Type Code: BGP-LS Attribute (29)
>             Length: 12
>             Link State
>                 Prefix SID TLV
>                     Type: 1158
>                     Length: 8
>                     Flags: 0x40, Node-SID (N)
>                         0... .... = Re-advertisement (R): Not set
>                         .1.. .... = Node-SID (N): Set
>                         ..0. .... = No-PHP (P): Not set
>                         ...0 .... = Explicit-Null (E): Not set
>                         .... 0... = Value (V): Not set
>                         .... .0.. = Local (L): Not set
>                     Algorithm: 0
>                     SID/Index: 3333
>
>
> Here is confusion, how, let's say, a controller, which receives such
> BGP-LS update, could  distinguish it from regular Prefix/Node SID?
>
> Unfortunately draft-ietf-idr-bgp-ls-segment-routing-ext-18 does not
> clarify this issue because it only describes Prefix SID TLV type 1158. Any
> suggestions?
>
> 2)  draft-ietf-idr-bgp-ls-segment-routing-ext-18 has two types of labels
> TLV: SID/Label TLV type 1161 and Prefix SID  TLV type 1158.  I checked
> several vendors implementations of BGP-LS, all of them use only Prefix SID
> TLV.
> Why the draft does not have some conditions or clarifications when to use
> SID/Label TLV and when Prefix SID TLV?
>
> Thank you.
>
> SY,
> Boris
>
>
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