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

Jeff Tantsura <jefftant.ietf@gmail.com> Thu, 22 July 2021 00:47 UTC

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From: Jeff Tantsura <jefftant.ietf@gmail.com>
Date: Wed, 21 Jul 2021 17:46:55 -0700
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To: Robert Raszuk <robert@raszuk.net>
Cc: Boris Hassanov <bhassanov=40yahoo.com@dmarc.ietf.org>, 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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What Robert said.
What is the use case to treat Anycast SID differently than a regular Prefix
SID(which it is)?

On Wed, Jul 21, 2021 at 4:20 AM Robert Raszuk <robert@raszuk.net> wrote:

> 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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